PIC


Virtual I/O Device (VIRTIO) Version 1.1

Committee Specification 01

11 April 2019

Specification URIs

This version:
https://docs.oasis-open.org/virtio/virtio/v1.1/cs01/tex/ (Authoritative)
https://docs.oasis-open.org/virtio/virtio/v1.1/cs01/virtio-v1.1-cs01.pdf
https://docs.oasis-open.org/virtio/virtio/v1.1/cs01/virtio-v1.1-cs01.html

Previous version:
N/A

Latest version:
https://docs.oasis-open.org/virtio/virtio/v1.1/virtio-v1.1.pdf
https://docs.oasis-open.org/virtio/virtio/v1.1/virtio-v1.1.html

Technical Committee:
OASIS Virtual I/O Device (VIRTIO) TC

Chair:
Michael S. Tsirkin (mst@redhat.com), Red Hat

Editors:
Michael S. Tsirkin (mst@redhat.com), Red Hat
Cornelia Huck (cohuck@redhat.com), Red Hat

Additional artifacts:
This prose specification is one component of a Work Product that also includes:

Related work:
This specification replaces or supersedes:

Abstract:
This document describes the specifications of the “virtio” family of devices. These devices are found in virtual environments, yet by design they look like physical devices to the guest within the virtual machine - and this document treats them as such. This similarity allows the guest to use standard drivers and discovery mechanisms.

The purpose of virtio and this specification is that virtual environments and guests should have a straightforward, efficient, standard and extensible mechanism for virtual devices, rather than boutique per-environment or per-OS mechanisms.

Status:
This document was last revised or approved by the Virtual I/O Device (VIRTIO) TC on the above date. The level of approval is also listed above. Check the “Latest version” location noted above for possible later revisions of this document. Any other numbered Versions and other technical work produced by the Technical Committee (TC) are listed at https://www.oasis-open.org/committees/tc_home.php?wg_abbrev=virtio#technical.

Technical Committee members should send comments on this specification to the Technical Committee’s email list. Others should send comments to the Technical Committee by using the “Send A Comment” button on the Technical Committee’s web page at https://www.oasis-open.org/committees/virtio/.

This specification is provided under the Non-Assertion Mode of the OASIS IPR Policy, the mode chosen when the Technical Committee was established. For information on whether any patents have been disclosed that may be essential to implementing this specification, and any offers of patent licensing terms, please refer to the Intellectual Property Rights section of the TC’s web page (https://github.com/oasis-tcs/virtio-admin/blob/master/IPR.md).

Note that any machine-readable content (Computer Language Definitions) declared Normative for this Work Product is provided in separate plain text files. In the event of a discrepancy between any such plain text file and display content in the Work Product’s prose narrative document(s), the content in the separate plain text file prevails.

Citation format:
When referencing this specification the following citation format should be used:

[VIRTIO-v1.1]
Virtual I/O Device (VIRTIO) Version 1.1. Edited by Michael S. Tsirkin and Cornelia Huck. 11 April 2019. OASIS Committee Specification 01. https://docs.oasis-open.org/virtio/virtio/v1.1/cs01/virtio-v1.1-cs01.html. Latest version: https://docs.oasis-open.org/virtio/virtio/v1.1/virtio-v1.1.html.

__________________________________________________________________

Notices

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This specification is provided under the Non-Assertion Mode of the OASIS IPR Policy, the mode chosen when the Technical Committee was established. For information on whether any patents have been disclosed that may be essential to implementing this specification, and any offers of patent licensing terms, please refer to the Intellectual Property Rights section of the TC’s web page (https://github.com/oasis-tcs/virtio-admin/blob/master/IPR.md).

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__________________________________________________________________

Table of Contents

1 Introduction
 1.1 Normative References
 1.2 Non-Normative References
 1.3 Terminology
  1.3.1 Legacy Interface: Terminology
  1.3.2 Transition from earlier specification drafts
 1.4 Structure Specifications
2 Basic Facilities of a Virtio Device
 2.1 Device Status Field
  2.1.1 Driver Requirements: Device Status Field
  2.1.2 Device Requirements: Device Status Field
 2.2 Feature Bits
  2.2.1 Driver Requirements: Feature Bits
  2.2.2 Device Requirements: Feature Bits
  2.2.3 Legacy Interface: A Note on Feature Bits
 2.3 Notifications
 2.4 Device Configuration Space
  2.4.1 Driver Requirements: Device Configuration Space
  2.4.2 Device Requirements: Device Configuration Space
  2.4.3 Legacy Interface: A Note on Device Configuration Space endian-ness
  2.4.4 Legacy Interface: Device Configuration Space
 2.5 Virtqueues
 2.6 Split Virtqueues
  2.6.1 Driver Requirements: Virtqueues
  2.6.2 Legacy Interfaces: A Note on Virtqueue Layout
  2.6.3 Legacy Interfaces: A Note on Virtqueue Endianness
  2.6.4 Message Framing
  2.6.5 The Virtqueue Descriptor Table
  2.6.6 The Virtqueue Available Ring
  2.6.7 Used Buffer Notification Suppression
  2.6.8 The Virtqueue Used Ring
  2.6.9 In-order use of descriptors
  2.6.10 Available Buffer Notification Suppression
  2.6.11 Helpers for Operating Virtqueues
  2.6.12 Virtqueue Operation
  2.6.13 Supplying Buffers to The Device
  2.6.14 Receiving Used Buffers From The Device
 2.7 Packed Virtqueues
  2.7.1 Driver and Device Ring Wrap Counters
  2.7.2 Polling of available and used descriptors
  2.7.3 Write Flag
  2.7.4 Element Address and Length
  2.7.5 Scatter-Gather Support
  2.7.6 Next Flag: Descriptor Chaining
  2.7.7 Indirect Flag: Scatter-Gather Support
  2.7.8 In-order use of descriptors
  2.7.9 Multi-buffer requests
  2.7.10 Driver and Device Event Suppression
  2.7.11 Driver Requirements: Virtqueues
  2.7.12 Device Requirements: Virtqueues
  2.7.13 The Virtqueue Descriptor Format
  2.7.14 Event Suppression Structure Format
  2.7.15 Device Requirements: The Virtqueue Descriptor Table
  2.7.16 Driver Requirements: The Virtqueue Descriptor Table
  2.7.17 Driver Requirements: Scatter-Gather Support
  2.7.18 Device Requirements: Scatter-Gather Support
  2.7.19 Driver Requirements: Indirect Descriptors
  2.7.20 Virtqueue Operation
  2.7.21 Supplying Buffers to The Device
  2.7.22 Receiving Used Buffers From The Device
  2.7.23 Driver notifications
 2.8 Shared Memory Regions
  2.8.1 Addressing within regions
  2.8.2 Device Requirements: Shared Memory Regions
3 General Initialization And Device Operation
 3.1 Device Initialization
  3.1.1 Driver Requirements: Device Initialization
  3.1.2 Legacy Interface: Device Initialization
 3.2 Device Operation
  3.2.1 Notification of Device Configuration Changes
 3.3 Device Cleanup
  3.3.1 Driver Requirements: Device Cleanup
4 Virtio Transport Options
 4.1 Virtio Over PCI Bus
  4.1.1 Device Requirements: Virtio Over PCI Bus
  4.1.2 PCI Device Discovery
  4.1.3 PCI Device Layout
  4.1.4 Virtio Structure PCI Capabilities
  4.1.5 PCI-specific Initialization And Device Operation
 4.2 Virtio Over MMIO
  4.2.1 MMIO Device Discovery
  4.2.2 MMIO Device Register Layout
  4.2.3 MMIO-specific Initialization And Device Operation
  4.2.4 Legacy interface
 4.3 Virtio Over Channel I/O
  4.3.1 Basic Concepts
  4.3.2 Device Initialization
  4.3.3 Device Operation
5 Device Types
 5.1 Network Device
  5.1.1 Device ID
  5.1.2 Virtqueues
  5.1.3 Feature bits
  5.1.4 Device configuration layout
  5.1.5 Device Initialization
  5.1.6 Device Operation
 5.2 Block Device
  5.2.1 Device ID
  5.2.2 Virtqueues
  5.2.3 Feature bits
  5.2.4 Device configuration layout
  5.2.5 Device Initialization
  5.2.6 Device Operation
 5.3 Console Device
  5.3.1 Device ID
  5.3.2 Virtqueues
  5.3.3 Feature bits
  5.3.4 Device configuration layout
  5.3.5 Device Initialization
  5.3.6 Device Operation
 5.4 Entropy Device
  5.4.1 Device ID
  5.4.2 Virtqueues
  5.4.3 Feature bits
  5.4.4 Device configuration layout
  5.4.5 Device Initialization
  5.4.6 Device Operation
 5.5 Traditional Memory Balloon Device
  5.5.1 Device ID
  5.5.2 Virtqueues
  5.5.3 Feature bits
  5.5.4 Device configuration layout
  5.5.5 Device Initialization
  5.5.6 Device Operation
 5.6 SCSI Host Device
  5.6.1 Device ID
  5.6.2 Virtqueues
  5.6.3 Feature bits
  5.6.4 Device configuration layout
  5.6.5 Device Requirements: Device Initialization
  5.6.6 Device Operation
 5.7 GPU Device
  5.7.1 Device ID
  5.7.2 Virtqueues
  5.7.3 Feature bits
  5.7.4 Device configuration layout
  5.7.5 Device Requirements: Device Initialization
  5.7.6 Device Operation
  5.7.7 VGA Compatibility
 5.8 Input Device
  5.8.1 Device ID
  5.8.2 Virtqueues
  5.8.3 Feature bits
  5.8.4 Device configuration layout
  5.8.5 Device Initialization
  5.8.6 Device Operation
 5.9 Crypto Device
  5.9.1 Device ID
  5.9.2 Virtqueues
  5.9.3 Feature bits
  5.9.4 Supported crypto services
  5.9.5 Device configuration layout
  5.9.6 Device Initialization
  5.9.7 Device Operation
 5.10 Socket Device
  5.10.1 Device ID
  5.10.2 Virtqueues
  5.10.3 Feature bits
  5.10.4 Device configuration layout
  5.10.5 Device Initialization
  5.10.6 Device Operation
 5.11 File System Device
  5.11.1 Device ID
  5.11.2 Virtqueues
  5.11.3 Feature bits
  5.11.4 Device configuration layout
  5.11.5 Device Initialization
  5.11.6 Device Operation
6 Reserved Feature Bits
 6.1 Driver Requirements: Reserved Feature Bits
 6.2 Device Requirements: Reserved Feature Bits
 6.3 Legacy Interface: Reserved Feature Bits
7 Conformance
 7.1 Conformance Targets
 7.2 Clause 1: Driver Conformance
  7.2.1 Clause 2: PCI Driver Conformance
  7.2.2 Clause 3: MMIO Driver Conformance
  7.2.3 Clause 4: Channel I/O Driver Conformance
  7.2.4 Clause 5: Network Driver Conformance
  7.2.5 Clause 6: Block Driver Conformance
  7.2.6 Clause 7: Console Driver Conformance
  7.2.7 Clause 8: Entropy Driver Conformance
  7.2.8 Clause 9: Traditional Memory Balloon Driver Conformance
  7.2.9 Clause 10: SCSI Host Driver Conformance
  7.2.10 Clause 11: Input Driver Conformance
  7.2.11 Clause 12: Crypto Driver Conformance
  7.2.12 Clause 13: Socket Driver Conformance
 7.3 Clause 14: Device Conformance
  7.3.1 Clause 15: PCI Device Conformance
  7.3.2 Clause 16: MMIO Device Conformance
  7.3.3 Clause 17: Channel I/O Device Conformance
  7.3.4 Clause 18: Network Device Conformance
  7.3.5 Clause 19: Block Device Conformance
  7.3.6 Clause 20: Console Device Conformance
  7.3.7 Clause 21: Entropy Device Conformance
  7.3.8 Clause 22: Traditional Memory Balloon Device Conformance
  7.3.9 Clause 23: SCSI Host Device Conformance
  7.3.10 Clause 24: Input Device Conformance
  7.3.11 Clause 25: Crypto Device Conformance
  7.3.12 Clause 26: Socket Device Conformance
 7.4 Clause 27: Legacy Interface: Transitional Device and Transitional Driver Conformance
A virtio_queue.h
B Creating New Device Types
 B.1 How Many Virtqueues?
 B.2 What Device Configuration Space Layout?
 B.3 What Device Number?
 B.4 How many MSI-X vectors? (for PCI)
 B.5 Device Improvements
C Acknowledgements
D Revision History


1 Introduction

This document describes the specifications of the “virtio” family of devices. These devices are found in virtual environments, yet by design they look like physical devices to the guest within the virtual machine - and this document treats them as such. This similarity allows the guest to use standard drivers and discovery mechanisms.

The purpose of virtio and this specification is that virtual environments and guests should have a straightforward, efficient, standard and extensible mechanism for virtual devices, rather than boutique per-environment or per-OS mechanisms.

Straightforward:
Virtio devices use normal bus mechanisms of interrupts and DMA which should be familiar to any device driver author. There is no exotic page-flipping or COW mechanism: it’s just a normal device.1
Efficient:
Virtio devices consist of rings of descriptors for both input and output, which are neatly laid out to avoid cache effects from both driver and device writing to the same cache lines.
Standard:
Virtio makes no assumptions about the environment in which it operates, beyond supporting the bus to which device is attached. In this specification, virtio devices are implemented over MMIO, Channel I/O and PCI bus transports 2, earlier drafts have been implemented on other buses not included here.
Extensible:
Virtio devices contain feature bits which are acknowledged by the guest operating system during device setup. This allows forwards and backwards compatibility: the device offers all the features it knows about, and the driver acknowledges those it understands and wishes to use.

1.1 Normative References

[RFC2119]

Bradner S., “Key words for use in RFCs to Indicate Requirement Levels”, BCP 14, RFC 2119, March 1997.
http://www.ietf.org/rfc/rfc2119.txt

[S390 PoP]

z/Architecture Principles of Operation, IBM Publication SA22-7832,
http://publibfi.boulder.ibm.com/epubs/pdf/dz9zr009.pdf, and any future revisions

[S390 Common I/O]

ESA/390 Common I/O-Device and Self-Description, IBM Publication SA22-7204,
http://publibfp.dhe.ibm.com/cgi-bin/bookmgr/BOOKS/dz9ar501/CCONTENTS, and any future revisions

[PCI]

Conventional PCI Specifications,
http://www.pcisig.com/specifications/conventional/, PCI-SIG

[PCIe]

PCI Express Specifications
http://www.pcisig.com/specifications/pciexpress/, PCI-SIG

[IEEE 802]

IEEE Standard for Local and Metropolitan Area Networks: Overview and Architecture,
http://www.ieee802.org/, IEEE

[SAM]

SCSI Architectural Model,
http://www.t10.org/cgi-bin/ac.pl?t=f&f=sam4r05.pdf

[SCSI MMC]

SCSI Multimedia Commands,
http://www.t10.org/cgi-bin/ac.pl?t=f&f=mmc6r00.pdf

[FUSE]

Linux FUSE interface,
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/uapi/linux/fuse.h

1.2 Non-Normative References

[Virtio PCI Draft]

Virtio PCI Draft Specification
http://ozlabs.org/~rusty/virtio-spec/virtio-0.9.5.pdf

1.3 Terminology

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in [RFC2119].

1.3.1 Legacy Interface: Terminology

Specification drafts preceding version 1.0 of this specification (e.g. see [Virtio PCI Draft]) defined a similar, but different interface between the driver and the device. Since these are widely deployed, this specification accommodates OPTIONAL features to simplify transition from these earlier draft interfaces.

Specifically devices and drivers MAY support:

Legacy Interface
is an interface specified by an earlier draft of this specification (before 1.0)
Legacy Device
is a device implemented before this specification was released, and implementing a legacy interface on the host side
Legacy Driver
is a driver implemented before this specification was released, and implementing a legacy interface on the guest side

Legacy devices and legacy drivers are not compliant with this specification.

To simplify transition from these earlier draft interfaces, a device MAY implement:

Transitional Device
a device supporting both drivers conforming to this specification, and allowing legacy drivers.

Similarly, a driver MAY implement:

Transitional Driver
a driver supporting both devices conforming to this specification, and legacy devices.
Note: Legacy interfaces are not required; ie. don’t implement them unless you have a need for backwards compatibility!

Devices or drivers with no legacy compatibility are referred to as non-transitional devices and drivers, respectively.

1.3.2 Transition from earlier specification drafts

For devices and drivers already implementing the legacy interface, some changes will have to be made to support this specification.

In this case, it might be beneficial for the reader to focus on sections tagged "Legacy Interface" in the section title. These highlight the changes made since the earlier drafts.

1.4 Structure Specifications

Many device and driver in-memory structure layouts are documented using the C struct syntax. All structures are assumed to be without additional padding. To stress this, cases where common C compilers are known to insert extra padding within structures are tagged using the GNU C __attribute__((packed)) syntax.

For the integer data types used in the structure definitions, the following conventions are used:

u8, u16, u32, u64
An unsigned integer of the specified length in bits.
le16, le32, le64
An unsigned integer of the specified length in bits, in little-endian byte order.
be16, be32, be64
An unsigned integer of the specified length in bits, in big-endian byte order.

Some of the fields to be defined in this specification don’t start or don’t end on a byte boundary. Such fields are called bit-fields. A set of bit-fields is always a sub-division of an integer typed field.

Bit-fields within integer fields are always listed in order, from the least significant to the most significant bit. The bit-fields are considered unsigned integers of the specified width with the next in significance relationship of the bits preserved.

For example:

struct S { 
        be16 { 
                A : 15; 
                B : 1; 
        } x; 
        be16 y; 
};

documents the value A stored in the low 15 bit of x and the value B stored in the high bit of x, the 16-bit integer x in turn stored using the big-endian byte order at the beginning of the structure S, and being followed immediately by an unsigned integer y stored in big-endian byte order at an offset of 2 bytes (16 bits) from the beginning of the structure.

Note that this notation somewhat resembles the C bitfield syntax but should not be naively converted to a bitfield notation for portable code: it matches the way bitfields are packed by C compilers on little-endian architectures but not the way bitfields are packed by C compilers on big-endian architectures.

Assuming that CPU_TO_BE16 converts a 16-bit integer from a native CPU to the big-endian byte order, the following is the equivalent portable C code to generate a value to be stored into x:

CPU_TO_BE16(B << 15 | A)


2 Basic Facilities of a Virtio Device

A virtio device is discovered and identified by a bus-specific method (see the bus specific sections: 4.1 Virtio Over PCI Bus, 4.2 Virtio Over MMIO and 4.3 Virtio Over Channel I/O). Each device consists of the following parts:

2.1 Device Status Field

During device initialization by a driver, the driver follows the sequence of steps specified in 3.1.

The device status field provides a simple low-level indication of the completed steps of this sequence. It’s most useful to imagine it hooked up to traffic lights on the console indicating the status of each device. The following bits are defined (listed below in the order in which they would be typically set):

ACKNOWLEDGE (1)
Indicates that the guest OS has found the device and recognized it as a valid virtio device.
DRIVER (2)
Indicates that the guest OS knows how to drive the device. Note: There could be a significant (or infinite) delay before setting this bit. For example, under Linux, drivers can be loadable modules.
FAILED (128)
Indicates that something went wrong in the guest, and it has given up on the device. This could be an internal error, or the driver didn’t like the device for some reason, or even a fatal error during device operation.
FEATURES_OK (8)
Indicates that the driver has acknowledged all the features it understands, and feature negotiation is complete.
DRIVER_OK (4)
Indicates that the driver is set up and ready to drive the device.
DEVICE_NEEDS_RESET (64)
Indicates that the device has experienced an error from which it can’t recover.

2.1.1 Driver Requirements: Device Status Field

The driver MUST update device status, setting bits to indicate the completed steps of the driver initialization sequence specified in 3.1. The driver MUST NOT clear a device status bit. If the driver sets the FAILED bit, the driver MUST later reset the device before attempting to re-initialize.

The driver SHOULD NOT rely on completion of operations of a device if DEVICE_NEEDS_RESET is set. Note: For example, the driver can’t assume requests in flight will be completed if DEVICE_NEEDS_RESET is set, nor can it assume that they have not been completed. A good implementation will try to recover by issuing a reset.

2.1.2 Device Requirements: Device Status Field

The device MUST initialize device status to 0 upon reset.

The device MUST NOT consume buffers or send any used buffer notifications to the driver before DRIVER_OK.

The device SHOULD set DEVICE_NEEDS_RESET when it enters an error state that a reset is needed. If DRIVER_OK is set, after it sets DEVICE_NEEDS_RESET, the device MUST send a device configuration change notification to the driver.

2.2 Feature Bits

Each virtio device offers all the features it understands. During device initialization, the driver reads this and tells the device the subset that it accepts. The only way to renegotiate is to reset the device.

This allows for forwards and backwards compatibility: if the device is enhanced with a new feature bit, older drivers will not write that feature bit back to the device. Similarly, if a driver is enhanced with a feature that the device doesn’t support, it see the new feature is not offered.

Feature bits are allocated as follows:

0 to 23
Feature bits for the specific device type
24 to 37
Feature bits reserved for extensions to the queue and feature negotiation mechanisms
38 and above
Feature bits reserved for future extensions.
Note: For example, feature bit 0 for a network device (i.e. Device ID 1) indicates that the device supports checksumming of packets.

In particular, new fields in the device configuration space are indicated by offering a new feature bit.

2.2.1 Driver Requirements: Feature Bits

The driver MUST NOT accept a feature which the device did not offer, and MUST NOT accept a feature which requires another feature which was not accepted.

The driver SHOULD go into backwards compatibility mode if the device does not offer a feature it understands, otherwise MUST set the FAILED device status bit and cease initialization.

2.2.2 Device Requirements: Feature Bits

The device MUST NOT offer a feature which requires another feature which was not offered. The device SHOULD accept any valid subset of features the driver accepts, otherwise it MUST fail to set the FEATURES_OK device status bit when the driver writes it.

If a device has successfully negotiated a set of features at least once (by accepting the FEATURES_OK device status bit during device initialization), then it SHOULD NOT fail re-negotiation of the same set of features after a device or system reset. Failure to do so would interfere with resuming from suspend and error recovery.

2.2.3 Legacy Interface: A Note on Feature Bits

Transitional Drivers MUST detect Legacy Devices by detecting that the feature bit VIRTIO_F_VERSION_1 is not offered. Transitional devices MUST detect Legacy drivers by detecting that VIRTIO_F_VERSION_1 has not been acknowledged by the driver.

In this case device is used through the legacy interface.

Legacy interface support is OPTIONAL. Thus, both transitional and non-transitional devices and drivers are compliant with this specification.

Requirements pertaining to transitional devices and drivers is contained in sections named ’Legacy Interface’ like this one.

When device is used through the legacy interface, transitional devices and transitional drivers MUST operate according to the requirements documented within these legacy interface sections. Specification text within these sections generally does not apply to non-transitional devices.

2.3 Notifications

The notion of sending a notification (driver to device or device to driver) plays an important role in this specification. The modus operandi of the notifications is transport specific.

There are three types of notifications:

Configuration change notifications and used buffer notifications are sent by the device, the recipient is the driver. A configuration change notification indicates that the device configuration space has changed; a used buffer notification indicates that a buffer may have been made used on the virtqueue designated by the notification.

Available buffer notifications are sent by the driver, the recipient is the device. This type of notification indicates that a buffer may have been made available on the virtqueue designated by the notification.

The semantics, the transport-specific implementations, and other important aspects of the different notifications are specified in detail in the following chapters.

Most transports implement notifications sent by the device to the driver using interrupts. Therefore, in previous versions of this specification, these notifications were often called interrupts. Some names defined in this specification still retain this interrupt terminology. Occasionally, the term event is used to refer to a notification or a receipt of a notification.

2.4 Device Configuration Space

Device configuration space is generally used for rarely-changing or initialization-time parameters. Where configuration fields are optional, their existence is indicated by feature bits: Future versions of this specification will likely extend the device configuration space by adding extra fields at the tail. Note: The device configuration space uses the little-endian format for multi-byte fields.

Each transport also provides a generation count for the device configuration space, which will change whenever there is a possibility that two accesses to the device configuration space can see different versions of that space.

2.4.1 Driver Requirements: Device Configuration Space

Drivers MUST NOT assume reads from fields greater than 32 bits wide are atomic, nor are reads from multiple fields: drivers SHOULD read device configuration space fields like so:

u32 before, after; 
do { 
        before = get_config_generation(device); 
        // read config entry/entries. 
        after = get_config_generation(device); 
} while (after != before);

For optional configuration space fields, the driver MUST check that the corresponding feature is offered before accessing that part of the configuration space. Note: See section 3.1 for details on feature negotiation.

Drivers MUST NOT limit structure size and device configuration space size. Instead, drivers SHOULD only check that device configuration space is large enough to contain the fields necessary for device operation. Note: For example, if the specification states that device configuration space ’includes a single 8-bit field’ drivers should understand this to mean that the device configuration space might also include an arbitrary amount of tail padding, and accept any device configuration space size equal to or greater than the specified 8-bit size.

2.4.2 Device Requirements: Device Configuration Space

The device MUST allow reading of any device-specific configuration field before FEATURES_OK is set by the driver. This includes fields which are conditional on feature bits, as long as those feature bits are offered by the device.

2.4.3 Legacy Interface: A Note on Device Configuration Space endian-ness

Note that for legacy interfaces, device configuration space is generally the guest’s native endian, rather than PCI’s little-endian. The correct endian-ness is documented for each device.

2.4.4 Legacy Interface: Device Configuration Space

Legacy devices did not have a configuration generation field, thus are susceptible to race conditions if configuration is updated. This affects the block capacity (see 5.2.4) and network mac (see 5.1.4) fields; when using the legacy interface, drivers SHOULD read these fields multiple times until two reads generate a consistent result.

2.5 Virtqueues

The mechanism for bulk data transport on virtio devices is pretentiously called a virtqueue. Each device can have zero or more virtqueues3.

Driver makes requests available to device by adding an available buffer to the queue, i.e., adding a buffer describing the request to a virtqueue, and optionally triggering a driver event, i.e., sending an available buffer notification to the device.

Device executes the requests and - when complete - adds a used buffer to the queue, i.e., lets the driver know by marking the buffer as used. Device can then trigger a device event, i.e., send a used buffer notification to the driver.

Device reports the number of bytes it has written to memory for each buffer it uses. This is referred to as “used length”.

Device is not generally required to use buffers in the same order in which they have been made available by the driver.

Some devices always use descriptors in the same order in which they have been made available. These devices can offer the VIRTIO_F_IN_ORDER feature. If negotiated, this knowledge might allow optimizations or simplify driver and/or device code.

Each virtqueue can consist of up to 3 parts:

Note: Note that previous versions of this spec used different names for these parts (following 2.6):

Two formats are supported: Split Virtqueues (see 2.6 Split Virtqueues) and Packed Virtqueues (see 2.7 Packed Virtqueues).

Every driver and device supports either the Packed or the Split Virtqueue format, or both.

2.6 Split Virtqueues

The split virtqueue format was the only format supported by the version 1.0 (and earlier) of this standard.

The split virtqueue format separates the virtqueue into several parts, where each part is write-able by either the driver or the device, but not both. Multiple parts and/or locations within a part need to be updated when making a buffer available and when marking it as used.

Each queue has a 16-bit queue size parameter, which sets the number of entries and implies the total size of the queue.

Each virtqueue consists of three parts:

where each part is physically-contiguous in guest memory, and has different alignment requirements.

The memory alignment and size requirements, in bytes, of each part of the virtqueue are summarized in the following table:




Virtqueue Part Alignment Size






Descriptor Table 16 16(Queue Size)



Available Ring 2 6 + 2(Queue Size)



Used Ring 4 6 + 8(Queue Size)



The Alignment column gives the minimum alignment for each part of the virtqueue.

The Size column gives the total number of bytes for each part of the virtqueue.

Queue Size corresponds to the maximum number of buffers in the virtqueue4. Queue Size value is always a power of 2. The maximum Queue Size value is 32768. This value is specified in a bus-specific way.

When the driver wants to send a buffer to the device, it fills in a slot in the descriptor table (or chains several together), and writes the descriptor index into the available ring. It then notifies the device. When the device has finished a buffer, it writes the descriptor index into the used ring, and sends a used buffer notification.

2.6.1 Driver Requirements: Virtqueues

The driver MUST ensure that the physical address of the first byte of each virtqueue part is a multiple of the specified alignment value in the above table.

2.6.2 Legacy Interfaces: A Note on Virtqueue Layout

For Legacy Interfaces, several additional restrictions are placed on the virtqueue layout:

Each virtqueue occupies two or more physically-contiguous pages (usually defined as 4096 bytes, but depending on the transport; henceforth referred to as Queue Align) and consists of three parts:




Descriptor Table Available Ring (…padding…) Used Ring



The bus-specific Queue Size field controls the total number of bytes for the virtqueue. When using the legacy interface, the transitional driver MUST retrieve the Queue Size field from the device and MUST allocate the total number of bytes for the virtqueue according to the following formula (Queue Align given in qalign and Queue Size given in qsz):

#define ALIGN(x) (((x) + qalign) & "qalign) 
static inline unsigned virtq_size(unsigned int qsz) 
{ 
     return ALIGN(sizeof(struct virtq_desc)*qsz + sizeof(u16)*(3 + qsz)) 
          + ALIGN(sizeof(u16)*3 + sizeof(struct virtq_used_elem)*qsz); 
}

This wastes some space with padding. When using the legacy interface, both transitional devices and drivers MUST use the following virtqueue layout structure to locate elements of the virtqueue:

struct virtq { 
        // The actual descriptors (16 bytes each) 
        struct virtq_desc desc[ Queue Size ]; 
 
        // A ring of available descriptor heads with free-running index. 
        struct virtq_avail avail; 
 
        // Padding to the next Queue Align boundary. 
        u8 pad[ Padding ]; 
 
        // A ring of used descriptor heads with free-running index. 
        struct virtq_used used; 
};

2.6.3 Legacy Interfaces: A Note on Virtqueue Endianness

Note that when using the legacy interface, transitional devices and drivers MUST use the native endian of the guest as the endian of fields and in the virtqueue. This is opposed to little-endian for non-legacy interface as specified by this standard. It is assumed that the host is already aware of the guest endian.

2.6.4 Message Framing

The framing of messages with descriptors is independent of the contents of the buffers. For example, a network transmit buffer consists of a 12 byte header followed by the network packet. This could be most simply placed in the descriptor table as a 12 byte output descriptor followed by a 1514 byte output descriptor, but it could also consist of a single 1526 byte output descriptor in the case where the header and packet are adjacent, or even three or more descriptors (possibly with loss of efficiency in that case).

Note that, some device implementations have large-but-reasonable restrictions on total descriptor size (such as based on IOV_MAX in the host OS). This has not been a problem in practice: little sympathy will be given to drivers which create unreasonably-sized descriptors such as by dividing a network packet into 1500 single-byte descriptors!

2.6.4.1 Device Requirements: Message Framing

The device MUST NOT make assumptions about the particular arrangement of descriptors. The device MAY have a reasonable limit of descriptors it will allow in a chain.

2.6.4.2 Driver Requirements: Message Framing

The driver MUST place any device-writable descriptor elements after any device-readable descriptor elements.

The driver SHOULD NOT use an excessive number of descriptors to describe a buffer.

2.6.4.3 Legacy Interface: Message Framing

Regrettably, initial driver implementations used simple layouts, and devices came to rely on it, despite this specification wording. In addition, the specification for virtio_blk SCSI commands required intuiting field lengths from frame boundaries (see 5.2.6.3 Legacy Interface: Device Operation)

Thus when using the legacy interface, the VIRTIO_F_ANY_LAYOUT feature indicates to both the device and the driver that no assumptions were made about framing. Requirements for transitional drivers when this is not negotiated are included in each device section.

2.6.5 The Virtqueue Descriptor Table

The descriptor table refers to the buffers the driver is using for the device. addr is a physical address, and the buffers can be chained via next. Each descriptor describes a buffer which is read-only for the device (“device-readable”) or write-only for the device (“device-writable”), but a chain of descriptors can contain both device-readable and device-writable buffers.

The actual contents of the memory offered to the device depends on the device type. Most common is to begin the data with a header (containing little-endian fields) for the device to read, and postfix it with a status tailer for the device to write.

struct virtq_desc { 
        /* Address (guest-physical). */ 
        le64 addr; 
        /* Length. */ 
        le32 len; 
 
/* This marks a buffer as continuing via the next field. */ 
#define VIRTQ_DESC_F_NEXT   1 
/* This marks a buffer as device write-only (otherwise device read-only). */ 
#define VIRTQ_DESC_F_WRITE     2 
/* This means the buffer contains a list of buffer descriptors. */ 
#define VIRTQ_DESC_F_INDIRECT   4 
        /* The flags as indicated above. */ 
        le16 flags; 
        /* Next field if flags & NEXT */ 
        le16 next; 
};

The number of descriptors in the table is defined by the queue size for this virtqueue: this is the maximum possible descriptor chain length.

If VIRTIO_F_IN_ORDER has been negotiated, driver uses descriptors in ring order: starting from offset 0 in the table, and wrapping around at the end of the table. Note: The legacy [Virtio PCI Draft] referred to this structure as vring_desc, and the constants as VRING_DESC_F_NEXT, etc, but the layout and values were identical.

2.6.5.1 Device Requirements: The Virtqueue Descriptor Table

A device MUST NOT write to a device-readable buffer, and a device SHOULD NOT read a device-writable buffer (it MAY do so for debugging or diagnostic purposes).

2.6.5.2 Driver Requirements: The Virtqueue Descriptor Table

Drivers MUST NOT add a descriptor chain longer than 232 bytes in total; this implies that loops in the descriptor chain are forbidden!

If VIRTIO_F_IN_ORDER has been negotiated, and when making a descriptor with VRING_DESC_F_NEXT set in flags at offset x in the table available to the device, driver MUST set next to 0 for the last descriptor in the table (where x = queue_size 1) and to x + 1 for the rest of the descriptors.

2.6.5.3 Indirect Descriptors

Some devices benefit by concurrently dispatching a large number of large requests. The VIRTIO_F_INDIRECT_DESC feature allows this (see A virtio_queue.h). To increase ring capacity the driver can store a table of indirect descriptors anywhere in memory, and insert a descriptor in main virtqueue (with flags&VIRTQ_DESC_F_INDIRECT on) that refers to memory buffer containing this indirect descriptor table; addr and len refer to the indirect table address and length in bytes, respectively.

The indirect table layout structure looks like this (len is the length of the descriptor that refers to this table, which is a variable, so this code won’t compile):

struct indirect_descriptor_table { 
        /* The actual descriptors (16 bytes each) */ 
        struct virtq_desc desc[len / 16]; 
};

The first indirect descriptor is located at start of the indirect descriptor table (index 0), additional indirect descriptors are chained by next. An indirect descriptor without a valid next (with flags&VIRTQ_DESC_F_NEXT off) signals the end of the descriptor. A single indirect descriptor table can include both device-readable and device-writable descriptors.

If VIRTIO_F_IN_ORDER has been negotiated, indirect descriptors use sequential indices, in-order: index 0 followed by index 1 followed by index 2, etc.

2.6.5.3.1 Driver Requirements: Indirect Descriptors
The driver MUST NOT set the VIRTQ_DESC_F_INDIRECT flag unless the VIRTIO_F_INDIRECT_DESC feature was negotiated. The driver MUST NOT set the VIRTQ_DESC_F_INDIRECT flag within an indirect descriptor (ie. only one table per descriptor).

A driver MUST NOT create a descriptor chain longer than the Queue Size of the device.

A driver MUST NOT set both VIRTQ_DESC_F_INDIRECT and VIRTQ_DESC_F_NEXT in flags.

If VIRTIO_F_IN_ORDER has been negotiated, indirect descriptors MUST appear sequentially, with next taking the value of 1 for the 1st descriptor, 2 for the 2nd one, etc.

2.6.5.3.2 Device Requirements: Indirect Descriptors
The device MUST ignore the write-only flag (flags&VIRTQ_DESC_F_WRITE) in the descriptor that refers to an indirect table.

The device MUST handle the case of zero or more normal chained descriptors followed by a single descriptor with flags&VIRTQ_DESC_F_INDIRECT. Note: While unusual (most implementations either create a chain solely using non-indirect descriptors, or use a single indirect element), such a layout is valid.

2.6.6 The Virtqueue Available Ring

The available ring has the following layout structure:

struct virtq_avail { 
#define VIRTQ_AVAIL_F_NO_INTERRUPT      1 
        le16 flags; 
        le16 idx; 
        le16 ring[ /* Queue Size */ ]; 
        le16 used_event; /* Only if VIRTIO_F_EVENT_IDX */ 
};

The driver uses the available ring to offer buffers to the device: each ring entry refers to the head of a descriptor chain. It is only written by the driver and read by the device.

idx field indicates where the driver would put the next descriptor entry in the ring (modulo the queue size). This starts at 0, and increases. Note: The legacy [Virtio PCI Draft] referred to this structure as vring_avail, and the constant as VRING_AVAIL_F_NO_INTERRUPT, but the layout and value were identical.

2.6.6.1 Driver Requirements: The Virtqueue Available Ring

A driver MUST NOT decrement the available idx on a virtqueue (ie. there is no way to “unexpose” buffers).

2.6.7 Used Buffer Notification Suppression

If the VIRTIO_F_EVENT_IDX feature bit is not negotiated, the flags field in the available ring offers a crude mechanism for the driver to inform the device that it doesn’t want notifications when buffers are used. Otherwise used_event is a more performant alternative where the driver specifies how far the device can progress before a notification is required.

Neither of these notification suppression methods are reliable, as they are not synchronized with the device, but they serve as useful optimizations.

2.6.7.1 Driver Requirements: Used Buffer Notification Suppression

If the VIRTIO_F_EVENT_IDX feature bit is not negotiated:

Otherwise, if the VIRTIO_F_EVENT_IDX feature bit is negotiated:

The driver MUST handle spurious notifications from the device.

2.6.7.2 Device Requirements: Used Buffer Notification Suppression

If the VIRTIO_F_EVENT_IDX feature bit is not negotiated:

Otherwise, if the VIRTIO_F_EVENT_IDX feature bit is negotiated:

Note: For example, if used_event is 0, then a device using

VIRTIO_F_EVENT_IDX would send a used buffer notification to the driver after the first buffer is used (and again after the 65536th buffer, etc).

2.6.8 The Virtqueue Used Ring

The used ring has the following layout structure:

struct virtq_used { 
#define VIRTQ_USED_F_NO_NOTIFY  1 
        le16 flags; 
        le16 idx; 
        struct virtq_used_elem ring[ /* Queue Size */]; 
        le16 avail_event; /* Only if VIRTIO_F_EVENT_IDX */ 
}; 
 
/* le32 is used here for ids for padding reasons. */ 
struct virtq_used_elem { 
        /* Index of start of used descriptor chain. */ 
        le32 id; 
        /* Total length of the descriptor chain which was used (written to) */ 
        le32 len; 
};

The used ring is where the device returns buffers once it is done with them: it is only written to by the device, and read by the driver.

Each entry in the ring is a pair: id indicates the head entry of the descriptor chain describing the buffer (this matches an entry placed in the available ring by the guest earlier), and len the total of bytes written into the buffer. Note: len is particularly useful for drivers using untrusted buffers: if a driver does not know exactly how much has been written by the device, the driver would have to zero the buffer in advance to ensure no data leakage occurs.

For example, a network driver may hand a received buffer directly to an unprivileged userspace application. If the network device has not overwritten the bytes which were in that buffer, this could leak the contents of freed memory from other processes to the application.

idx field indicates where the device would put the next descriptor entry in the ring (modulo the queue size). This starts at 0, and increases. Note: The legacy [Virtio PCI Draft] referred to these structures as vring_used and vring_used_elem, and the constant as VRING_USED_F_NO_NOTIFY, but the layout and value were identical.

2.6.8.1 Legacy Interface: The Virtqueue Used Ring

Historically, many drivers ignored the len value, as a result, many devices set len incorrectly. Thus, when using the legacy interface, it is generally a good idea to ignore the len value in used ring entries if possible. Specific known issues are listed per device type.

2.6.8.2 Device Requirements: The Virtqueue Used Ring

The device MUST set len prior to updating the used idx.

The device MUST write at least len bytes to descriptor, beginning at the first device-writable buffer, prior to updating the used idx.

The device MAY write more than len bytes to descriptor. Note: There are potential error cases where a device might not know what parts of the buffers have been written. This is why len is permitted to be an underestimate: that’s preferable to the driver believing that uninitialized memory has been overwritten when it has not.

2.6.8.3 Driver Requirements: The Virtqueue Used Ring

The driver MUST NOT make assumptions about data in device-writable buffers beyond the first len bytes, and SHOULD ignore this data.

2.6.9 In-order use of descriptors

Some devices always use descriptors in the same order in which they have been made available. These devices can offer the VIRTIO_F_IN_ORDER feature. If negotiated, this knowledge allows devices to notify the use of a batch of buffers to the driver by only writing out a single used ring entry with the id corresponding to the head entry of the descriptor chain describing the last buffer in the batch.

The device then skips forward in the ring according to the size of the batch. Accordingly, it increments the used idx by the size of the batch.

The driver needs to look up the used id and calculate the batch size to be able to advance to where the next used ring entry will be written by the device.

This will result in the used ring entry at an offset matching the first available ring entry in the batch, the used ring entry for the next batch at an offset matching the first available ring entry in the next batch, etc.

The skipped buffers (for which no used ring entry was written) are assumed to have been used (read or written) by the device completely.

2.6.10 Available Buffer Notification Suppression

The device can suppress available buffer notifications in a manner analogous to the way drivers can suppress used buffer notifications as detailed in section 2.6.7. The device manipulates flags or avail_event in the used ring the same way the driver manipulates flags or used_event in the available ring.

2.6.10.1 Driver Requirements: Available Buffer Notification Suppression

The driver MUST initialize flags in the used ring to 0 when allocating the used ring.

If the VIRTIO_F_EVENT_IDX feature bit is not negotiated:

Otherwise, if the VIRTIO_F_EVENT_IDX feature bit is negotiated:

2.6.10.2 Device Requirements: Available Buffer Notification Suppression

If the VIRTIO_F_EVENT_IDX feature bit is not negotiated:

Otherwise, if the VIRTIO_F_EVENT_IDX feature bit is negotiated:

The device MUST handle spurious notifications from the driver.

2.6.11 Helpers for Operating Virtqueues

The Linux Kernel Source code contains the definitions above and helper routines in a more usable form, in include/uapi/linux/virtio_ring.h. This was explicitly licensed by IBM and Red Hat under the (3-clause) BSD license so that it can be freely used by all other projects, and is reproduced (with slight variation) in A virtio_queue.h.

2.6.12 Virtqueue Operation

There are two parts to virtqueue operation: supplying new available buffers to the device, and processing used buffers from the device. Note: As an example, the simplest virtio network device has two virtqueues: the transmit virtqueue and the receive virtqueue. The driver adds outgoing (device-readable) packets to the transmit virtqueue, and then frees them after they are used. Similarly, incoming (device-writable) buffers are added to the receive virtqueue, and processed after they are used.

What follows is the requirements of each of these two parts when using the split virtqueue format in more detail.

2.6.13 Supplying Buffers to The Device

The driver offers buffers to one of the device’s virtqueues as follows:

  1. The driver places the buffer into free descriptor(s) in the descriptor table, chaining as necessary (see 2.6.5 The Virtqueue Descriptor Table).
  2. The driver places the index of the head of the descriptor chain into the next ring entry of the available ring.
  3. Steps 1 and 2 MAY be performed repeatedly if batching is possible.
  4. The driver performs a suitable memory barrier to ensure the device sees the updated descriptor table and available ring before the next step.
  5. The available idx is increased by the number of descriptor chain heads added to the available ring.
  6. The driver performs a suitable memory barrier to ensure that it updates the idx field before checking for notification suppression.
  7. The driver sends an available buffer notification to the device if such notifications are not suppressed.

Note that the above code does not take precautions against the available ring buffer wrapping around: this is not possible since the ring buffer is the same size as the descriptor table, so step (1) will prevent such a condition.

In addition, the maximum queue size is 32768 (the highest power of 2 which fits in 16 bits), so the 16-bit idx value can always distinguish between a full and empty buffer.

What follows is the requirements of each stage in more detail.

2.6.13.1 Placing Buffers Into The Descriptor Table

A buffer consists of zero or more device-readable physically-contiguous elements followed by zero or more physically-contiguous device-writable elements (each has at least one element). This algorithm maps it into the descriptor table to form a descriptor chain:

for each buffer element, b:

  1. Get the next free descriptor table entry, d
  2. Set d.addr to the physical address of the start of b
  3. Set d.len to the length of b.
  4. If b is device-writable, set d.flags to VIRTQ_DESC_F_WRITE, otherwise 0.
  5. If there is a buffer element after this:
    1. Set d.next to the index of the next free descriptor element.
    2. Set the VIRTQ_DESC_F_NEXT bit in d.flags.

In practice, d.next is usually used to chain free descriptors, and a separate count kept to check there are enough free descriptors before beginning the mappings.

2.6.13.2 Updating The Available Ring

The descriptor chain head is the first d in the algorithm above, ie. the index of the descriptor table entry referring to the first part of the buffer. A naive driver implementation MAY do the following (with the appropriate conversion to-and-from little-endian assumed):

avail->ring[avail->idx % qsz] = head;

However, in general the driver MAY add many descriptor chains before it updates idx (at which point they become visible to the device), so it is common to keep a counter of how many the driver has added:

avail->ring[(avail->idx + added++) % qsz] = head;
2.6.13.3 Updating idx

idx always increments, and wraps naturally at 65536:

avail->idx += added;

Once available idx is updated by the driver, this exposes the descriptor and its contents. The device MAY access the descriptor chains the driver created and the memory they refer to immediately.

2.6.13.3.1 Driver Requirements: Updating idx
The driver MUST perform a suitable memory barrier before the idx update, to ensure the device sees the most up-to-date copy.
2.6.13.4 Notifying The Device

The actual method of device notification is bus-specific, but generally it can be expensive. So the device MAY suppress such notifications if it doesn’t need them, as detailed in section 2.6.10.

The driver has to be careful to expose the new idx value before checking if notifications are suppressed.

2.6.13.4.1 Driver Requirements: Notifying The Device
The driver MUST perform a suitable memory barrier before reading flags or avail_event, to avoid missing a notification.

2.6.14 Receiving Used Buffers From The Device

Once the device has used buffers referred to by a descriptor (read from or written to them, or parts of both, depending on the nature of the virtqueue and the device), it sends a used buffer notification to the driver as detailed in section 2.6.7. Note:

For optimal performance, a driver MAY disable used buffer notifications while processing the used ring, but beware the problem of missing notifications between emptying the ring and reenabling notifications. This is usually handled by re-checking for more used buffers after notifications are re-enabled:

virtq_disable_used_buffer_notifications(vq); 
 
for (;;) { 
        if (vq->last_seen_used != le16_to_cpu(virtq->used.idx)) { 
                virtq_enable_used_buffer_notifications(vq); 
                mb(); 
 
                if (vq->last_seen_used != le16_to_cpu(virtq->used.idx)) 
                        break; 
 
                virtq_disable_used_buffer_notifications(vq); 
        } 
 
        struct virtq_used_elem *e = virtq.used->ring[vq->last_seen_used%vsz]; 
        process_buffer(e); 
        vq->last_seen_used++; 
}

2.7 Packed Virtqueues

Packed virtqueues is an alternative compact virtqueue layout using read-write memory, that is memory that is both read and written by both host and guest.

Use of packed virtqueues is negotiated by the VIRTIO_F_RING_PACKED feature bit.

Packed virtqueues support up to 215 entries each.

With current transports, virtqueues are located in guest memory allocated by the driver. Each packed virtqueue consists of three parts:

Where the Descriptor Ring in turn consists of descriptors, and where each descriptor can contain the following parts:

A buffer consists of zero or more device-readable physically-contiguous elements followed by zero or more physically-contiguous device-writable elements (each buffer has at least one element).

When the driver wants to send such a buffer to the device, it writes at least one available descriptor describing elements of the buffer into the Descriptor Ring. The descriptor(s) are associated with a buffer by means of a Buffer ID stored within the descriptor.

The driver then notifies the device. When the device has finished processing the buffer, it writes a used device descriptor including the Buffer ID into the Descriptor Ring (overwriting a driver descriptor previously made available), and sends a used event notification.

The Descriptor Ring is used in a circular manner: the driver writes descriptors into the ring in order. After reaching the end of the ring, the next descriptor is placed at the head of the ring. Once the ring is full of driver descriptors, the driver stops sending new requests and waits for the device to start processing descriptors and to write out some used descriptors before making new driver descriptors available.

Similarly, the device reads descriptors from the ring in order and detects that a driver descriptor has been made available. As processing of descriptors is completed, used descriptors are written by the device back into the ring.

Note: after reading driver descriptors and starting their processing in order, the device might complete their processing out of order. Used device descriptors are written in the order in which their processing is complete.

The Device Event Suppression data structure is write-only by the device. It includes information for reducing the number of device events, i.e., sending fewer available buffer notifications to the device.

The Driver Event Suppression data structure is read-only by the device. It includes information for reducing the number of driver events, i.e., sending fewer used buffer notifications to the driver.

2.7.1 Driver and Device Ring Wrap Counters

Each of the driver and the device are expected to maintain, internally, a single-bit ring wrap counter initialized to 1.

The counter maintained by the driver is called the Driver Ring Wrap Counter. The driver changes the value of this counter each time it makes available the last descriptor in the ring (after making the last descriptor available).

The counter maintained by the device is called the Device Ring Wrap Counter. The device changes the value of this counter each time it uses the last descriptor in the ring (after marking the last descriptor used).

It is easy to see that the Driver Ring Wrap Counter in the driver matches the Device Ring Wrap Counter in the device when both are processing the same descriptor, or when all available descriptors have been used.

To mark a descriptor as available and used, both the driver and the device use the following two flags:

#define VIRTQ_DESC_F_AVAIL     (1 << 7) 
#define VIRTQ_DESC_F_USED      (1 << 15)

To mark a descriptor as available, the driver sets the VIRTQ_DESC_F_AVAIL bit in Flags to match the internal Driver Ring Wrap Counter. It also sets the VIRTQ_DESC_F_USED bit to match the inverse value (i.e. to not match the internal Driver Ring Wrap Counter).

To mark a descriptor as used, the device sets the VIRTQ_DESC_F_USED bit in Flags to match the internal Device Ring Wrap Counter. It also sets the VIRTQ_DESC_F_AVAIL bit to match the same value.

Thus VIRTQ_DESC_F_AVAIL and VIRTQ_DESC_F_USED bits are different for an available descriptor and equal for a used descriptor.

Note that this observation is mostly useful for sanity-checking as these are necessary but not sufficient conditions - for example, all descriptors are zero-initialized. To detect used and available descriptors it is possible for drivers and devices to keep track of the last observed value of VIRTQ_DESC_F_USED/VIRTQ_DESC_F_AVAIL. Other techniques to detect VIRTQ_DESC_F_AVAIL/VIRTQ_DESC_F_USED bit changes might also be possible.

2.7.2 Polling of available and used descriptors

Writes of device and driver descriptors can generally be reordered, but each side (driver and device) are only required to poll (or test) a single location in memory: the next device descriptor after the one they processed previously, in circular order.

Sometimes the device needs to only write out a single used descriptor after processing a batch of multiple available descriptors. As described in more detail below, this can happen when using descriptor chaining or with in-order use of descriptors. In this case, the device writes out a used descriptor with the buffer id of the last descriptor in the group. After processing the used descriptor, both device and driver then skip forward in the ring the number of the remaining descriptors in the group until processing (reading for the driver and writing for the device) the next used descriptor.

2.7.3 Write Flag

In an available descriptor, the VIRTQ_DESC_F_WRITE bit within Flags is used to mark a descriptor as corresponding to a write-only or read-only element of a buffer.

/* This marks a descriptor as device write-only (otherwise device read-only). */ 
#define VIRTQ_DESC_F_WRITE     2

In a used descriptor, this bit is used to specify whether any data has been written by the device into any parts of the buffer.

2.7.4 Element Address and Length

In an available descriptor, Element Address corresponds to the physical address of the buffer element. The length of the element assumed to be physically contiguous is stored in Element Length.

In a used descriptor, Element Address is unused. Element Length specifies the length of the buffer that has been initialized (written to) by the device.

Element Length is reserved for used descriptors without the VIRTQ_DESC_F_WRITE flag, and is ignored by drivers.

2.7.5 Scatter-Gather Support

Some drivers need an ability to supply a list of multiple buffer elements (also known as a scatter/gather list) with a request. Two features support this: descriptor chaining and indirect descriptors.

If neither feature is in use by the driver, each buffer is physically-contiguous, either read-only or write-only and is described completely by a single descriptor.

While unusual (most implementations either create all lists solely using non-indirect descriptors, or always use a single indirect element), if both features have been negotiated, mixing indirect and non-indirect descriptors in a ring is valid, as long as each list only contains descriptors of a given type.

Scatter/gather lists only apply to available descriptors. A single used descriptor corresponds to the whole list.

The device limits the number of descriptors in a list through a transport-specific and/or device-specific value. If not limited, the maximum number of descriptors in a list is the virt queue size.

2.7.6 Next Flag: Descriptor Chaining

The packed ring format allows the driver to supply a scatter/gather list to the device by using multiple descriptors, and setting the VIRTQ_DESC_F_NEXT bit in Flags for all but the last available descriptor.

/* This marks a buffer as continuing. */ 
#define VIRTQ_DESC_F_NEXT   1

Buffer ID is included in the last descriptor in the list.

The driver always makes the first descriptor in the list available after the rest of the list has been written out into the ring. This guarantees that the device will never observe a partial scatter/gather list in the ring.

Note: all flags, including VIRTQ_DESC_F_AVAIL, VIRTQ_DESC_F_USED, VIRTQ_DESC_F_WRITE must be set/cleared correctly in all descriptors in the list, not just the first one.

The device only writes out a single used descriptor for the whole list. It then skips forward according to the number of descriptors in the list. The driver needs to keep track of the size of the list corresponding to each buffer ID, to be able to skip to where the next used descriptor is written by the device.

For example, if descriptors are used in the same order in which they are made available, this will result in the used descriptor overwriting the first available descriptor in the list, the used descriptor for the next list overwriting the first available descriptor in the next list, etc.

VIRTQ_DESC_F_NEXT is reserved in used descriptors, and should be ignored by drivers.

2.7.7 Indirect Flag: Scatter-Gather Support

Some devices benefit by concurrently dispatching a large number of large requests. The VIRTIO_F_INDIRECT_DESC feature allows this. To increase ring capacity the driver can store a (read-only by the device) table of indirect descriptors anywhere in memory, and insert a descriptor in the main virtqueue (with Flags bit VIRTQ_DESC_F_INDIRECT on) that refers to a buffer element containing this indirect descriptor table; addr and len refer to the indirect table address and length in bytes, respectively.

/* This means the element contains a table of descriptors. */ 
#define VIRTQ_DESC_F_INDIRECT   4

The indirect table layout structure looks like this (len is the Buffer Length of the descriptor that refers to this table, which is a variable):

struct pvirtq_indirect_descriptor_table { 
        /* The actual descriptor structures (struct pvirtq_desc each) */ 
        struct pvirtq_desc desc[len / sizeof(struct pvirtq_desc)]; 
};

The first descriptor is located at the start of the indirect descriptor table, additional indirect descriptors come immediately afterwards. The VIRTQ_DESC_F_WRITE flags bit is the only valid flag for descriptors in the indirect table. Others are reserved and are ignored by the device. Buffer ID is also reserved and is ignored by the device.

In descriptors with VIRTQ_DESC_F_INDIRECT set VIRTQ_DESC_F_WRITE is reserved and is ignored by the device.

2.7.8 In-order use of descriptors

Some devices always use descriptors in the same order in which they have been made available. These devices can offer the VIRTIO_F_IN_ORDER feature. If negotiated, this knowledge allows devices to notify the use of a batch of buffers to the driver by only writing out a single used descriptor with the Buffer ID corresponding to the last descriptor in the batch.

The device then skips forward in the ring according to the size of the batch. The driver needs to look up the used Buffer ID and calculate the batch size to be able to advance to where the next used descriptor will be written by the device.

This will result in the used descriptor overwriting the first available descriptor in the batch, the used descriptor for the next batch overwriting the first available descriptor in the next batch, etc.

The skipped buffers (for which no used descriptor was written) are assumed to have been used (read or written) by the device completely.

2.7.9 Multi-buffer requests

Some devices combine multiple buffers as part of processing of a single request. These devices always mark the descriptor corresponding to the first buffer in the request used after the rest of the descriptors (corresponding to rest of the buffers) in the request - which follow the first descriptor in ring order - has been marked used and written out into the ring. This guarantees that the driver will never observe a partial request in the ring.

2.7.10 Driver and Device Event Suppression

In many systems used and available buffer notifications involve significant overhead. To mitigate this overhead, each virtqueue includes two identical structures used for controlling notifications between the device and the driver.

The Driver Event Suppression structure is read-only by the device and controls the used buffer notifications sent by the device to the driver.

The Device Event Suppression structure is read-only by the driver and controls the available buffer notifications sent by the driver to the device.

Each of these Event Suppression structures includes the following fields:

Descriptor Ring Change Event Flags
Takes values:
/* Enable events */ 
#define RING_EVENT_FLAGS_ENABLE 0x0 
/* Disable events */ 
#define RING_EVENT_FLAGS_DISABLE 0x1 
/* 
 * Enable events for a specific descriptor 
 * (as specified by Descriptor Ring Change Event Offset/Wrap Counter). 
 * Only valid if VIRTIO_F_RING_EVENT_IDX has been negotiated. 
 */ 
#define RING_EVENT_FLAGS_DESC 0x2 
/* The value 0x3 is reserved */
Descriptor Ring Change Event Offset
If Event Flags set to descriptor specific event: offset within the ring (in units of descriptor size). Event will only trigger when this descriptor is made available/used respectively.
Descriptor Ring Change Event Wrap Counter
If Event Flags set to descriptor specific event: offset within the ring (in units of descriptor size). Event will only trigger when Ring Wrap Counter matches this value and a descriptor is made available/used respectively.

After writing out some descriptors, both the device and the driver are expected to consult the relevant structure to find out whether a used respectively an available buffer notification should be sent.

2.7.10.1 Structure Size and Alignment

Each part of the virtqueue is physically-contiguous in guest memory, and has different alignment requirements.

The memory alignment and size requirements, in bytes, of each part of the virtqueue are summarized in the following table:




Virtqueue Part Alignment Size






Descriptor Ring 16 16(Queue Size)



Device Event Suppression 4 4



Driver Event Suppression 4 4



The Alignment column gives the minimum alignment for each part of the virtqueue.

The Size column gives the total number of bytes for each part of the virtqueue.

Queue Size corresponds to the maximum number of descriptors in the virtqueue5. The Queue Size value does not have to be a power of 2.

2.7.11 Driver Requirements: Virtqueues

The driver MUST ensure that the physical address of the first byte of each virtqueue part is a multiple of the specified alignment value in the above table.

2.7.12 Device Requirements: Virtqueues

The device MUST start processing driver descriptors in the order in which they appear in the ring. The device MUST start writing device descriptors into the ring in the order in which they complete. The device MAY reorder descriptor writes once they are started.

2.7.13 The Virtqueue Descriptor Format

The available descriptor refers to the buffers the driver is sending to the device. addr is a physical address, and the descriptor is identified with a buffer using the id field.

struct pvirtq_desc { 
        /* Buffer Address. */ 
        le64 addr; 
        /* Buffer Length. */ 
        le32 len; 
        /* Buffer ID. */ 
        le16 id; 
        /* The flags depending on descriptor type. */ 
        le16 flags; 
};

The descriptor ring is zero-initialized.

2.7.14 Event Suppression Structure Format

The following structure is used to reduce the number of notifications sent between driver and device.

struct pvirtq_event_suppress { 
        le16 { 
             desc_event_off : 15; /* Descriptor Ring Change Event Offset */ 
             desc_event_wrap : 1; /* Descriptor Ring Change Event Wrap Counter */ 
        } desc; /* If desc_event_flags set to RING_EVENT_FLAGS_DESC */ 
        le16 { 
             desc_event_flags : 2, /* Descriptor Ring Change Event Flags */ 
             reserved : 14; /* Reserved, set to 0 */ 
        } flags; 
};

2.7.15 Device Requirements: The Virtqueue Descriptor Table

A device MUST NOT write to a device-readable buffer, and a device SHOULD NOT read a device-writable buffer. A device MUST NOT use a descriptor unless it observes the VIRTQ_DESC_F_AVAIL bit in its flags being changed (e.g. as compared to the initial zero value). A device MUST NOT change a descriptor after changing it’s the VIRTQ_DESC_F_USED bit in its flags.

2.7.16 Driver Requirements: The Virtqueue Descriptor Table

A driver MUST NOT change a descriptor unless it observes the VIRTQ_DESC_F_USED bit in its flags being changed. A driver MUST NOT change a descriptor after changing the VIRTQ_DESC_F_AVAIL bit in its flags. When notifying the device, driver MUST set next_off and next_wrap to match the next descriptor not yet made available to the device. A driver MAY send multiple available buffer notifications without making any new descriptors available to the device.

2.7.17 Driver Requirements: Scatter-Gather Support

A driver MUST NOT create a descriptor list longer than allowed by the device.

A driver MUST NOT create a descriptor list longer than the Queue Size.

This implies that loops in the descriptor list are forbidden!

The driver MUST place any device-writable descriptor elements after any device-readable descriptor elements.

A driver MUST NOT depend on the device to use more descriptors to be able to write out all descriptors in a list. A driver MUST make sure there’s enough space in the ring for the whole list before making the first descriptor in the list available to the device.

A driver MUST NOT make the first descriptor in the list available before all subsequent descriptors comprising the list are made available.

2.7.18 Device Requirements: Scatter-Gather Support

The device MUST use descriptors in a list chained by the VIRTQ_DESC_F_NEXT flag in the same order that they were made available by the driver.

The device MAY limit the number of buffers it will allow in a list.

2.7.19 Driver Requirements: Indirect Descriptors

The driver MUST NOT set the DESC_F_INDIRECT flag unless the VIRTIO_F_INDIRECT_DESC feature was negotiated. The driver MUST NOT set any flags except DESC_F_WRITE within an indirect descriptor.

A driver MUST NOT create a descriptor chain longer than allowed by the device.

A driver MUST NOT write direct descriptors with DESC_F_INDIRECT set in a scatter-gather list linked by VIRTQ_DESC_F_NEXT. flags.

2.7.20 Virtqueue Operation

There are two parts to virtqueue operation: supplying new available buffers to the device, and processing used buffers from the device.

What follows is the requirements of each of these two parts when using the packed virtqueue format in more detail.

2.7.21 Supplying Buffers to The Device

The driver offers buffers to one of the device’s virtqueues as follows:

  1. The driver places the buffer into free descriptor(s) in the Descriptor Ring.
  2. The driver performs a suitable memory barrier to ensure that it updates the descriptor(s) before checking for notification suppression.
  3. If notifications are not suppressed, the driver notifies the device of the new available buffers.

What follows are the requirements of each stage in more detail.

2.7.21.1 Placing Available Buffers Into The Descriptor Ring

For each buffer element, b:

  1. Get the next descriptor table entry, d
  2. Get the next free buffer id value
  3. Set d.addr to the physical address of the start of b
  4. Set d.len to the length of b.
  5. Set d.id to the buffer id
  6. Calculate the flags as follows:
    1. If b is device-writable, set the VIRTQ_DESC_F_WRITE bit to 1, otherwise 0
    2. Set the VIRTQ_DESC_F_AVAIL bit to the current value of the Driver Ring Wrap Counter
    3. Set the VIRTQ_DESC_F_USED bit to inverse value
  7. Perform a memory barrier to ensure that the descriptor has been initialized
  8. Set d.flags to the calculated flags value
  9. If d is the last descriptor in the ring, toggle the Driver Ring Wrap Counter
  10. Otherwise, increment d to point at the next descriptor

This makes a single descriptor buffer available. However, in general the driver MAY make use of a batch of descriptors as part of a single request. In that case, it defers updating the descriptor flags for the first descriptor (and the previous memory barrier) until after the rest of the descriptors have been initialized.

Once the descriptor flags field is updated by the driver, this exposes the descriptor and its contents. The device MAY access the descriptor and any following descriptors the driver created and the memory they refer to immediately.

2.7.21.1.1 Driver Requirements: Updating flags
The driver MUST perform a suitable memory barrier before the flags update, to ensure the device sees the most up-to-date copy.
2.7.21.2 Sending Available Buffer Notifications

The actual method of device notification is bus-specific, but generally it can be expensive. So the device MAY suppress such notifications if it doesn’t need them, using the Event Suppression structure comprising the Device Area as detailed in section 2.7.14.

The driver has to be careful to expose the new flags value before checking if notifications are suppressed.

2.7.21.3 Implementation Example

Below is a driver code example. It does not attempt to reduce the number of available buffer notifications, neither does it support the VIRTIO_F_RING_EVENT_IDX feature.

/* Note: vq->avail_wrap_count is initialized to 1 */ 
/* Note: vq->sgs is an array same size as the ring */ 
 
id = alloc_id(vq); 
 
first = vq->next_avail; 
sgs = 0; 
for (each buffer element b) { 
        sgs++; 
 
        vq->ids[vq->next_avail] = -1; 
        vq->desc[vq->next_avail].address = get_addr(b); 
        vq->desc[vq->next_avail].len = get_len(b); 
 
        avail = vq->avail_wrap_count ? VIRTQ_DESC_F_AVAIL : 0; 
        used = !vq->avail_wrap_count ? VIRTQ_DESC_F_USED : 0; 
        f = get_flags(b) | avail | used; 
        if (b is not the last buffer element) { 
                f |= VIRTQ_DESC_F_NEXT; 
        } 
 
        /* Dont mark the 1st descriptor available until all of them are ready. */ 
        if (vq->next_avail == first) { 
                flags = f; 
        } else { 
                vq->desc[vq->next_avail].flags = f; 
        } 
 
        last = vq->next_avail; 
 
        vq->next_avail++; 
 
        if (vq->next_avail >= vq->size) { 
                vq->next_avail = 0; 
                vq->avail_wrap_count \^= 1; 
        } 
} 
vq->sgs[id] = sgs; 
/* ID included in the last descriptor in the list */ 
vq->desc[last].id = id; 
write_memory_barrier(); 
vq->desc[first].flags = flags; 
 
memory_barrier(); 
 
if (vq->device_event.flags != RING_EVENT_FLAGS_DISABLE) { 
        notify_device(vq); 
}
2.7.21.3.1 Driver Requirements: Sending Available Buffer Notifications
The driver MUST perform a suitable memory barrier before reading the Event Suppression structure occupying the Device Area. Failing to do so could result in mandatory available buffer notifications not being sent.

2.7.22 Receiving Used Buffers From The Device

Once the device has used buffers referred to by a descriptor (read from or written to them, or parts of both, depending on the nature of the virtqueue and the device), it sends a used buffer notification to the driver as detailed in section 2.7.14. Note:

For optimal performance, a driver MAY disable used buffer notifications while processing the used buffers, but beware the problem of missing notifications between emptying the ring and reenabling used buffer notifications. This is usually handled by re-checking for more used buffers after notifications are re-enabled:

/* Note: vq->used_wrap_count is initialized to 1 */ 
 
vq->driver_event.flags = RING_EVENT_FLAGS_DISABLE; 
 
for (;;) { 
        struct pvirtq_desc *d = vq->desc[vq->next_used]; 
 
        /* 
         * Check that 
         * 1. Descriptor has been made available. This check is necessary 
         *    if the driver is making new descriptors available in parallel 
         *    with this processing of used descriptors (e.g. from another thread). 
         *    Note: there are many other ways to check this, e.g. 
         *    track the number of outstanding available descriptors or buffers 
         *    and check that its not 0. 
         * 2. Descriptor has been used by the device. 
         */ 
        flags = d->flags; 
        bool avail = flags & VIRTQ_DESC_F_AVAIL; 
        bool used = flags & VIRTQ_DESC_F_USED; 
        if (avail != vq->used_wrap_count || used != vq->used_wrap_count) { 
                vq->driver_event.flags = RING_EVENT_FLAGS_ENABLE; 
                memory_barrier(); 
 
                /* 
                 * Re-test in case the driver made more descriptors available in 
                 * parallel with the used descriptor processing (e.g. from another 
                 * thread) and/or the device used more descriptors before the driver 
                 * enabled events. 
                 */ 
                flags = d->flags; 
                bool avail = flags & VIRTQ_DESC_F_AVAIL; 
                bool used = flags & VIRTQ_DESC_F_USED; 
                if (avail != vq->used_wrap_count || used != vq->used_wrap_count) { 
                        break; 
                } 
 
                vq->driver_event.flags = RING_EVENT_FLAGS_DISABLE; 
        } 
 
        read_memory_barrier(); 
 
        /* skip descriptors until the next buffer */ 
        id = d->id; 
        assert(id < vq->size); 
        sgs = vq->sgs[id]; 
        vq->next_used += sgs; 
        if (vq->next_used >= vq->size) { 
                vq->next_used -= vq->size; 
                vq->used_wrap_count \^= 1; 
        } 
 
        free_id(vq, id); 
 
        process_buffer(d); 
}

2.7.23 Driver notifications

The driver is sometimes required to send an available buffer notification to the device.

When VIRTIO_F_NOTIFICATION_DATA has not been negotiated, this notification involves sending the virtqueue number to the device (method depending on the transport).

However, some devices benefit from the ability to find out the amount of available data in the queue without accessing the virtqueue in memory: for efficiency or as a debugging aid.

To help with these optimizations, when VIRTIO_F_NOTIFICATION_DATA has been negotiated, driver notifications to the device include the following information:

vqn
VQ number to be notified.
next_off
Offset within the ring where the next available ring entry will be written. When VIRTIO_F_RING_PACKED has not been negotiated this refers to the 15 least significant bits of the available index. When VIRTIO_F_RING_PACKED has been negotiated this refers to the offset (in units of descriptor entries) within the descriptor ring where the next available descriptor will be written.
next_wrap
Wrap Counter. With VIRTIO_F_RING_PACKED this is the wrap counter referring to the next available descriptor. Without VIRTIO_F_RING_PACKED this is the most significant bit (bit 15) of the available index.

Note that the driver can send multiple notifications even without making any more buffers available. When VIRTIO_F_NOTIFICATION_DATA has been negotiated, these notifications would then have identical next_off and next_wrap values.

2.8 Shared Memory Regions

Shared memory regions are an additional facility available to devices that need a region of memory that’s continuously shared between the device and the driver, rather than passed between them in the way virtqueue elements are.

Example uses include shared caches and version pools for versioned data structures.

The memory region is allocated by the device and presented to the driver. Where the device is implemented in software on a host, this arrangement allows the memory region to be allocated by a library on the host, which the device may not have full control over.

A device may have multiple shared memory regions associated with it. Each region has a shmid to identify it, the meaning of which is device-specific.

Enumeration and location of shared memory regions is performed in a transport-specific way.

Memory consistency rules vary depending on the region and the device and they will be specified as required by each device.

2.8.1 Addressing within regions

References into shared memory regions are represented as offsets from the beginning of the region instead of absolute memory addresses. Offsets are used both for references between structures stored within shared memory and for requests placed in virtqueues that refer to shared memory. The shmid may be explicit or may be inferred from the context of the reference.

2.8.2 Device Requirements: Shared Memory Regions

Shared memory regions MUST NOT expose shared memory regions which are used to control the operation of the device, nor to stream data.


3 General Initialization And Device Operation

We start with an overview of device initialization, then expand on the details of the device and how each step is preformed. This section is best read along with the bus-specific section which describes how to communicate with the specific device.

3.1 Device Initialization

3.1.1 Driver Requirements: Device Initialization

The driver MUST follow this sequence to initialize a device:

  1. Reset the device.
  2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the device.
  3. Set the DRIVER status bit: the guest OS knows how to drive the device.
  4. Read device feature bits, and write the subset of feature bits understood by the OS and driver to the device. During this step the driver MAY read (but MUST NOT write) the device-specific configuration fields to check that it can support the device before accepting it.
  5. Set the FEATURES_OK status bit. The driver MUST NOT accept new feature bits after this step.
  6. Re-read device status to ensure the FEATURES_OK bit is still set: otherwise, the device does not support our subset of features and the device is unusable.
  7. Perform device-specific setup, including discovery of virtqueues for the device, optional per-bus setup, reading and possibly writing the device’s virtio configuration space, and population of virtqueues.
  8. Set the DRIVER_OK status bit. At this point the device is “live”.

If any of these steps go irrecoverably wrong, the driver SHOULD set the FAILED status bit to indicate that it has given up on the device (it can reset the device later to restart if desired). The driver MUST NOT continue initialization in that case.

The driver MUST NOT send any buffer available notifications to the device before setting DRIVER_OK.

3.1.2 Legacy Interface: Device Initialization

Legacy devices did not support the FEATURES_OK status bit, and thus did not have a graceful way for the device to indicate unsupported feature combinations. They also did not provide a clear mechanism to end feature negotiation, which meant that devices finalized features on first-use, and no features could be introduced which radically changed the initial operation of the device.

Legacy driver implementations often used the device before setting the DRIVER_OK bit, and sometimes even before writing the feature bits to the device.

The result was the steps 5 and 6 were omitted, and steps 4, 7 and 8 were conflated.

Therefore, when using the legacy interface:

3.2 Device Operation

When operating the device, each field in the device configuration space can be changed by either the driver or the device.

Whenever such a configuration change is triggered by the device, driver is notified. This makes it possible for drivers to cache device configuration, avoiding expensive configuration reads unless notified.

3.2.1 Notification of Device Configuration Changes

For devices where the device-specific configuration information can be changed, a configuration change notification is sent when a device-specific configuration change occurs.

In addition, this notification is triggered by the device setting DEVICE_NEEDS_RESET (see 2.1.2).

3.3 Device Cleanup

Once the driver has set the DRIVER_OK status bit, all the configured virtqueue of the device are considered live. None of the virtqueues of a device are live once the device has been reset.

3.3.1 Driver Requirements: Device Cleanup

A driver MUST NOT alter virtqueue entries for exposed buffers, i.e., buffers which have been made available to the device (and not been used by the device) of a live virtqueue.

Thus a driver MUST ensure a virtqueue isn’t live (by device reset) before removing exposed buffers.


4 Virtio Transport Options

Virtio can use various different buses, thus the standard is split into virtio general and bus-specific sections.

4.1 Virtio Over PCI Bus

Virtio devices are commonly implemented as PCI devices.

A Virtio device can be implemented as any kind of PCI device: a Conventional PCI device or a PCI Express device. To assure designs meet the latest level requirements, see the PCI-SIG home page at http://www.pcisig.com for any approved changes.

4.1.1 Device Requirements: Virtio Over PCI Bus

A Virtio device using Virtio Over PCI Bus MUST expose to guest an interface that meets the specification requirements of the appropriate PCI specification: [PCI] and [PCIe] respectively.

4.1.2 PCI Device Discovery

Any PCI device with PCI Vendor ID 0x1AF4, and PCI Device ID 0x1000 through 0x107F inclusive is a virtio device. The actual value within this range indicates which virtio device is supported by the device. The PCI Device ID is calculated by adding 0x1040 to the Virtio Device ID, as indicated in section 5. Additionally, devices MAY utilize a Transitional PCI Device ID range, 0x1000 to 0x103F depending on the device type.

4.1.2.1 Device Requirements: PCI Device Discovery

Devices MUST have the PCI Vendor ID 0x1AF4. Devices MUST either have the PCI Device ID calculated by adding 0x1040 to the Virtio Device ID, as indicated in section 5 or have the Transitional PCI Device ID depending on the device type, as follows:



Transitional PCI Device ID Virtio Device




0x1000 network card


0x1001 block device


0x1002 memory ballooning (traditional)


0x1003 console


0x1004 SCSI host


0x1005 entropy source


0x1009 9P transport


For example, the network card device with the Virtio Device ID 1 has the PCI Device ID 0x1041 or the Transitional PCI Device ID 0x1000.

The PCI Subsystem Vendor ID and the PCI Subsystem Device ID MAY reflect the PCI Vendor and Device ID of the environment (for informational purposes by the driver).

Non-transitional devices SHOULD have a PCI Device ID in the range 0x1040 to 0x107f. Non-transitional devices SHOULD have a PCI Revision ID of 1 or higher. Non-transitional devices SHOULD have a PCI Subsystem Device ID of 0x40 or higher.

This is to reduce the chance of a legacy driver attempting to drive the device.

4.1.2.2 Driver Requirements: PCI Device Discovery

Drivers MUST match devices with the PCI Vendor ID 0x1AF4 and the PCI Device ID in the range 0x1040 to 0x107f, calculated by adding 0x1040 to the Virtio Device ID, as indicated in section 5. Drivers for device types listed in section 4.1.2 MUST match devices with the PCI Vendor ID 0x1AF4 and the Transitional PCI Device ID indicated in section 4.1.2.

Drivers MUST match any PCI Revision ID value. Drivers MAY match any PCI Subsystem Vendor ID and any PCI Subsystem Device ID value.

4.1.2.3 Legacy Interfaces: A Note on PCI Device Discovery

Transitional devices MUST have a PCI Revision ID of 0. Transitional devices MUST have the PCI Subsystem Device ID matching the Virtio Device ID, as indicated in section 5. Transitional devices MUST have the Transitional PCI Device ID in the range 0x1000 to 0x103f.

This is to match legacy drivers.

4.1.3 PCI Device Layout

The device is configured via I/O and/or memory regions (though see 4.1.4.8 for access via the PCI configuration space), as specified by Virtio Structure PCI Capabilities.

Fields of different sizes are present in the device configuration regions. All 64-bit, 32-bit and 16-bit fields are little-endian. 64-bit fields are to be treated as two 32-bit fields, with low 32 bit part followed by the high 32 bit part.

4.1.3.1 Driver Requirements: PCI Device Layout

For device configuration access, the driver MUST use 8-bit wide accesses for 8-bit wide fields, 16-bit wide and aligned accesses for 16-bit wide fields and 32-bit wide and aligned accesses for 32-bit and 64-bit wide fields. For 64-bit fields, the driver MAY access each of the high and low 32-bit parts of the field independently.

4.1.3.2 Device Requirements: PCI Device Layout

For 64-bit device configuration fields, the device MUST allow driver independent access to high and low 32-bit parts of the field.

4.1.4 Virtio Structure PCI Capabilities

The virtio device configuration layout includes several structures:

Each structure can be mapped by a Base Address register (BAR) belonging to the function, or accessed via the special VIRTIO_PCI_CAP_PCI_CFG field in the PCI configuration space.

The location of each structure is specified using a vendor-specific PCI capability located on the capability list in PCI configuration space of the device. This virtio structure capability uses little-endian format; all fields are read-only for the driver unless stated otherwise:

struct virtio_pci_cap { 
        u8 cap_vndr;    /* Generic PCI field: PCI_CAP_ID_VNDR */ 
        u8 cap_next;    /* Generic PCI field: next ptr. */ 
        u8 cap_len;     /* Generic PCI field: capability length */ 
        u8 cfg_type;    /* Identifies the structure. */ 
        u8 bar;         /* Where to find it. */ 
        u8 id;          /* Multiple capabilities of the same type */ 
        u8 padding[2];  /* Pad to full dword. */ 
        le32 offset;    /* Offset within bar. */ 
        le32 length;    /* Length of the structure, in bytes. */ 
};

This structure can be followed by extra data, depending on cfg_type, as documented below.

The fields are interpreted as follows:

cap_vndr
0x09; Identifies a vendor-specific capability.
cap_next
Link to next capability in the capability list in the PCI configuration space.
cap_len
Length of this capability structure, including the whole of struct virtio_pci_cap, and extra data if any. This length MAY include padding, or fields unused by the driver.
cfg_type
identifies the structure, according to the following table:
/* Common configuration */ 
#define VIRTIO_PCI_CAP_COMMON_CFG        1 
/* Notifications */ 
#define VIRTIO_PCI_CAP_NOTIFY_CFG        2 
/* ISR Status */ 
#define VIRTIO_PCI_CAP_ISR_CFG           3 
/* Device specific configuration */ 
#define VIRTIO_PCI_CAP_DEVICE_CFG        4 
/* PCI configuration access */ 
#define VIRTIO_PCI_CAP_PCI_CFG           5 
/* Shared memory region */ 
#define VIRTIO_PCI_CAP_SHARED_MEMORY_CFG 8

Any other value is reserved for future use.

Each structure is detailed individually below.

The device MAY offer more than one structure of any type - this makes it possible for the device to expose multiple interfaces to drivers. The order of the capabilities in the capability list specifies the order of preference suggested by the device. A device may specify that this ordering mechanism be overridden by the use of the id field. Note: For example, on some hypervisors, notifications using IO accesses are faster than memory accesses. In this case, the device would expose two capabilities with cfg_type set to VIRTIO_PCI_CAP_NOTIFY_CFG: the first one addressing an I/O BAR, the second one addressing a memory BAR. In this example, the driver would use the I/O BAR if I/O resources are available, and fall back on memory BAR when I/O resources are unavailable.

bar
values 0x0 to 0x5 specify a Base Address register (BAR) belonging to the function located beginning at 10h in PCI Configuration Space and used to map the structure into Memory or I/O Space. The BAR is permitted to be either 32-bit or 64-bit, it can map Memory Space or I/O Space.

Any other value is reserved for future use.

id
Used by some device types to uniquely identify multiple capabilities of a certain type. If the device type does not specify the meaning of this field, its contents are undefined.
offset
indicates where the structure begins relative to the base address associated with the BAR. The alignment requirements of offset are indicated in each structure-specific section below.
length
indicates the length of the structure.

length MAY include padding, or fields unused by the driver, or future extensions. Note: For example, a future device might present a large structure size of several MBytes. As current devices never utilize structures larger than 4KBytes in size, driver MAY limit the mapped structure size to e.g. 4KBytes (thus ignoring parts of structure after the first 4KBytes) to allow forward compatibility with such devices without loss of functionality and without wasting resources.

A variant of this type, struct virtio_pci_cap64, is defined for those capaibilites that require offsets or lengths larger than 4GiB:

struct virtio_pci_cap64 { 
        struct virtio_pci_cap cap; 
        u32 offset_hi; 
        u32 length_hi; 
};

Given that the cap.length and cap.offset fields are only 32 bit, the additional offset_hi and length_hi fields provide the most significant 32 bits of a total 64 bit offset and length within the bar specified by cap.bar.

4.1.4.1 Driver Requirements: Virtio Structure PCI Capabilities

The driver MUST ignore any vendor-specific capability structure which has a reserved cfg_type value.

The driver SHOULD use the first instance of each virtio structure type they can support.

The driver MUST accept a cap_len value which is larger than specified here.

The driver MUST ignore any vendor-specific capability structure which has a reserved bar value.

The drivers SHOULD only map part of configuration structure large enough for device operation. The drivers MUST handle an unexpectedly large length, but MAY check that length is large enough for device operation.

The driver MUST NOT write into any field of the capability structure, with the exception of those with cap_type VIRTIO_PCI_CAP_PCI_CFG as detailed in 4.1.4.8.2.

4.1.4.2 Device Requirements: Virtio Structure PCI Capabilities

The device MUST include any extra data (from the beginning of the cap_vndr field through end of the extra data fields if any) in cap_len. The device MAY append extra data or padding to any structure beyond that.

If the device presents multiple structures of the same type, it SHOULD order them from optimal (first) to least-optimal (last).

4.1.4.3 Common configuration structure layout

The common configuration structure is found at the bar and offset within the VIRTIO_PCI_CAP_COMMON_CFG capability; its layout is below.

struct virtio_pci_common_cfg { 
        /* About the whole device. */ 
        le32 device_feature_select;     /* read-write */ 
        le32 device_feature;            /* read-only for driver */ 
        le32 driver_feature_select;     /* read-write */ 
        le32 driver_feature;            /* read-write */ 
        le16 msix_config;               /* read-write */ 
        le16 num_queues;                /* read-only for driver */ 
        u8 device_status;               /* read-write */ 
        u8 config_generation;           /* read-only for driver */ 
 
        /* About a specific virtqueue. */ 
        le16 queue_select;              /* read-write */ 
        le16 queue_size;                /* read-write */ 
        le16 queue_msix_vector;         /* read-write */ 
        le16 queue_enable;              /* read-write */ 
        le16 queue_notify_off;          /* read-only for driver */ 
        le64 queue_desc;                /* read-write */ 
        le64 queue_driver;              /* read-write */ 
        le64 queue_device;              /* read-write */ 
};
device_feature_select
The driver uses this to select which feature bits device_feature shows. Value 0x0 selects Feature Bits 0 to 31, 0x1 selects Feature Bits 32 to 63, etc.
device_feature
The device uses this to report which feature bits it is offering to the driver: the driver writes to device_feature_select to select which feature bits are presented.
driver_feature_select
The driver uses this to select which feature bits driver_feature shows. Value 0x0 selects Feature Bits 0 to 31, 0x1 selects Feature Bits 32 to 63, etc.
driver_feature
The driver writes this to accept feature bits offered by the device. Driver Feature Bits selected by driver_feature_select.
config_msix_vector
The driver sets the Configuration Vector for MSI-X.
num_queues
The device specifies the maximum number of virtqueues supported here.
device_status
The driver writes the device status here (see 2.1). Writing 0 into this field resets the device.
config_generation
Configuration atomicity value. The device changes this every time the configuration noticeably changes.
queue_select
Queue Select. The driver selects which virtqueue the following fields refer to.
queue_size
Queue Size. On reset, specifies the maximum queue size supported by the device. This can be modified by the driver to reduce memory requirements. A 0 means the queue is unavailable.
queue_msix_vector
The driver uses this to specify the queue vector for MSI-X.
queue_enable
The driver uses this to selectively prevent the device from executing requests from this virtqueue. 1 - enabled; 0 - disabled.
queue_notify_off
The driver reads this to calculate the offset from start of Notification structure at which this virtqueue is located. Note: this is not an offset in bytes. See 4.1.4.4 below.
queue_desc
The driver writes the physical address of Descriptor Area here. See section 2.5.
queue_driver
The driver writes the physical address of Driver Area here. See section 2.5.
queue_device
The driver writes the physical address of Device Area here. See section 2.5.
4.1.4.3.1 Device Requirements: Common configuration structure layout
offset MUST be 4-byte aligned.

The device MUST present at least one common configuration capability.

The device MUST present the feature bits it is offering in device_feature, starting at bit device_feature_select 32 for any device_feature_select written by the driver. Note: This means that it will present 0 for any device_feature_select other than 0 or 1, since no feature defined here exceeds 63.

The device MUST present any valid feature bits the driver has written in driver_feature, starting at bit driver_feature_select 32 for any driver_feature_select written by the driver. Valid feature bits are those which are subset of the corresponding device_feature bits. The device MAY present invalid bits written by the driver. Note: This means that a device can ignore writes for feature bits it never offers, and simply present 0 on reads. Or it can just mirror what the driver wrote (but it will still have to check them when the driver sets FEATURES_OK). Note: A driver shouldn’t write invalid bits anyway, as per 3.1.1, but this attempts to handle it.

The device MUST present a changed config_generation after the driver has read a device-specific configuration value which has changed since any part of the device-specific configuration was last read. Note: As config_generation is an 8-bit value, simply incrementing it on every configuration change could violate this requirement due to wrap. Better would be to set an internal flag when it has changed, and if that flag is set when the driver reads from the device-specific configuration, increment config_generation and clear the flag.

The device MUST reset when 0 is written to device_status, and present a 0 in device_status once that is done.

The device MUST present a 0 in queue_enable on reset.

The device MUST present a 0 in queue_size if the virtqueue corresponding to the current queue_select is unavailable.

If VIRTIO_F_RING_PACKED has not been negotiated, the device MUST present either a value of 0 or a power of 2 in queue_size.

4.1.4.3.2 Driver Requirements: Common configuration structure layout
The driver MUST NOT write to device_feature, num_queues, config_generation or queue_notify_off.

If VIRTIO_F_RING_PACKED has been negotiated, the driver MUST NOT write the value 0 to queue_size. If VIRTIO_F_RING_PACKED has not been negotiated, the driver MUST NOT write a value which is not a power of 2 to queue_size.

The driver MUST configure the other virtqueue fields before enabling the virtqueue with queue_enable.

After writing 0 to device_status, the driver MUST wait for a read of device_status to return 0 before reinitializing the device.

The driver MUST NOT write a 0 to queue_enable.

4.1.4.4 Notification structure layout

The notification location is found using the VIRTIO_PCI_CAP_NOTIFY_CFG capability. This capability is immediately followed by an additional field, like so:

struct virtio_pci_notify_cap { 
        struct virtio_pci_cap cap; 
        le32 notify_off_multiplier; /* Multiplier for queue_notify_off. */ 
};

notify_off_multiplier is combined with the queue_notify_off to derive the Queue Notify address within a BAR for a virtqueue:

        cap.offset + queue_notify_off * notify_off_multiplier

The cap.offset and notify_off_multiplier are taken from the notification capability structure above, and the queue_notify_off is taken from the common configuration structure. Note: For example, if notifier_off_multiplier is 0, the device uses the same Queue Notify address for all queues.

4.1.4.4.1 Device Requirements: Notification capability
The device MUST present at least one notification capability.

For devices not offering VIRTIO_F_NOTIFICATION_DATA:

The cap.offset MUST be 2-byte aligned.

The device MUST either present notify_off_multiplier as an even power of 2, or present notify_off_multiplier as 0.

The value cap.length presented by the device MUST be at least 2 and MUST be large enough to support queue notification offsets for all supported queues in all possible configurations.

For all queues, the value cap.length presented by the device MUST satisfy:

cap.length >= queue_notify_off * notify_off_multiplier + 2

For devices offering VIRTIO_F_NOTIFICATION_DATA:

The device MUST either present notify_off_multiplier as a number that is a power of 2 that is also a multiple 4, or present notify_off_multiplier as 0.

The cap.offset MUST be 4-byte aligned.

The value cap.length presented by the device MUST be at least 4 and MUST be large enough to support queue notification offsets for all supported queues in all possible configurations.

For all queues, the value cap.length presented by the device MUST satisfy:

cap.length >= queue_notify_off * notify_off_multiplier + 4
4.1.4.5 ISR status capability

The VIRTIO_PCI_CAP_ISR_CFG capability refers to at least a single byte, which contains the 8-bit ISR status field to be used for INT#x interrupt handling.

The offset for the ISR status has no alignment requirements.

The ISR bits allow the device to distinguish between device-specific configuration change interrupts and normal virtqueue interrupts:





Bits 0 1 2 to 31




Purpose Queue Interrupt Device Configuration Interrupt Reserved




To avoid an extra access, simply reading this register resets it to 0 and causes the device to de-assert the interrupt.

In this way, driver read of ISR status causes the device to de-assert an interrupt.

See sections 4.1.5.3 and 4.1.5.4 for how this is used.

4.1.4.5.1 Device Requirements: ISR status capability
The device MUST present at least one VIRTIO_PCI_CAP_ISR_CFG capability.

The device MUST set the Device Configuration Interrupt bit in ISR status before sending a device configuration change notification to the driver.

If MSI-X capability is disabled, the device MUST set the Queue Interrupt bit in ISR status before sending a virtqueue notification to the driver.

If MSI-X capability is disabled, the device MUST set the Interrupt Status bit in the PCI Status register in the PCI Configuration Header of the device to the logical OR of all bits in ISR status of the device. The device then asserts/deasserts INT#x interrupts unless masked according to standard PCI rules [PCI].

The device MUST reset ISR status to 0 on driver read.

4.1.4.5.2 Driver Requirements: ISR status capability
If MSI-X capability is enabled, the driver SHOULD NOT access ISR status upon detecting a Queue Interrupt.
4.1.4.6 Device-specific configuration

The device MUST present at least one VIRTIO_PCI_CAP_DEVICE_CFG capability for any device type which has a device-specific configuration.

4.1.4.6.1 Device Requirements: Device-specific configuration
The offset for the device-specific configuration MUST be 4-byte aligned.
4.1.4.7 Shared memory capability

Shared memory regions 2.8 are enumerated on the PCI transport as a sequence of VIRTIO_PCI_CAP_SHARED_MEMORY_CFG capabilities, one per region.

The capability is defined by a struct virtio_pci_cap64 and utilises the cap.id to allow multiple shared memory regions per device. The identifier in cap.id does not denote a certain order of preference; it is only used to uniquely identify a region.

4.1.4.7.1 Device Requirements: Device-specific configuration
The region defined by the combination of the cap.offset, cap.offset_hi, and cap.length, cap.length_hi fields MUST be contained within the declared bar.

The cap.id MUST be unique for any one device instance.

4.1.4.8 PCI configuration access capability

The VIRTIO_PCI_CAP_PCI_CFG capability creates an alternative (and likely suboptimal) access method to the common configuration, notification, ISR and device-specific configuration regions.

The capability is immediately followed by an additional field like so:

struct virtio_pci_cfg_cap { 
        struct virtio_pci_cap cap; 
        u8 pci_cfg_data[4]; /* Data for BAR access. */ 
};

The fields cap.bar, cap.length, cap.offset and pci_cfg_data are read-write (RW) for the driver.

To access a device region, the driver writes into the capability structure (ie. within the PCI configuration space) as follows:

At that point, pci_cfg_data will provide a window of size cap.length into the given cap.bar at offset cap.offset.

4.1.4.8.1 Device Requirements: PCI configuration access capability
The device MUST present at least one VIRTIO_PCI_CAP_PCI_CFG capability.

Upon detecting driver write access to pci_cfg_data, the device MUST execute a write access at offset cap.offset at BAR selected by cap.bar using the first cap.length bytes from pci_cfg_data.

Upon detecting driver read access to pci_cfg_data, the device MUST execute a read access of length cap.length at offset cap.offset at BAR selected by cap.bar and store the first cap.length bytes in pci_cfg_data.

4.1.4.8.2 Driver Requirements: PCI configuration access capability
The driver MUST NOT write a cap.offset which is not a multiple of cap.length (ie. all accesses MUST be aligned).

The driver MUST NOT read or write pci_cfg_data unless cap.bar, cap.length and cap.offset address cap.length bytes within a BAR range specified by some other Virtio Structure PCI Capability of type other than VIRTIO_PCI_CAP_PCI_CFG.

4.1.4.9 Legacy Interfaces: A Note on PCI Device Layout

Transitional devices MUST present part of configuration registers in a legacy configuration structure in BAR0 in the first I/O region of the PCI device, as documented below. When using the legacy interface, transitional drivers MUST use the legacy configuration structure in BAR0 in the first I/O region of the PCI device, as documented below.

When using the legacy interface the driver MAY access the device-specific configuration region using any width accesses, and a transitional device MUST present driver with the same results as when accessed using the “natural” access method (i.e. 32-bit accesses for 32-bit fields, etc).

Note that this is possible because while the virtio common configuration structure is PCI (i.e. little) endian, when using the legacy interface the device-specific configuration region is encoded in the native endian of the guest (where such distinction is applicable).

When used through the legacy interface, the virtio common configuration structure looks as follows:










Bits 32 32 32 16 16 16 8 8









Read / Write R R+W R+W R R+W R+W R+W R









Purpose Device Features bits 0:31 Driver Features bits 0:31 Queue Address queue_sizequeue_selectQueue Notify Device Status ISR
Status









If MSI-X is enabled for the device, two additional fields immediately follow this header:




Bits 16 16



Read/Write R+W R+W



Purpose (MSI-X) config_msix_vector queue_msix_vector



Note: When MSI-X capability is enabled, device-specific configuration starts at byte offset 24 in virtio common configuration structure structure. When MSI-X capability is not enabled, device-specific configuration starts at byte offset 20 in virtio header. ie. once you enable MSI-X on the device, the other fields move. If you turn it off again, they move back!

Any device-specific configuration space immediately follows these general headers:




Bits Device Specific


Read / Write Device Specific


Purpose Device Specific



When accessing the device-specific configuration space using the legacy interface, transitional drivers MUST access the device-specific configuration space at an offset immediately following the general headers.

When using the legacy interface, transitional devices MUST present the device-specific configuration space if any at an offset immediately following the general headers.

Note that only Feature Bits 0 to 31 are accessible through the Legacy Interface. When used through the Legacy Interface, Transitional Devices MUST assume that Feature Bits 32 to 63 are not acknowledged by Driver.

As legacy devices had no config_generation field, see 2.4.4 Legacy Interface: Device Configuration Space for workarounds.

4.1.4.10 Non-transitional Device With Legacy Driver: A Note on PCI Device Layout

All known legacy drivers check either the PCI Revision or the Device and Vendor IDs, and thus won’t attempt to drive a non-transitional device.

A buggy legacy driver might mistakenly attempt to drive a non-transitional device. If support for such drivers is required (as opposed to fixing the bug), the following would be the recommended way to detect and handle them. Note: Such buggy drivers are not currently known to be used in production.

4.1.4.10.0.1 Device Requirements: Non-transitional Device With Legacy Driver
Non-transitional devices, on a platform where a legacy driver for a legacy device with the same ID (including PCI Revision, Device and Vendor IDs) is known to have previously existed, SHOULD take the following steps to cause the legacy driver to fail gracefully when it attempts to drive them:
  1. Present an I/O BAR in BAR0, and
  2. Respond to a single-byte zero write to offset 18 (corresponding to Device Status register in the legacy layout) of BAR0 by presenting zeroes on every BAR and ignoring writes.

4.1.5 PCI-specific Initialization And Device Operation

4.1.5.1 Device Initialization

This documents PCI-specific steps executed during Device Initialization.

4.1.5.1.1 Virtio Device Configuration Layout Detection
As a prerequisite to device initialization, the driver scans the PCI capability list, detecting virtio configuration layout using Virtio Structure PCI capabilities as detailed in 4.1.4
4.1.5.1.1.1 Legacy Interface: A Note on Device Layout Detection
Legacy drivers skipped the Device Layout Detection step, assuming legacy device configuration space in BAR0 in I/O space unconditionally.

Legacy devices did not have the Virtio PCI Capability in their capability list.

Therefore:

Transitional devices MUST expose the Legacy Interface in I/O space in BAR0.

Transitional drivers MUST look for the Virtio PCI Capabilities on the capability list. If these are not present, driver MUST assume a legacy device, and use it through the legacy interface.

Non-transitional drivers MUST look for the Virtio PCI Capabilities on the capability list. If these are not present, driver MUST assume a legacy device, and fail gracefully.

4.1.5.1.2 MSI-X Vector Configuration
When MSI-X capability is present and enabled in the device (through standard PCI configuration space) config_msix_vector and queue_msix_vector are used to map configuration change and queue interrupts to MSI-X vectors. In this case, the ISR Status is unused.

Writing a valid MSI-X Table entry number, 0 to 0x7FF, to config_msix_vector/queue_msix_vector maps interrupts triggered by the configuration change/selected queue events respectively to the corresponding MSI-X vector. To disable interrupts for an event type, the driver unmaps this event by writing a special NO_VECTOR value:

/* Vector value used to disable MSI for queue */ 
#define VIRTIO_MSI_NO_VECTOR            0xffff

Note that mapping an event to vector might require device to allocate internal device resources, and thus could fail.

4.1.5.1.2.1 Device Requirements: MSI-X Vector Configuration
A device that has an MSI-X capability SHOULD support at least 2 and at most 0x800 MSI-X vectors. Device MUST report the number of vectors supported in Table Size in the MSI-X Capability as specified in [PCI]. The device SHOULD restrict the reported MSI-X Table Size field to a value that might benefit system performance. Note: For example, a device which does not expect to send interrupts at a high rate might only specify 2 MSI-X vectors.

Device MUST support mapping any event type to any valid vector 0 to MSI-X Table Size. Device MUST support unmapping any event type.

The device MUST return vector mapped to a given event, (NO_VECTOR if unmapped) on read of config_msix_vector/queue_msix_vector. The device MUST have all queue and configuration change events are unmapped upon reset.

Devices SHOULD NOT cause mapping an event to vector to fail unless it is impossible for the device to satisfy the mapping request. Devices MUST report mapping failures by returning the NO_VECTOR value when the relevant config_msix_vector/queue_msix_vector field is read.

4.1.5.1.2.2 Driver Requirements: MSI-X Vector Configuration
Driver MUST support device with any MSI-X Table Size 0 to 0x7FF. Driver MAY fall back on using INT#x interrupts for a device which only supports one MSI-X vector (MSI-X Table Size = 0).

Driver MAY intepret the Table Size as a hint from the device for the suggested number of MSI-X vectors to use.

Driver MUST NOT attempt to map an event to a vector outside the MSI-X Table supported by the device, as reported by Table Size in the MSI-X Capability.

After mapping an event to vector, the driver MUST verify success by reading the Vector field value: on success, the previously written value is returned, and on failure, NO_VECTOR is returned. If a mapping failure is detected, the driver MAY retry mapping with fewer vectors, disable MSI-X or report device failure.

4.1.5.1.3 Virtqueue Configuration
As a device can have zero or more virtqueues for bulk data transport6, the driver needs to configure them as part of the device-specific configuration.

The driver typically does this as follows, for each virtqueue a device has:

  1. Write the virtqueue index (first queue is 0) to queue_select.
  2. Read the virtqueue size from queue_size. This controls how big the virtqueue is (see 2.5 Virtqueues). If this field is 0, the virtqueue does not exist.
  3. Optionally, select a smaller virtqueue size and write it to queue_size.
  4. Allocate and zero Descriptor Table, Available and Used rings for the virtqueue in contiguous physical memory.
  5. Optionally, if MSI-X capability is present and enabled on the device, select a vector to use to request interrupts triggered by virtqueue events. Write the MSI-X Table entry number corresponding to this vector into queue_msix_vector. Read queue_msix_vector: on success, previously written value is returned; on failure, NO_VECTOR value is returned.
4.1.5.1.3.1 Legacy Interface: A Note on Virtqueue Configuration
When using the legacy interface, the queue layout follows 2.6.2 Legacy Interfaces: A Note on Virtqueue Layout with an alignment of 4096. Driver writes the physical address, divided by 4096 to the Queue Address field7. There was no mechanism to negotiate the queue size.
4.1.5.2 Available Buffer Notifications

When VIRTIO_F_NOTIFICATION_DATA has not been negotiated, the driver sends an available buffer notification to the device by writing the 16-bit virtqueue index of this virtqueue to the Queue Notify address.

When VIRTIO_F_NOTIFICATION_DATA has been negotiated, the driver sends an available buffer notification to the device by writing the following 32-bit value to the Queue Notify address:

 
le32 { 
  vqn : 16; 
  next_off : 15; 
  next_wrap : 1; 
};

See 2.7.23 Driver notifications for the definition of the components.

See 4.1.4.4 for how to calculate the Queue Notify address.

4.1.5.3 Used Buffer Notifications

If a used buffer notification is necessary for a virtqueue, the device would typically act as follows:

4.1.5.3.1 Device Requirements: Used Buffer Notifications
If MSI-X capability is enabled and queue_msix_vector is NO_VECTOR for a virtqueue, the device MUST NOT deliver an interrupt for that virtqueue.
4.1.5.4 Notification of Device Configuration Changes

Some virtio PCI devices can change the device configuration state, as reflected in the device-specific configuration region of the device. In this case:

A single interrupt MAY indicate both that one or more virtqueue has been used and that the configuration space has changed.

4.1.5.4.1 Device Requirements: Notification of Device Configuration Changes
If MSI-X capability is enabled and config_msix_vector is NO_VECTOR, the device MUST NOT deliver an interrupt for device configuration space changes.
4.1.5.4.2 Driver Requirements: Notification of Device Configuration Changes
A driver MUST handle the case where the same interrupt is used to indicate both device configuration space change and one or more virtqueues being used.
4.1.5.5 Driver Handling Interrupts

The driver interrupt handler would typically:

4.2 Virtio Over MMIO

Virtual environments without PCI support (a common situation in embedded devices models) might use simple memory mapped device (“virtio-mmio”) instead of the PCI device.

The memory mapped virtio device behaviour is based on the PCI device specification. Therefore most operations including device initialization, queues configuration and buffer transfers are nearly identical. Existing differences are described in the following sections.

4.2.1 MMIO Device Discovery

Unlike PCI, MMIO provides no generic device discovery mechanism. For each device, the guest OS will need to know the location of the registers and interrupt(s) used. The suggested binding for systems using flattened device trees is shown in this example:

// EXAMPLE: virtio_block device taking 512 bytes at 0x1e000, interrupt 42. 
virtio_block@1e000 { 
        compatible = "virtio,mmio"; 
        reg = <0x1e000 0x200>; 
        interrupts = <42>; 
}

4.2.2 MMIO Device Register Layout

MMIO virtio devices provide a set of memory mapped control registers followed by a device-specific configuration space, described in the table 4.1.

All register values are organized as Little Endian.

Table 4.1: MMIO Device Register Layout


Name
Offset from base
Direction

Function
Description



MagicValue
0x000
R

Magic value
0x74726976 (a Little Endian equivalent of the “virt” string).



Version
0x004
R

Device version number
0x2. Note: Legacy devices (see 4.2.4 Legacy interface) used 0x1.



DeviceID
0x008
R

Virtio Subsystem Device ID
See 5 Device Types for possible values. Value zero (0x0) is used to define a system memory map with placeholder devices at static, well known addresses, assigning functions to them depending on user’s needs.



VendorID
0x00c
R

Virtio Subsystem Vendor ID



DeviceFeatures
0x010
R

Flags representing features the device supports
Reading from this register returns 32 consecutive flag bits, the least significant bit depending on the last value written to DeviceFeaturesSel. Access to this register returns bits DeviceFeaturesSel 32 to (DeviceFeaturesSel 32) + 31, eg. feature bits 0 to 31 if DeviceFeaturesSel is set to 0 and features bits 32 to 63 if DeviceFeaturesSel is set to 1. Also see 2.2 Feature Bits.



DeviceFeaturesSel
0x014
W

Device (host) features word selection.
Writing to this register selects a set of 32 device feature bits accessible by reading from DeviceFeatures.



DriverFeatures
0x020
W

Flags representing device features understood and activated by the driver
Writing to this register sets 32 consecutive flag bits, the least significant bit depending on the last value written to DriverFeaturesSel. Access to this register sets bits DriverFeaturesSel 32 to (DriverFeaturesSel 32) + 31, eg. feature bits 0 to 31 if DriverFeaturesSel is set to 0 and features bits 32 to 63 if DriverFeaturesSel is set to 1. Also see 2.2 Feature Bits.



DriverFeaturesSel
0x024
W

Activated (guest) features word selection
Writing to this register selects a set of 32 activated feature bits accessible by writing to DriverFeatures.



QueueSel
0x030
W

Virtual queue index
Writing to this register selects the virtual queue that the following operations on QueueNumMax, QueueNum, QueueReady, QueueDescLow, QueueDescHigh, QueueAvailLow, QueueAvailHigh, QueueUsedLow and QueueUsedHigh apply to. The index number of the first queue is zero (0x0).



QueueNumMax
0x034
R

Maximum virtual queue size
Reading from the register returns the maximum size (number of elements) of the queue the device is ready to process or zero (0x0) if the queue is not available. This applies to the queue selected by writing to QueueSel.



QueueNum
0x038
W

Virtual queue size
Queue size is the number of elements in the queue. Writing to this register notifies the device what size of the queue the driver will use. This applies to the queue selected by writing to QueueSel.



QueueReady
0x044
RW

Virtual queue ready bit
Writing one (0x1) to this register notifies the device that it can execute requests from this virtual queue. Reading from this register returns the last value written to it. Both read and write accesses apply to the queue selected by writing to QueueSel.



QueueNotify
0x050
W

Queue notifier
Writing a value to this register notifies the device that there are new buffers to process in a queue.

When VIRTIO_F_NOTIFICATION_DATA has not been negotiated, the value written is the queue index.

When VIRTIO_F_NOTIFICATION_DATA has been negotiated, the Notification data value has the following format:

 
le32 { 
  vqn : 16; 
  next_off : 15; 
  next_wrap : 1; 
};

See 2.7.23 Driver notifications for the definition of the components.



InterruptStatus
0x60
R

Interrupt status
Reading from this register returns a bit mask of events that caused the device interrupt to be asserted. The following events are possible:

Used Buffer Notification
- bit 0 - the interrupt was asserted because the device has used a buffer in at least one of the active virtual queues.
Configuration Change Notification
- bit 1 - the interrupt was asserted because the configuration of the device has changed.



InterruptACK
0x064
W

Interrupt acknowledge
Writing a value with bits set as defined in InterruptStatus to this register notifies the device that events causing the interrupt have been handled.



Status
0x070
RW

Device status
Reading from this register returns the current device status flags. Writing non-zero values to this register sets the status flags, indicating the driver progress. Writing zero (0x0) to this register triggers a device reset. See also p. 4.2.3.1 Device Initialization.



QueueDescLow
0x080
QueueDescHigh
0x084
W

Virtual queue’s Descriptor Area 64 bit long physical address
Writing to these two registers (lower 32 bits of the address to QueueDescLow, higher 32 bits to QueueDescHigh) notifies the device about location of the Descriptor Area of the queue selected by writing to QueueSel register.



QueueDriverLow
0x090
QueueDriverHigh
0x094
W

Virtual queue’s Driver Area 64 bit long physical address
Writing to these two registers (lower 32 bits of the address to QueueAvailLow, higher 32 bits to QueueAvailHigh) notifies the device about location of the Driver Area of the queue selected by writing to QueueSel.



QueueDeviceLow
0x0a0
QueueDeviceHigh
0x0a4
W

Virtual queue’s Device Area 64 bit long physical address
Writing to these two registers (lower 32 bits of the address to QueueUsedLow, higher 32 bits to QueueUsedHigh) notifies the device about location of the Device Area of the queue selected by writing to QueueSel.



SHMSel
0x0ac
W

Shared memory id
Writing to this register selects the shared memory region 2.8 following operations on SHMLenLow, SHMLenHigh, SHMBaseLow and SHMBaseHigh apply to.



SHMLenLow
0x0b0
SHMLenHigh
0x0xb4
R

Shared memory region 64 bit long length
These registers return the length of the shared memory region in bytes, as defined by the device for the region selected by the SHMSel register. The lower 32 bits of the length are read from SHMLenLow and the higher 32 bits from SHMLenHigh. Reading from a non-existent region (i.e. where the ID written to SHMSel is unused) results in a length of -1.



SHMBaseLow
0x0b8
SHMBaseHigh
0x0xbc
R

Shared memory region 64 bit long physical address
The driver reads these registers to discover the base address of the region in physical address space. This address is chosen by the device (or other part of the VMM). The lower 32 bits of the address are read from SHMBaseLow with the higher 32 bits from SHMBaseHigh. Reading from a non-existent region (i.e. where the ID written to SHMSel is unused) results in a base address of 0xffffffffffffffff.



ConfigGeneration
0x0fc
R

Configuration atomicity value
Reading from this register returns a value describing a version of the device-specific configuration space (see Config). The driver can then access the configuration space and, when finished, read ConfigGeneration again. If no part of the configuration space has changed between these two ConfigGeneration reads, the returned values are identical. If the values are different, the configuration space accesses were not atomic and the driver has to perform the operations again. See also 2.4.



Config
0x100+
RW

Configuration space
Device-specific configuration space starts at the offset 0x100 and is accessed with byte alignment. Its meaning and size depend on the device and the driver.



4.2.2.1 Device Requirements: MMIO Device Register Layout

The device MUST return 0x74726976 in MagicValue.

The device MUST return value 0x2 in Version.

The device MUST present each event by setting the corresponding bit in InterruptStatus from the moment it takes place, until the driver acknowledges the interrupt by writing a corresponding bit mask to the InterruptACK register. Bits which do not represent events which took place MUST be zero.

Upon reset, the device MUST clear all bits in InterruptStatus and ready bits in the QueueReady register for all queues in the device.

The device MUST change value returned in ConfigGeneration if there is any risk of a driver seeing an inconsistent configuration state.

The device MUST NOT access virtual queue contents when QueueReady is zero (0x0).

4.2.2.2 Driver Requirements: MMIO Device Register Layout

The driver MUST NOT access memory locations not described in the table 4.1 (or, in case of the configuration space, described in the device specification), MUST NOT write to the read-only registers (direction R) and MUST NOT read from the write-only registers (direction W).

The driver MUST only use 32 bit wide and aligned reads and writes to access the control registers described in table 4.1. For the device-specific configuration space, the driver MUST use 8 bit wide accesses for 8 bit wide fields, 16 bit wide and aligned accesses for 16 bit wide fields and 32 bit wide and aligned accesses for 32 and 64 bit wide fields.

The driver MUST ignore a device with MagicValue which is not 0x74726976, although it MAY report an error.

The driver MUST ignore a device with Version which is not 0x2, although it MAY report an error.

The driver MUST ignore a device with DeviceID 0x0, but MUST NOT report any error.

Before reading from DeviceFeatures, the driver MUST write a value to DeviceFeaturesSel.

Before writing to the DriverFeatures register, the driver MUST write a value to the DriverFeaturesSel register.

The driver MUST write a value to QueueNum which is less than or equal to the value presented by the device in QueueNumMax.

When QueueReady is not zero, the driver MUST NOT access QueueNum, QueueDescLow, QueueDescHigh, QueueAvailLow, QueueAvailHigh, QueueUsedLow, QueueUsedHigh.

To stop using the queue the driver MUST write zero (0x0) to this QueueReady and MUST read the value back to ensure synchronization.

The driver MUST ignore undefined bits in InterruptStatus.

The driver MUST write a value with a bit mask describing events it handled into InterruptACK when it finishes handling an interrupt and MUST NOT set any of the undefined bits in the value.

4.2.3 MMIO-specific Initialization And Device Operation

4.2.3.1 Device Initialization
4.2.3.1.1 Driver Requirements: Device Initialization
The driver MUST start the device initialization by reading and checking values from MagicValue and Version. If both values are valid, it MUST read DeviceID and if its value is zero (0x0) MUST abort initialization and MUST NOT access any other register.

Drivers not expecting shared memory MUST NOT use the shared memory registers.

Further initialization MUST follow the procedure described in 3.1 Device Initialization.

4.2.3.2 Virtqueue Configuration

The driver will typically initialize the virtual queue in the following way:

  1. Select the queue writing its index (first queue is 0) to QueueSel.
  2. Check if the queue is not already in use: read QueueReady, and expect a returned value of zero (0x0).
  3. Read maximum queue size (number of elements) from QueueNumMax. If the returned value is zero (0x0) the queue is not available.
  4. Allocate and zero the queue memory, making sure the memory is physically contiguous.
  5. Notify the device about the queue size by writing the size to QueueNum.
  6. Write physical addresses of the queue’s Descriptor Area, Driver Area and Device Area to (respectively) the QueueDescLow/QueueDescHigh, QueueDriverLow/QueueDriverHigh and QueueDeviceLow/QueueDeviceHigh register pairs.
  7. Write 0x1 to QueueReady.
4.2.3.3 Available Buffer Notifications

When VIRTIO_F_NOTIFICATION_DATA has not been negotiated, the driver sends an available buffer notification to the device by writing the 16-bit virtqueue index of the queue to be notified to QueueNotify.

When VIRTIO_F_NOTIFICATION_DATA has been negotiated, the driver sends an available buffer notification to the device by writing the following 32-bit value to QueueNotify:

 
le32 { 
  vqn : 16; 
  next_off : 15; 
  next_wrap : 1; 
};

See 2.7.23 Driver notifications for the definition of the components.

4.2.3.4 Notifications From The Device

The memory mapped virtio device is using a single, dedicated interrupt signal, which is asserted when at least one of the bits described in the description of InterruptStatus is set. This is how the device sends a used buffer notification or a configuration change notification to the device.

4.2.3.4.1 Driver Requirements: Notifications From The Device
After receiving an interrupt, the driver MUST read InterruptStatus to check what caused the interrupt (see the register description). The used buffer notification bit being set SHOULD be interpreted as a used buffer notification for each active virtqueue. After the interrupt is handled, the driver MUST acknowledge it by writing a bit mask corresponding to the handled events to the InterruptACK register.

4.2.4 Legacy interface

The legacy MMIO transport used page-based addressing, resulting in a slightly different control register layout, the device initialization and the virtual queue configuration procedure.

Table 4.2 presents control registers layout, omitting descriptions of registers which did not change their function nor behaviour:

Table 4.2: MMIO Device Legacy Register Layout


Name
Offset from base
Direction

Function
Description



MagicValue
0x000
R

Magic value



Version
0x004
R

Device version number
Legacy device returns value 0x1.



DeviceID
0x008
R

Virtio Subsystem Device ID



VendorID
0x00c
R

Virtio Subsystem Vendor ID



HostFeatures
0x010
R

Flags representing features the device supports



HostFeaturesSel
0x014
W

Device (host) features word selection.



GuestFeatures
0x020
W

Flags representing device features understood and activated by the driver



GuestFeaturesSel
0x024
W

Activated (guest) features word selection



GuestPageSize
0x028
W

Guest page size
The driver writes the guest page size in bytes to the register during initialization, before any queues are used. This value should be a power of 2 and is used by the device to calculate the Guest address of the first queue page (see QueuePFN).



QueueSel
0x030
W

Virtual queue index
Writing to this register selects the virtual queue that the following operations on the QueueNumMax, QueueNum, QueueAlign and QueuePFN registers apply to. The index number of the first queue is zero (0x0). .



QueueNumMax
0x034
R

Maximum virtual queue size
Reading from the register returns the maximum size of the queue the device is ready to process or zero (0x0) if the queue is not available. This applies to the queue selected by writing to QueueSel and is allowed only when QueuePFN is set to zero (0x0), so when the queue is not actively used.



QueueNum
0x038
W

Virtual queue size
Queue size is the number of elements in the queue. Writing to this register notifies the device what size of the queue the driver will use. This applies to the queue selected by writing to QueueSel.



QueueAlign
0x03c
W

Used Ring alignment in the virtual queue
Writing to this register notifies the device about alignment boundary of the Used Ring in bytes. This value should be a power of 2 and applies to the queue selected by writing to QueueSel.



QueuePFN
0x040
RW

Guest physical page number of the virtual queue
Writing to this register notifies the device about location of the virtual queue in the Guest’s physical address space. This value is the index number of a page starting with the queue Descriptor Table. Value zero (0x0) means physical address zero (0x00000000) and is illegal. When the driver stops using the queue it writes zero (0x0) to this register. Reading from this register returns the currently used page number of the queue, therefore a value other than zero (0x0) means that the queue is in use. Both read and write accesses apply to the queue selected by writing to QueueSel.



QueueNotify
0x050
W

Queue notifier



InterruptStatus
0x60
R

Interrupt status



InterruptACK
0x064
W

Interrupt acknowledge



Status
0x070
RW

Device status
Reading from this register returns the current device status flags. Writing non-zero values to this register sets the status flags, indicating the OS/driver progress. Writing zero (0x0) to this register triggers a device reset. The device sets QueuePFN to zero (0x0) for all queues in the device. Also see 3.1 Device Initialization.



Config
0x100+
RW

Configuration space



The virtual queue page size is defined by writing to GuestPageSize, as written by the guest. The driver does this before the virtual queues are configured.

The virtual queue layout follows p. 2.6.2 Legacy Interfaces: A Note on Virtqueue Layout, with the alignment defined in QueueAlign.

The virtual queue is configured as follows:

  1. Select the queue writing its index (first queue is 0) to QueueSel.
  2. Check if the queue is not already in use: read QueuePFN, expecting a returned value of zero (0x0).
  3. Read maximum queue size (number of elements) from QueueNumMax. If the returned value is zero (0x0) the queue is not available.
  4. Allocate and zero the queue pages in contiguous virtual memory, aligning the Used Ring to an optimal boundary (usually page size). The driver should choose a queue size smaller than or equal to QueueNumMax.
  5. Notify the device about the queue size by writing the size to QueueNum.
  6. Notify the device about the used alignment by writing its value in bytes to QueueAlign.
  7. Write the physical number of the first page of the queue to the QueuePFN register.

Notification mechanisms did not change.

4.3 Virtio Over Channel I/O

S/390 based virtual machines support neither PCI nor MMIO, so a different transport is needed there.

virtio-ccw uses the standard channel I/O based mechanism used for the majority of devices on S/390. A virtual channel device with a special control unit type acts as proxy to the virtio device (similar to the way virtio-pci uses a PCI device) and configuration and operation of the virtio device is accomplished (mostly) via channel commands. This means virtio devices are discoverable via standard operating system algorithms, and adding virtio support is mainly a question of supporting a new control unit type.

As the S/390 is a big endian machine, the data structures transmitted via channel commands are big-endian: this is made clear by use of the types be16, be32 and be64.

4.3.1 Basic Concepts

As a proxy device, virtio-ccw uses a channel-attached I/O control unit with a special control unit type (0x3832) and a control unit model corresponding to the attached virtio device’s subsystem device ID, accessed via a virtual I/O subchannel and a virtual channel path of type 0x32. This proxy device is discoverable via normal channel subsystem device discovery (usually a STORE SUBCHANNEL loop) and answers to the basic channel commands:

For a virtio-ccw proxy device, SENSE ID will return the following information:




Bytes Description Contents






0 reserved 0xff



1-2 control unit type 0x3832



3 control unit model



4-5 device type zeroes (unset)



6 device model zeroes (unset)



7-255 extended SenseId data zeroes (unset)



A virtio-ccw proxy device facilitates:

4.3.1.1 Channel Commands for Virtio

In addition to the basic channel commands, virtio-ccw defines a set of channel commands related to configuration and operation of virtio:

#define CCW_CMD_SET_VQ 0x13 
#define CCW_CMD_VDEV_RESET 0x33 
#define CCW_CMD_SET_IND 0x43 
#define CCW_CMD_SET_CONF_IND 0x53 
#define CCW_CMD_SET_IND_ADAPTER 0x73 
#define CCW_CMD_READ_FEAT 0x12 
#define CCW_CMD_WRITE_FEAT 0x11 
#define CCW_CMD_READ_CONF 0x22 
#define CCW_CMD_WRITE_CONF 0x21 
#define CCW_CMD_WRITE_STATUS 0x31 
#define CCW_CMD_READ_VQ_CONF 0x32 
#define CCW_CMD_SET_VIRTIO_REV 0x83 
#define CCW_CMD_READ_STATUS 0x72
4.3.1.2 Notifications

Available buffer notifications are realized as a hypercall. No additional setup by the driver is needed. The operation of available buffer notifications is described in section 4.3.3.2.

Used buffer notifications are realized either as so-called classic or adapter I/O interrupts depending on a transport level negotiation. The initialization is described in sections 4.3.2.6.1 and 4.3.2.6.3 respectively. The operation of each flavor is described in sections 4.3.3.1.1 and 4.3.3.1.2 respectively.

Configuration change notifications are done using so-called classic I/O interrupts. The initialization is described in section 4.3.2.6.2 and the operation in section 4.3.3.1.1.

4.3.1.3 Device Requirements: Basic Concepts

The virtio-ccw device acts like a normal channel device, as specified in [S390 PoP] and [S390 Common I/O]. In particular:

4.3.1.4 Driver Requirements: Basic Concepts

A driver for virtio-ccw devices MUST check for a control unit type of 0x3832 and MUST ignore the device type and model.

A driver SHOULD attempt to provide the correct length in a channel command even if it suppresses length checks for that command.

4.3.2 Device Initialization

virtio-ccw uses several channel commands to set up a device.

4.3.2.1 Setting the Virtio Revision

CCW_CMD_SET_VIRTIO_REV is issued by the driver to set the revision of the virtio-ccw transport it intends to drive the device with. It uses the following communication structure:

struct virtio_rev_info { 
        be16 revision; 
        be16 length; 
        u8 data[]; 
};

revision contains the desired revision id, length the length of the data portion and data revision-dependent additional desired options.

The following values are supported:





revision length data remarks








0 0 legacy interface; transitional devices only




1 0 Virtio 1




2 0 CCW_CMD_READ_STATUS support




3-n reserved for later revisions




Note that a change in the virtio standard does not necessarily correspond to a change in the virtio-ccw revision.

4.3.2.1.1 Device Requirements: Setting the Virtio Revision
A device MUST post a unit check with command reject for any revision it does not support. For any invalid combination of revision, length and data, it MUST post a unit check with command reject as well. A non-transitional device MUST reject revision id 0.

A device MUST answer with command reject to any virtio-ccw specific channel command that is not contained in the revision selected by the driver.

A device MUST answer with command reject to any attempt to select a different revision after a revision has been successfully selected by the driver.

A device MUST treat the revision as unset from the time the associated subchannel has been enabled until a revision has been successfully set by the driver. This implies that revisions are not persistent across disabling and enabling of the associated subchannel.

4.3.2.1.2 Driver Requirements: Setting the Virtio Revision
A driver SHOULD start with trying to set the highest revision it supports and continue with lower revisions if it gets a command reject.

A driver MUST NOT issue any other virtio-ccw specific channel commands prior to setting the revision.

After a revision has been successfully selected by the driver, it MUST NOT attempt to select a different revision.

4.3.2.1.3 Legacy Interfaces: A Note on Setting the Virtio Revision
A legacy device will not support the CCW_CMD_SET_VIRTIO_REV and answer with a command reject. A non-transitional driver MUST stop trying to operate this device in that case. A transitional driver MUST operate the device as if it had been able to set revision 0.

A legacy driver will not issue the CCW_CMD_SET_VIRTIO_REV prior to issuing other virtio-ccw specific channel commands. A non-transitional device therefore MUST answer any such attempts with a command reject. A transitional device MUST assume in this case that the driver is a legacy driver and continue as if the driver selected revision 0. This implies that the device MUST reject any command not valid for revision 0, including a subsequent CCW_CMD_SET_VIRTIO_REV.

4.3.2.2 Configuring a Virtqueue

CCW_CMD_READ_VQ_CONF is issued by the driver to obtain information about a queue. It uses the following structure for communicating:

struct vq_config_block { 
        be16 index; 
        be16 max_num; 
};

The requested number of buffers for queue index is returned in max_num.

Afterwards, CCW_CMD_SET_VQ is issued by the driver to inform the device about the location used for its queue. The transmitted structure is

struct vq_info_block { 
        be64 desc; 
        be32 res0; 
        be16 index; 
        be16 num; 
        be64 driver; 
        be64 device; 
};

desc, driver and device contain the guest addresses for the descriptor area, available area and used area for queue index, respectively. The actual virtqueue size (number of allocated buffers) is transmitted in num.

4.3.2.2.1 Device Requirements: Configuring a Virtqueue
res0 is reserved and MUST be ignored by the device.
4.3.2.2.2 Legacy Interface: A Note on Configuring a Virtqueue
For a legacy driver or for a driver that selected revision 0, CCW_CMD_SET_VQ uses the following communication block:
struct vq_info_block_legacy { 
        be64 queue; 
        be32 align; 
        be16 index; 
        be16 num; 
};

queue contains the guest address for queue index, num the number of buffers and align the alignment. The queue layout follows 2.6.2 Legacy Interfaces: A Note on Virtqueue Layout.

4.3.2.3 Communicating Status Information

The driver changes the status of a device via the CCW_CMD_WRITE_STATUS command, which transmits an 8 bit status value.

As described in 2.2.2, a device sometimes fails to set the device status field: For example, it might fail to accept the FEATURES_OK status bit during device initialization.

With revision 2, CCW_CMD_READ_STATUS is defined: It reads an 8 bit status value from the device and acts as a reverse operation to CCW_CMD_WRITE_STATUS.

4.3.2.3.1 Driver Requirements: Communicating Status Information
If the device posts a unit check with command reject in response to the CCW_CMD_WRITE_STATUS command, the driver MUST assume that the device failed to set the status and the device status field retained its previous value.

If at least revision 2 has been negotiated, the driver SHOULD use the CCW_CMD_READ_STATUS command to retrieve the device status field after a configuration change has been detected.

If not at least revision 2 has been negotiated, the driver MUST NOT attempt to issue the CCW_CMD_READ_STATUS command.

4.3.2.3.2 Device Requirements: Communicating Status Information
If the device fails to set the device status field to the value written by the driver, the device MUST assure that the device status field is left unchanged and MUST post a unit check with command reject.

If at least revision 2 has been negotiated, the device MUST return the current device status field if the CCW_CMD_READ_STATUS command is issued.

4.3.2.4 Handling Device Features

Feature bits are arranged in an array of 32 bit values, making for a total of 8192 feature bits. Feature bits are in little-endian byte order.

The CCW commands dealing with features use the following communication block:

struct virtio_feature_desc { 
        le32 features; 
        u8 index; 
};

features are the 32 bits of features currently accessed, while index describes which of the feature bit values is to be accessed. No padding is added at the end of the structure, it is exactly 5 bytes in length.

The guest obtains the device’s device feature set via the CCW_CMD_READ_FEAT command. The device stores the features at index to features.

For communicating its supported features to the device, the driver uses the CCW_CMD_WRITE_FEAT command, denoting a features/index combination.

4.3.2.5 Device Configuration

The device’s configuration space is located in host memory.

To obtain information from the configuration space, the driver uses CCW_CMD_READ_CONF, specifying the guest memory for the device to write to.

For changing configuration information, the driver uses CCW_CMD_WRITE_CONF, specifying the guest memory for the device to read from.

In both cases, the complete configuration space is transmitted. This allows the driver to compare the new configuration space with the old version, and keep a generation count internally whenever it changes.

4.3.2.6 Setting Up Indicators

In order to set up the indicator bits for host->guest notification, the driver uses different channel commands depending on whether it wishes to use traditional I/O interrupts tied to a subchannel or adapter I/O interrupts for virtqueue notifications. For any given device, the two mechanisms are mutually exclusive.

For the configuration change indicators, only a mechanism using traditional I/O interrupts is provided, regardless of whether traditional or adapter I/O interrupts are used for virtqueue notifications.

4.3.2.6.1 Setting Up Classic Queue Indicators
Indicators for notification via classic I/O interrupts are contained in a 64 bit value per virtio-ccw proxy device.

To communicate the location of the indicator bits for host->guest notification, the driver uses the CCW_CMD_SET_IND command, pointing to a location containing the guest address of the indicators in a 64 bit value.

If the driver has already set up two-staged queue indicators via the CCW_CMD_SET_IND_ADAPTER command, the device MUST post a unit check with command reject to any subsequent CCW_CMD_SET_IND command.

4.3.2.6.2 Setting Up Configuration Change Indicators
Indicators for configuration change host->guest notification are contained in a 64 bit value per virtio-ccw proxy device.

To communicate the location of the indicator bits used in the configuration change host->guest notification, the driver issues the CCW_CMD_SET_CONF_IND command, pointing to a location containing the guest address of the indicators in a 64 bit value.

4.3.2.6.3 Setting Up Two-Stage Queue Indicators
Indicators for notification via adapter I/O interrupts consist of two stages:

To communicate the location of the summary and queue indicator bits, the driver uses the CCW_CMD_SET_IND_ADAPTER command with the following payload:

struct virtio_thinint_area { 
        be64 summary_indicator; 
        be64 indicator; 
        be64 bit_nr; 
        u8 isc; 
} __attribute__ ((packed));

summary_indicator contains the guest address of the 8 bit summary indicator. indicator contains the guest address of an area wherein the indicators for the devices are contained, starting at bit_nr, one bit per virtqueue of the device. Bit numbers start at the left, i.e. the most significant bit in the first byte is assigned the bit number 0. isc contains the I/O interruption subclass to be used for the adapter I/O interrupt. It MAY be different from the isc used by the proxy virtio-ccw device’s subchannel. No padding is added at the end of the structure, it is exactly 25 bytes in length.

4.3.2.6.3.1 Device Requirements: Setting Up Two-Stage Queue Indicators
If the driver has already set up classic queue indicators via the CCW_CMD_SET_IND command, the device MUST post a unit check with command reject to any subsequent CCW_CMD_SET_IND_ADAPTER command.
4.3.2.6.4 Legacy Interfaces: A Note on Setting Up Indicators
In some cases, legacy devices will only support classic queue indicators; in that case, they will reject CCW_CMD_SET_IND_ADAPTER as they don’t know that command. Some legacy devices will support two-stage queue indicators, though, and a driver will be able to successfully use CCW_CMD_SET_IND_ADAPTER to set them up.

4.3.3 Device Operation

4.3.3.1 Host->Guest Notification

There are two modes of operation regarding host->guest notification, classic I/O interrupts and adapter I/O interrupts. The mode to be used is determined by the driver by using CCW_CMD_SET_IND respectively CCW_CMD_SET_IND_ADAPTER to set up queue indicators.

For configuration changes, the driver always uses classic I/O interrupts.

4.3.3.1.1 Notification via Classic I/O Interrupts
If the driver used the CCW_CMD_SET_IND command to set up queue indicators, the device will use classic I/O interrupts for host->guest notification about virtqueue activity.

For notifying the driver of virtqueue buffers, the device sets the corresponding bit in the guest-provided indicators. If an interrupt is not already pending for the subchannel, the device generates an unsolicited I/O interrupt.

If the device wants to notify the driver about configuration changes, it sets bit 0 in the configuration indicators and generates an unsolicited I/O interrupt, if needed. This also applies if adapter I/O interrupts are used for queue notifications.

4.3.3.1.2 Notification via Adapter I/O Interrupts
If the driver used the CCW_CMD_SET_IND_ADAPTER command to set up queue indicators, the device will use adapter I/O interrupts for host->guest notification about virtqueue activity.

For notifying the driver of virtqueue buffers, the device sets the bit in the guest-provided indicator area at the corresponding offset. The guest-provided summary indicator is set to 0x01. An adapter I/O interrupt for the corresponding interruption subclass is generated.

The recommended way to process an adapter I/O interrupt by the driver is as follows:

4.3.3.1.2.1 Device Requirements: Notification via Adapter I/O Interrupts
The device SHOULD only generate an adapter I/O interrupt if the summary indicator had not been set prior to notification.
4.3.3.1.2.2 Driver Requirements: Notification via Adapter I/O Interrupts
The driver MUST clear the summary indicator after receiving an adapter I/O interrupt before it processes the queue indicators.
4.3.3.1.3 Legacy Interfaces: A Note on Host->Guest Notification
As legacy devices and drivers support only classic queue indicators, host->guest notification will always be done via classic I/O interrupts.
4.3.3.2 Guest->Host Notification

For notifying the device of virtqueue buffers, the driver unfortunately can’t use a channel command (the asynchronous characteristics of channel I/O interact badly with the host block I/O backend). Instead, it uses a diagnose 0x500 call with subcode 3 specifying the queue, as follows:




GPR Input Value Output Value






1 0x3



2 Subchannel ID Host Cookie



3 Notification data



4 Host Cookie



When VIRTIO_F_NOTIFICATION_DATA has not been negotiated, the Notification data contains the Virtqueue number.

When VIRTIO_F_NOTIFICATION_DATA has been negotiated, the value has the following format:

 
be32 { 
  vqn : 16; 
  next_off : 15; 
  next_wrap : 1; 
};

See 2.7.23 Driver notifications for the definition of the components.

4.3.3.2.1 Device Requirements: Guest->Host Notification
The device MUST ignore bits 0-31 (counting from the left) of GPR2. This aligns passing the subchannel ID with the way it is passed for the existing I/O instructions.

The device MAY return a 64-bit host cookie in GPR2 to speed up the notification execution.

4.3.3.2.2 Driver Requirements: Guest->Host Notification
For each notification, the driver SHOULD use GPR4 to pass the host cookie received in GPR2 from the previous notication. Note: For example:
info->cookie = do_notify(schid, 
                         virtqueue_get_queue_index(vq), 
                         info->cookie);
4.3.3.3 Resetting Devices

In order to reset a device, a driver sends the CCW_CMD_VDEV_RESET command.


5 Device Types

On top of the queues, config space and feature negotiation facilities built into virtio, several devices are defined.

The following device IDs are used to identify different types of virtio devices. Some device IDs are reserved for devices which are not currently defined in this standard.

Discovering what devices are available and their type is bus-dependent.



Device ID Virtio Device




0 reserved (invalid)


1 network card


2 block device


3 console


4 entropy source


5 memory ballooning (traditional)


6 ioMemory


7 rpmsg


8 SCSI host


9 9P transport


10 mac80211 wlan


11 rproc serial


12 virtio CAIF


13 memory balloon


16 GPU device


17 Timer/Clock device


18 Input device


19 Socket device


20 Crypto device


21 Signal Distribution Module


22 pstore device


23 IOMMU device


24 Memory device


26 file system device


27 PMEM device


Some of the devices above are unspecified by this document, because they are seen as immature or especially niche. Be warned that some are only specified by the sole existing implementation; they could become part of a future specification, be abandoned entirely, or live on outside this standard. We shall speak of them no further.

5.1 Network Device

The virtio network device is a virtual ethernet card, and is the most complex of the devices supported so far by virtio. It has enhanced rapidly and demonstrates clearly how support for new features are added to an existing device. Empty buffers are placed in one virtqueue for receiving packets, and outgoing packets are enqueued into another for transmission in that order. A third command queue is used to control advanced filtering features.

5.1.1 Device ID

1

5.1.2 Virtqueues

0
receiveq1
1
transmitq1
2(N-1)
receiveqN
2(N-1)+1
transmitqN
2N
controlq

N=1 if VIRTIO_NET_F_MQ is not negotiated, otherwise N is set by max_virtqueue_pairs.

controlq only exists if VIRTIO_NET_F_CTRL_VQ set.

5.1.3 Feature bits

VIRTIO_NET_F_CSUM (0)
Device handles packets with partial checksum. This “checksum offload” is a common feature on modern network cards.
VIRTIO_NET_F_GUEST_CSUM (1)
Driver handles packets with partial checksum.
VIRTIO_NET_F_CTRL_GUEST_OFFLOADS (2)
Control channel offloads reconfiguration support.
VIRTIO_NET_F_MTU(3)
Device maximum MTU reporting is supported. If offered by the device, device advises driver about the value of its maximum MTU. If negotiated, the driver uses mtu as the maximum MTU value.
VIRTIO_NET_F_MAC (5)
Device has given MAC address.
VIRTIO_NET_F_GUEST_TSO4 (7)
Driver can receive TSOv4.
VIRTIO_NET_F_GUEST_TSO6 (8)
Driver can receive TSOv6.
VIRTIO_NET_F_GUEST_ECN (9)
Driver can receive TSO with ECN.
VIRTIO_NET_F_GUEST_UFO (10)
Driver can receive UFO.
VIRTIO_NET_F_HOST_TSO4 (11)
Device can receive TSOv4.
VIRTIO_NET_F_HOST_TSO6 (12)
Device can receive TSOv6.
VIRTIO_NET_F_HOST_ECN (13)
Device can receive TSO with ECN.
VIRTIO_NET_F_HOST_UFO (14)
Device can receive UFO.
VIRTIO_NET_F_MRG_RXBUF (15)
Driver can merge receive buffers.
VIRTIO_NET_F_STATUS (16)
Configuration status field is available.
VIRTIO_NET_F_CTRL_VQ (17)
Control channel is available.
VIRTIO_NET_F_CTRL_RX (18)
Control channel RX mode support.
VIRTIO_NET_F_CTRL_VLAN (19)
Control channel VLAN filtering.
VIRTIO_NET_F_GUEST_ANNOUNCE(21)
Driver can send gratuitous packets.
VIRTIO_NET_F_MQ(22)
Device supports multiqueue with automatic receive steering.
VIRTIO_NET_F_CTRL_MAC_ADDR(23)
Set MAC address through control channel.
VIRTIO_NET_F_RSC_EXT(61)
Device can process duplicated ACKs and report number of coalesced segments and duplicated ACKs
VIRTIO_NET_F_STANDBY(62)
Device may act as a standby for a primary device with the same MAC address.
5.1.3.1 Feature bit requirements

Some networking feature bits require other networking feature bits (see 2.2.1):

VIRTIO_NET_F_GUEST_TSO4
Requires VIRTIO_NET_F_GUEST_CSUM.
VIRTIO_NET_F_GUEST_TSO6
Requires VIRTIO_NET_F_GUEST_CSUM.
VIRTIO_NET_F_GUEST_ECN
Requires VIRTIO_NET_F_GUEST_TSO4 or VIRTIO_NET_F_GUEST_TSO6.
VIRTIO_NET_F_GUEST_UFO
Requires VIRTIO_NET_F_GUEST_CSUM.
VIRTIO_NET_F_HOST_TSO4
Requires VIRTIO_NET_F_CSUM.
VIRTIO_NET_F_HOST_TSO6
Requires VIRTIO_NET_F_CSUM.
VIRTIO_NET_F_HOST_ECN
Requires VIRTIO_NET_F_HOST_TSO4 or VIRTIO_NET_F_HOST_TSO6.
VIRTIO_NET_F_HOST_UFO
Requires VIRTIO_NET_F_CSUM.
VIRTIO_NET_F_CTRL_RX
Requires VIRTIO_NET_F_CTRL_VQ.
VIRTIO_NET_F_CTRL_VLAN
Requires VIRTIO_NET_F_CTRL_VQ.
VIRTIO_NET_F_GUEST_ANNOUNCE
Requires VIRTIO_NET_F_CTRL_VQ.
VIRTIO_NET_F_MQ
Requires VIRTIO_NET_F_CTRL_VQ.
VIRTIO_NET_F_CTRL_MAC_ADDR
Requires VIRTIO_NET_F_CTRL_VQ.
VIRTIO_NET_F_RSC_EXT
Requires VIRTIO_NET_F_HOST_TSO4 or VIRTIO_NET_F_HOST_TSO6.
5.1.3.2 Legacy Interface: Feature bits
VIRTIO_NET_F_GSO (6)
Device handles packets with any GSO type. This was supposed to indicate segmentation offload support, but upon further investigation it became clear that multiple bits were needed.
VIRTIO_NET_F_GUEST_RSC4 (41)
Device coalesces TCPIP v4 packets. This was implemented by hypervisor patch for certification purposes and current Windows driver depends on it. It will not function if virtio-net device reports this feature.
VIRTIO_NET_F_GUEST_RSC6 (42)
Device coalesces TCPIP v6 packets. Similar to VIRTIO_NET_F_GUEST_RSC4.

5.1.4 Device configuration layout

Three driver-read-only configuration fields are currently defined. The mac address field always exists (though is only valid if VIRTIO_NET_F_MAC is set), and status only exists if VIRTIO_NET_F_STATUS is set. Two read-only bits (for the driver) are currently defined for the status field: VIRTIO_NET_S_LINK_UP and VIRTIO_NET_S_ANNOUNCE.

#define VIRTIO_NET_S_LINK_UP     1 
#define VIRTIO_NET_S_ANNOUNCE    2

The following driver-read-only field, max_virtqueue_pairs only exists if VIRTIO_NET_F_MQ is set. This field specifies the maximum number of each of transmit and receive virtqueues (receiveq1…receiveqN and transmitq1…transmitqN respectively) that can be configured once VIRTIO_NET_F_MQ is negotiated.

The following driver-read-only field, mtu only exists if VIRTIO_NET_F_MTU is set. This field specifies the maximum MTU for the driver to use.

struct virtio_net_config { 
        u8 mac[6]; 
        le16 status; 
        le16 max_virtqueue_pairs; 
        le16 mtu; 
};
5.1.4.1 Device Requirements: Device configuration layout

The device MUST set max_virtqueue_pairs to between 1 and 0x8000 inclusive, if it offers VIRTIO_NET_F_MQ.

The device MUST set mtu to between 68 and 65535 inclusive, if it offers VIRTIO_NET_F_MTU.

The device SHOULD set mtu to at least 1280, if it offers VIRTIO_NET_F_MTU.

The device MUST NOT modify mtu once it has been set.

The device MUST NOT pass received packets that exceed mtu (plus low level ethernet header length) size with gso_type NONE or ECN after VIRTIO_NET_F_MTU has been successfully negotiated.

The device MUST forward transmitted packets of up to mtu (plus low level ethernet header length) size with gso_type NONE or ECN, and do so without fragmentation, after VIRTIO_NET_F_MTU has been successfully negotiated.

If the driver negotiates the VIRTIO_NET_F_STANDBY feature, the device MAY act as a standby device for a primary device with the same MAC address.

5.1.4.2 Driver Requirements: Device configuration layout

A driver SHOULD negotiate VIRTIO_NET_F_MAC if the device offers it. If the driver negotiates the VIRTIO_NET_F_MAC feature, the driver MUST set the physical address of the NIC to mac. Otherwise, it SHOULD use a locally-administered MAC address (see IEEE 802, “9.2 48-bit universal LAN MAC addresses”).

If the driver does not negotiate the VIRTIO_NET_F_STATUS feature, it SHOULD assume the link is active, otherwise it SHOULD read the link status from the bottom bit of status.

A driver SHOULD negotiate VIRTIO_NET_F_MTU if the device offers it.

If the driver negotiates VIRTIO_NET_F_MTU, it MUST supply enough receive buffers to receive at least one receive packet of size mtu (plus low level ethernet header length) with gso_type NONE or ECN.

If the driver negotiates VIRTIO_NET_F_MTU, it MUST NOT transmit packets of size exceeding the value of mtu (plus low level ethernet header length) with gso_type NONE or ECN.

A driver SHOULD negotiate the VIRTIO_NET_F_STANDBY feature if the device offers it.

5.1.4.3 Legacy Interface: Device configuration layout

When using the legacy interface, transitional devices and drivers MUST format status and max_virtqueue_pairs in struct virtio_net_config according to the native endian of the guest rather than (necessarily when not using the legacy interface) little-endian.

When using the legacy interface, mac is driver-writable which provided a way for drivers to update the MAC without negotiating VIRTIO_NET_F_CTRL_MAC_ADDR.

5.1.5 Device Initialization

A driver would perform a typical initialization routine like so:

  1. Identify and initialize the receive and transmission virtqueues, up to N of each kind. If VIRTIO_NET_F_MQ feature bit is negotiated, N=max_virtqueue_pairs, otherwise identify N=1.
  2. If the VIRTIO_NET_F_CTRL_VQ feature bit is negotiated, identify the control virtqueue.
  3. Fill the receive queues with buffers: see 5.1.6.3.
  4. Even with VIRTIO_NET_F_MQ, only receiveq1, transmitq1 and controlq are used by default. The driver would send the VIRTIO_NET_CTRL_MQ_VQ_PAIRS_SET command specifying the number of the transmit and receive queues to use.
  5. If the VIRTIO_NET_F_MAC feature bit is set, the configuration space mac entry indicates the “physical” address of the network card, otherwise the driver would typically generate a random local MAC address.
  6. If the VIRTIO_NET_F_STATUS feature bit is negotiated, the link status comes from the bottom bit of status. Otherwise, the driver assumes it’s active.
  7. A performant driver would indicate that it will generate checksumless packets by negotating the VIRTIO_NET_F_CSUM feature.
  8. If that feature is negotiated, a driver can use TCP or UDP segmentation offload by negotiating the VIRTIO_NET_F_HOST_TSO4 (IPv4 TCP), VIRTIO_NET_F_HOST_TSO6 (IPv6 TCP) and VIRTIO_NET_F_HOST_UFO (UDP fragmentation) features.
  9. The converse features are also available: a driver can save the virtual device some work by negotiating these features. Note: For example, a network packet transported between two guests on the same system might not need checksumming at all, nor segmentation, if both guests are amenable. The VIRTIO_NET_F_GUEST_CSUM feature indicates that partially checksummed packets can be received, and if it can do that then the VIRTIO_NET_F_GUEST_TSO4, VIRTIO_NET_F_GUEST_TSO6, VIRTIO_NET_F_GUEST_UFO and VIRTIO_NET_F_GUEST_ECN are the input equivalents of the features described above. See 5.1.6.3 Setting Up Receive Buffers and 5.1.6.4 Processing of Incoming Packets below.

    A truly minimal driver would only accept VIRTIO_NET_F_MAC and ignore everything else.

    5.1.6 Device Operation

    Packets are transmitted by placing them in the transmitq1…transmitqN, and buffers for incoming packets are placed in the receiveq1…receiveqN. In each case, the packet itself is preceded by a header:

    struct virtio_net_hdr { 
    #define VIRTIO_NET_HDR_F_NEEDS_CSUM    1 
    #define VIRTIO_NET_HDR_F_DATA_VALID    2 
    #define VIRTIO_NET_HDR_F_RSC_INFO      4 
            u8 flags; 
    #define VIRTIO_NET_HDR_GSO_NONE        0 
    #define VIRTIO_NET_HDR_GSO_TCPV4       1 
    #define VIRTIO_NET_HDR_GSO_UDP         3 
    #define VIRTIO_NET_HDR_GSO_TCPV6       4 
    #define VIRTIO_NET_HDR_GSO_ECN      0x80 
            u8 gso_type; 
            le16 hdr_len; 
            le16 gso_size; 
            le16 csum_start; 
            le16 csum_offset; 
            le16 num_buffers; 
    };

    The controlq is used to control device features such as filtering.

    5.1.6.1 Legacy Interface: Device Operation

    When using the legacy interface, transitional devices and drivers MUST format the fields in struct virtio_net_hdr according to the native endian of the guest rather than (necessarily when not using the legacy interface) little-endian.

    The legacy driver only presented num_buffers in the struct virtio_net_hdr when VIRTIO_NET_F_MRG_RXBUF was negotiated; without that feature the structure was 2 bytes shorter.

    When using the legacy interface, the driver SHOULD ignore the used length for the transmit queues and the controlq queue. Note: Historically, some devices put the total descriptor length there, even though no data was actually written.

    5.1.6.2 Packet Transmission

    Transmitting a single packet is simple, but varies depending on the different features the driver negotiated.

    1. The driver can send a completely checksummed packet. In this case, flags will be zero, and gso_type will be VIRTIO_NET_HDR_GSO_NONE.
    2. If the driver negotiated VIRTIO_NET_F_CSUM, it can skip checksumming the packet:
      • flags has the VIRTIO_NET_HDR_F_NEEDS_CSUM set,
      • csum_start is set to the offset within the packet to begin checksumming, and
      • csum_offset indicates how many bytes after the csum_start the new (16 bit ones’ complement) checksum is placed by the device.
      • The TCP checksum field in the packet is set to the sum of the TCP pseudo header, so that replacing it by the ones’ complement checksum of the TCP header and body will give the correct result.
      Note: For example, consider a partially checksummed TCP (IPv4) packet. It will have a 14 byte ethernet header and 20 byte IP header followed by the TCP header (with the TCP checksum field 16 bytes into that header). csum_start will be 14+20 = 34 (the TCP checksum includes the header), and csum_offset will be 16.
    3. If the driver negotiated VIRTIO_NET_F_HOST_TSO4, TSO6 or UFO, and the packet requires TCP segmentation or UDP fragmentation, then gso_type is set to VIRTIO_NET_HDR_GSO_TCPV4, TCPV6 or UDP. (Otherwise, it is set to VIRTIO_NET_HDR_GSO_NONE). In this case, packets larger than 1514 bytes can be transmitted: the metadata indicates how to replicate the packet header to cut it into smaller packets. The other gso fields are set:
      • hdr_len is a hint to the device as to how much of the header needs to be kept to copy into each packet, usually set to the length of the headers, including the transport header8.
      • gso_size is the maximum size of each packet beyond that header (ie. MSS).
      • If the driver negotiated the VIRTIO_NET_F_HOST_ECN feature, the VIRTIO_NET_HDR_GSO_ECN bit in gso_type indicates that the TCP packet has the ECN bit set9.
    4. num_buffers is set to zero. This field is unused on transmitted packets.
    5. The header and packet are added as one output descriptor to the transmitq, and the device is notified of the new entry (see 5.1.5 Device Initialization).
    5.1.6.2.1 Driver Requirements: Packet Transmission
    The driver MUST set num_buffers to zero.

    If VIRTIO_NET_F_CSUM is not negotiated, the driver MUST set flags to zero and SHOULD supply a fully checksummed packet to the device.

    If VIRTIO_NET_F_HOST_TSO4 is negotiated, the driver MAY set gso_type to VIRTIO_NET_HDR_GSO_TCPV4 to request TCPv4 segmentation, otherwise the driver MUST NOT set gso_type to VIRTIO_NET_HDR_GSO_TCPV4.

    If VIRTIO_NET_F_HOST_TSO6 is negotiated, the driver MAY set gso_type to VIRTIO_NET_HDR_GSO_TCPV6 to request TCPv6 segmentation, otherwise the driver MUST NOT set gso_type to VIRTIO_NET_HDR_GSO_TCPV6.

    If VIRTIO_NET_F_HOST_UFO is negotiated, the driver MAY set gso_type to VIRTIO_NET_HDR_GSO_UDP to request UDP segmentation, otherwise the driver MUST NOT set gso_type to VIRTIO_NET_HDR_GSO_UDP.

    The driver SHOULD NOT send to the device TCP packets requiring segmentation offload which have the Explicit Congestion Notification bit set, unless the VIRTIO_NET_F_HOST_ECN feature is negotiated, in which case the driver MUST set the VIRTIO_NET_HDR_GSO_ECN bit in gso_type.

    If the VIRTIO_NET_F_CSUM feature has been negotiated, the driver MAY set the VIRTIO_NET_HDR_F_NEEDS_CSUM bit in flags, if so:

    1. the driver MUST validate the packet checksum at offset csum_offset from csum_start as well as all preceding offsets;
    2. the driver MUST set the packet checksum stored in the buffer to the TCP/UDP pseudo header;
    3. the driver MUST set csum_start and csum_offset such that calculating a ones’ complement checksum from csum_start up until the end of the packet and storing the result at offset csum_offset from csum_start will result in a fully checksummed packet;

    If none of the VIRTIO_NET_F_HOST_TSO4, TSO6 or UFO options have been negotiated, the driver MUST set gso_type to VIRTIO_NET_HDR_GSO_NONE.

    If gso_type differs from VIRTIO_NET_HDR_GSO_NONE, then the driver MUST also set the VIRTIO_NET_HDR_F_NEEDS_CSUM bit in flags and MUST set gso_size to indicate the desired MSS.

    If one of the VIRTIO_NET_F_HOST_TSO4, TSO6 or UFO options have been negotiated, the driver SHOULD set hdr_len to a value not less than the length of the headers, including the transport header.

    The driver MUST NOT set the VIRTIO_NET_HDR_F_DATA_VALID and VIRTIO_NET_HDR_F_RSC_INFO bits in flags.

    5.1.6.2.2 Device Requirements: Packet Transmission
    The device MUST ignore flag bits that it does not recognize.

    If VIRTIO_NET_HDR_F_NEEDS_CSUM bit in flags is not set, the device MUST NOT use the csum_start and csum_offset.

    If one of the VIRTIO_NET_F_HOST_TSO4, TSO6 or UFO options have been negotiated, the device MAY use hdr_len only as a hint about the transport header size. The device MUST NOT rely on hdr_len to be correct. Note: This is due to various bugs in implementations.

    If VIRTIO_NET_HDR_F_NEEDS_CSUM is not set, the device MUST NOT rely on the packet checksum being correct.

    5.1.6.2.3 Packet Transmission Interrupt
    Often a driver will suppress transmission virtqueue interrupts and check for used packets in the transmit path of following packets.

    The normal behavior in this interrupt handler is to retrieve used buffers from the virtqueue and free the corresponding headers and packets.

    5.1.6.3 Setting Up Receive Buffers

    It is generally a good idea to keep the receive virtqueue as fully populated as possible: if it runs out, network performance will suffer.

    If the VIRTIO_NET_F_GUEST_TSO4, VIRTIO_NET_F_GUEST_TSO6 or VIRTIO_NET_F_GUEST_UFO features are used, the maximum incoming packet will be to 65550 bytes long (the maximum size of a TCP or UDP packet, plus the 14 byte ethernet header), otherwise 1514 bytes. The 12-byte struct virtio_net_hdr is prepended to this, making for 65562 or 1526 bytes.

    5.1.6.3.1 Driver Requirements: Setting Up Receive Buffers
    Note: Obviously each buffer can be split across multiple descriptor elements.

    If VIRTIO_NET_F_MQ is negotiated, each of receiveq1…receiveqN that will be used SHOULD be populated with receive buffers.

    5.1.6.3.2 Device Requirements: Setting Up Receive Buffers
    The device MUST set num_buffers to the number of descriptors used to hold the incoming packet.

    The device MUST use only a single descriptor if VIRTIO_NET_F_MRG_RXBUF was not negotiated. Note: This means that num_buffers will always be 1 if VIRTIO_NET_F_MRG_RXBUF is not negotiated.

    5.1.6.4 Processing of Incoming Packets

    When a packet is copied into a buffer in the receiveq, the optimal path is to disable further used buffer notifications for the receiveq and process packets until no more are found, then re-enable them.

    Processing incoming packets involves:

    1. num_buffers indicates how many descriptors this packet is spread over (including this one): this will always be 1 if VIRTIO_NET_F_MRG_RXBUF was not negotiated. This allows receipt of large packets without having to allocate large buffers: a packet that does not fit in a single buffer can flow over to the next buffer, and so on. In this case, there will be at least num_buffers used buffers in the virtqueue, and the device chains them together to form a single packet in a way similar to how it would store it in a single buffer spread over multiple descriptors. The other buffers will not begin with a struct virtio_net_hdr.
    2. If num_buffers is one, then the entire packet will be contained within this buffer, immediately following the struct virtio_net_hdr.
    3. If the VIRTIO_NET_F_GUEST_CSUM feature was negotiated, the VIRTIO_NET_HDR_F_DATA_VALID bit in flags can be set: if so, device has validated the packet checksum. In case of multiple encapsulated protocols, one level of checksums has been validated.

    Additionally, VIRTIO_NET_F_GUEST_CSUM, TSO4, TSO6, UDP and ECN features enable receive checksum, large receive offload and ECN support which are the input equivalents of the transmit checksum, transmit segmentation offloading and ECN features, as described in 5.1.6.2:

    1. If the VIRTIO_NET_F_GUEST_TSO4, TSO6 or UFO options were negotiated, then gso_type MAY be something other than VIRTIO_NET_HDR_GSO_NONE, and gso_size field indicates the desired MSS (see Packet Transmission point 2).
    2. If the VIRTIO_NET_F_RSC_EXT option was negotiated (this implies one of VIRTIO_NET_F_GUEST_TSO4, TSO6), the device processes also duplicated ACK segments, reports number of coalesced TCP segments in csum_start field and number of duplicated ACK segments in csum_offset field and sets bit VIRTIO_NET_HDR_F_RSC_INFO in flags.
    3. If the VIRTIO_NET_F_GUEST_CSUM feature was negotiated, the VIRTIO_NET_HDR_F_NEEDS_CSUM bit in flags can be set: if so, the packet checksum at offset csum_offset from csum_start and any preceding checksums have been validated. The checksum on the packet is incomplete and if bit VIRTIO_NET_HDR_F_RSC_INFO is not set in flags, then csum_start and csum_offset indicate how to calculate it (see Packet Transmission point 1).
    5.1.6.4.1 Device Requirements: Processing of Incoming Packets
    If VIRTIO_NET_F_MRG_RXBUF has not been negotiated, the device MUST set num_buffers to 1.

    If VIRTIO_NET_F_MRG_RXBUF has been negotiated, the device MUST set num_buffers to indicate the number of buffers the packet (including the header) is spread over.

    If a receive packet is spread over multiple buffers, the device MUST use all buffers but the last (i.e. the first numbuffers1 buffers) completely up to the full length of each buffer supplied by the driver.

    The device MUST use all buffers used by a single receive packet together, such that at least num_buffers are observed by driver as used.

    If VIRTIO_NET_F_GUEST_CSUM is not negotiated, the device MUST set flags to zero and SHOULD supply a fully checksummed packet to the driver.

    If VIRTIO_NET_F_GUEST_TSO4 is not negotiated, the device MUST NOT set gso_type to VIRTIO_NET_HDR_GSO_TCPV4.

    If VIRTIO_NET_F_GUEST_UDP is not negotiated, the device MUST NOT set gso_type to VIRTIO_NET_HDR_GSO_UDP.

    If VIRTIO_NET_F_GUEST_TSO6 is not negotiated, the device MUST NOT set gso_type to VIRTIO_NET_HDR_GSO_TCPV6.

    The device SHOULD NOT send to the driver TCP packets requiring segmentation offload which have the Explicit Congestion Notification bit set, unless the VIRTIO_NET_F_GUEST_ECN feature is negotiated, in which case the device MUST set the VIRTIO_NET_HDR_GSO_ECN bit in gso_type.

    If the VIRTIO_NET_F_GUEST_CSUM feature has been negotiated, the device MAY set the VIRTIO_NET_HDR_F_NEEDS_CSUM bit in flags, if so:

    1. the device MUST validate the packet checksum at offset csum_offset from csum_start as well as all preceding offsets;
    2. the device MUST set the packet checksum stored in the receive buffer to the TCP/UDP pseudo header;
    3. the device MUST set csum_start and csum_offset such that calculating a ones’ complement checksum from csum_start up until the end of the packet and storing the result at offset csum_offset from csum_start will result in a fully checksummed packet;

    If none of the VIRTIO_NET_F_GUEST_TSO4, TSO6 or UFO options have been negotiated, the device MUST set gso_type to VIRTIO_NET_HDR_GSO_NONE.

    If gso_type differs from VIRTIO_NET_HDR_GSO_NONE, then the device MUST also set the VIRTIO_NET_HDR_F_NEEDS_CSUM bit in flags MUST set gso_size to indicate the desired MSS. If VIRTIO_NET_F_RSC_EXT was negotiated, the device MUST also set VIRTIO_NET_HDR_F_RSC_INFO bit in flags, set csum_start to number of coalesced TCP segments and set csum_offset to number of received duplicated ACK segments.

    If VIRTIO_NET_F_RSC_EXT was not negotiated, the device MUST not set VIRTIO_NET_HDR_F_RSC_INFO bit in flags.

    If one of the VIRTIO_NET_F_GUEST_TSO4, TSO6 or UFO options have been negotiated, the device SHOULD set hdr_len to a value not less than the length of the headers, including the transport header.

    If the VIRTIO_NET_F_GUEST_CSUM feature has been negotiated, the device MAY set the VIRTIO_NET_HDR_F_DATA_VALID bit in flags, if so, the device MUST validate the packet checksum (in case of multiple encapsulated protocols, one level of checksums is validated).

    5.1.6.4.2 Driver Requirements: Processing of Incoming Packets
    The driver MUST ignore flag bits that it does not recognize.

    If VIRTIO_NET_HDR_F_NEEDS_CSUM bit in flags is not set or if VIRTIO_NET_HDR_F_RSC_INFO bit flags is set, the driver MUST NOT use the csum_start and csum_offset.

    If one of the VIRTIO_NET_F_GUEST_TSO4, TSO6 or UFO options have been negotiated, the driver MAY use hdr_len only as a hint about the transport header size. The driver MUST NOT rely on hdr_len to be correct. Note: This is due to various bugs in implementations.

    If neither VIRTIO_NET_HDR_F_NEEDS_CSUM nor VIRTIO_NET_HDR_F_DATA_VALID is set, the driver MUST NOT rely on the packet checksum being correct.

    5.1.6.5 Control Virtqueue

    The driver uses the control virtqueue (if VIRTIO_NET_F_CTRL_VQ is negotiated) to send commands to manipulate various features of the device which would not easily map into the configuration space.

    All commands are of the following form:

    struct virtio_net_ctrl { 
            u8 class; 
            u8 command; 
            u8 command-specific-data[]; 
            u8 ack; 
    }; 
     
    /* ack values */ 
    #define VIRTIO_NET_OK     0 
    #define VIRTIO_NET_ERR    1

    The class, command and command-specific-data are set by the driver, and the device sets the ack byte. There is little it can do except issue a diagnostic if ack is not VIRTIO_NET_OK.

    5.1.6.5.1 Packet Receive Filtering
    If the VIRTIO_NET_F_CTRL_RX and VIRTIO_NET_F_CTRL_RX_EXTRA features are negotiated, the driver can send control commands for promiscuous mode, multicast, unicast and broadcast receiving. Note: In general, these commands are best-effort: unwanted packets could still arrive.
    #define VIRTIO_NET_CTRL_RX    0 
     #define VIRTIO_NET_CTRL_RX_PROMISC      0 
     #define VIRTIO_NET_CTRL_RX_ALLMULTI     1 
     #define VIRTIO_NET_CTRL_RX_ALLUNI       2 
     #define VIRTIO_NET_CTRL_RX_NOMULTI      3 
     #define VIRTIO_NET_CTRL_RX_NOUNI        4 
     #define VIRTIO_NET_CTRL_RX_NOBCAST      5
    5.1.6.5.1.1 Device Requirements: Packet Receive Filtering
    If the VIRTIO_NET_F_CTRL_RX feature has been negotiated, the device MUST support the following VIRTIO_NET_CTRL_RX class commands:

    If the VIRTIO_NET_F_CTRL_RX_EXTRA feature has been negotiated, the device MUST support the following VIRTIO_NET_CTRL_RX class commands:

    5.1.6.5.1.2 Driver Requirements: Packet Receive Filtering
    If the VIRTIO_NET_F_CTRL_RX feature has not been negotiated, the driver MUST NOT issue commands VIRTIO_NET_CTRL_RX_PROMISC or VIRTIO_NET_CTRL_RX_ALLMULTI.

    If the VIRTIO_NET_F_CTRL_RX_EXTRA feature has not been negotiated, the driver MUST NOT issue commands VIRTIO_NET_CTRL_RX_ALLUNI, VIRTIO_NET_CTRL_RX_NOMULTI, VIRTIO_NET_CTRL_RX_NOUNI or VIRTIO_NET_CTRL_RX_NOBCAST.

    5.1.6.5.2 Setting MAC Address Filtering
    If the VIRTIO_NET_F_CTRL_RX feature is negotiated, the driver can send control commands for MAC address filtering.
    struct virtio_net_ctrl_mac { 
            le32 entries; 
            u8 macs[entries][6]; 
    }; 
     
    #define VIRTIO_NET_CTRL_MAC    1 
     #define VIRTIO_NET_CTRL_MAC_TABLE_SET        0 
     #define VIRTIO_NET_CTRL_MAC_ADDR_SET         1

    The device can filter incoming packets by any number of destination MAC addresses10. This table is set using the class VIRTIO_NET_CTRL_MAC and the command VIRTIO_NET_CTRL_MAC_TABLE_SET. The command-specific-data is two variable length tables of 6-byte MAC addresses (as described in struct virtio_net_ctrl_mac). The first table contains unicast addresses, and the second contains multicast addresses.

    The VIRTIO_NET_CTRL_MAC_ADDR_SET command is used to set the default MAC address which rx filtering accepts (and if VIRTIO_NET_F_MAC_ADDR has been negotiated, this will be reflected in mac in config space).

    The command-specific-data for VIRTIO_NET_CTRL_MAC_ADDR_SET is the 6-byte MAC address.

    5.1.6.5.2.1 Device Requirements: Setting MAC Address Filtering
    The device MUST have an empty MAC filtering table on reset.

    The device MUST update the MAC filtering table before it consumes the VIRTIO_NET_CTRL_MAC_TABLE_SET command.

    The device MUST update mac in config space before it consumes the VIRTIO_NET_CTRL_MAC_ADDR_SET command, if VIRTIO_NET_F_MAC_ADDR has been negotiated.

    The device SHOULD drop incoming packets which have a destination MAC which matches neither the mac (or that set with VIRTIO_NET_CTRL_MAC_ADDR_SET) nor the MAC filtering table.

    5.1.6.5.2.2 Driver Requirements: Setting MAC Address Filtering
    If VIRTIO_NET_F_CTRL_RX has not been negotiated, the driver MUST NOT issue VIRTIO_NET_CTRL_MAC class commands.

    If VIRTIO_NET_F_CTRL_RX has been negotiated, the driver SHOULD issue VIRTIO_NET_CTRL_MAC_ADDR_SET to set the default mac if it is different from mac.

    The driver MUST follow the VIRTIO_NET_CTRL_MAC_TABLE_SET command by a le32 number, followed by that number of non-multicast MAC addresses, followed by another le32 number, followed by that number of multicast addresses. Either number MAY be 0.

    5.1.6.5.2.3 Legacy Interface: Setting MAC Address Filtering
    When using the legacy interface, transitional devices and drivers MUST format entries in struct virtio_net_ctrl_mac according to the native endian of the guest rather than (necessarily when not using the legacy interface) little-endian.

    Legacy drivers that didn’t negotiate VIRTIO_NET_F_CTRL_MAC_ADDR changed mac in config space when NIC is accepting incoming packets. These drivers always wrote the mac value from first to last byte, therefore after detecting such drivers, a transitional device MAY defer MAC update, or MAY defer processing incoming packets until driver writes the last byte of mac in the config space.

    5.1.6.5.3 VLAN Filtering
    If the driver negotiates the VIRTION_NET_F_CTRL_VLAN feature, it can control a VLAN filter table in the device.
    #define VIRTIO_NET_CTRL_VLAN       2 
     #define VIRTIO_NET_CTRL_VLAN_ADD             0 
     #define VIRTIO_NET_CTRL_VLAN_DEL             1

    Both the VIRTIO_NET_CTRL_VLAN_ADD and VIRTIO_NET_CTRL_VLAN_DEL command take a little-endian 16-bit VLAN id as the command-specific-data.

    5.1.6.5.3.1 Legacy Interface: VLAN Filtering
    When using the legacy interface, transitional devices and drivers MUST format the VLAN id according to the native endian of the guest rather than (necessarily when not using the legacy interface) little-endian.
    5.1.6.5.4 Gratuitous Packet Sending
    If the driver negotiates the VIRTIO_NET_F_GUEST_ANNOUNCE (depends on VIRTIO_NET_F_CTRL_VQ), the device can ask the driver to send gratuitous packets; this is usually done after the guest has been physically migrated, and needs to announce its presence on the new network links. (As hypervisor does not have the knowledge of guest network configuration (eg. tagged vlan) it is simplest to prod the guest in this way).
    #define VIRTIO_NET_CTRL_ANNOUNCE       3 
     #define VIRTIO_NET_CTRL_ANNOUNCE_ACK             0

    The driver checks VIRTIO_NET_S_ANNOUNCE bit in the device configuration status field when it notices the changes of device configuration. The command VIRTIO_NET_CTRL_ANNOUNCE_ACK is used to indicate that driver has received the notification and device clears the VIRTIO_NET_S_ANNOUNCE bit in status.

    Processing this notification involves:

    1. Sending the gratuitous packets (eg. ARP) or marking there are pending gratuitous packets to be sent and letting deferred routine to send them.
    2. Sending VIRTIO_NET_CTRL_ANNOUNCE_ACK command through control vq.
    5.1.6.5.4.1 Driver Requirements: Gratuitous Packet Sending
    If the driver negotiates VIRTIO_NET_F_GUEST_ANNOUNCE, it SHOULD notify network peers of its new location after it sees the VIRTIO_NET_S_ANNOUNCE bit in status. The driver MUST send a command on the command queue with class VIRTIO_NET_CTRL_ANNOUNCE and command VIRTIO_NET_CTRL_ANNOUNCE_ACK.
    5.1.6.5.4.2 Device Requirements: Gratuitous Packet Sending
    If VIRTIO_NET_F_GUEST_ANNOUNCE is negotiated, the device MUST clear the VIRTIO_NET_S_ANNOUNCE bit in status upon receipt of a command buffer with class VIRTIO_NET_CTRL_ANNOUNCE and command VIRTIO_NET_CTRL_ANNOUNCE_ACK before marking the buffer as used.
    5.1.6.5.5 Automatic receive steering in multiqueue mode
    If the driver negotiates the VIRTIO_NET_F_MQ feature bit (depends on VIRTIO_NET_F_CTRL_VQ), it MAY transmit outgoing packets on one of the multiple transmitq1…transmitqN and ask the device to queue incoming packets into one of the multiple receiveq1…receiveqN depending on the packet flow.
    struct virtio_net_ctrl_mq { 
            le16 virtqueue_pairs; 
    }; 
     
    #define VIRTIO_NET_CTRL_MQ    4 
     #define VIRTIO_NET_CTRL_MQ_VQ_PAIRS_SET        0 
     #define VIRTIO_NET_CTRL_MQ_VQ_PAIRS_MIN        1 
     #define VIRTIO_NET_CTRL_MQ_VQ_PAIRS_MAX        0x8000

    Multiqueue is disabled by default. The driver enables multiqueue by executing the VIRTIO_NET_CTRL_MQ_VQ_PAIRS_SET command, specifying the number of the transmit and receive queues to be used up to max_virtqueue_pairs; subsequently, transmitq1…transmitqn and receiveq1…receiveqn where n=virtqueue_pairs MAY be used.

    When multiqueue is enabled, the device MUST use automatic receive steering based on packet flow. Programming of the receive steering classificator is implicit. After the driver transmitted a packet of a flow on transmitqX, the device SHOULD cause incoming packets for that flow to be steered to receiveqX. For uni-directional protocols, or where no packets have been transmitted yet, the device MAY steer a packet to a random queue out of the specified receiveq1…receiveqn.

    Multiqueue is disabled by setting virtqueue_pairs to 1 (this is the default) and waiting for the device to use the command buffer.

    5.1.6.5.5.1 Driver Requirements: Automatic receive steering in multiqueue mode
    The driver MUST configure the virtqueues before enabling them with the VIRTIO_NET_CTRL_MQ_VQ_PAIRS_SET command.

    The driver MUST NOT request a virtqueue_pairs of 0 or greater than max_virtqueue_pairs in the device configuration space.

    The driver MUST queue packets only on any transmitq1 before the VIRTIO_NET_CTRL_MQ_VQ_PAIRS_SET command.

    The driver MUST NOT queue packets on transmit queues greater than virtqueue_pairs once it has placed the VIRTIO_NET_CTRL_MQ_VQ_PAIRS_SET command in the available ring.

    5.1.6.5.5.2 Device Requirements: Automatic receive steering in multiqueue mode
    The device MUST queue packets only on any receiveq1 before the VIRTIO_NET_CTRL_MQ_VQ_PAIRS_SET command.

    The device MUST NOT queue packets on receive queues greater than virtqueue_pairs once it has placed the VIRTIO_NET_CTRL_MQ_VQ_PAIRS_SET command in a used buffer.

    5.1.6.5.5.3 Legacy Interface: Automatic receive steering in multiqueue mode
    When using the legacy interface, transitional devices and drivers MUST format virtqueue_pairs according to the native endian of the guest rather than (necessarily when not using the legacy interface) little-endian.
    5.1.6.5.6 Offloads State Configuration
    If the VIRTIO_NET_F_CTRL_GUEST_OFFLOADS feature is negotiated, the driver can send control commands for dynamic offloads state configuration.
    5.1.6.5.6.1 Setting Offloads State
    To configure the offloads, the following layout structure and definitions are used:
    le64 offloads; 
     
    #define VIRTIO_NET_F_GUEST_CSUM       1 
    #define VIRTIO_NET_F_GUEST_TSO4       7 
    #define VIRTIO_NET_F_GUEST_TSO6       8 
    #define VIRTIO_NET_F_GUEST_ECN        9 
    #define VIRTIO_NET_F_GUEST_UFO        10 
     
    #define VIRTIO_NET_CTRL_GUEST_OFFLOADS       5 
     #define VIRTIO_NET_CTRL_GUEST_OFFLOADS_SET   0

    The class VIRTIO_NET_CTRL_GUEST_OFFLOADS has one command: VIRTIO_NET_CTRL_GUEST_OFFLOADS_SET applies the new offloads configuration.

    le64 value passed as command data is a bitmask, bits set define offloads to be enabled, bits cleared - offloads to be disabled.

    There is a corresponding device feature for each offload. Upon feature negotiation corresponding offload gets enabled to preserve backward compartibility.

    5.1.6.5.6.2 Driver Requirements: Setting Offloads State
    A driver MUST NOT enable an offload for which the appropriate feature has not been negotiated.
    5.1.6.5.6.3 Legacy Interface: Setting Offloads State
    When using the legacy interface, transitional devices and drivers MUST format offloads according to the native endian of the guest rather than (necessarily when not using the legacy interface) little-endian.
    5.1.6.6 Legacy Interface: Framing Requirements

    When using legacy interfaces, transitional drivers which have not negotiated VIRTIO_F_ANY_LAYOUT MUST use a single descriptor for the struct virtio_net_hdr on both transmit and receive, with the network data in the following descriptors.

    Additionally, when using the control virtqueue (see 5.1.6.5) , transitional drivers which have not negotiated VIRTIO_F_ANY_LAYOUT MUST:

    See 2.6.4.

    5.2 Block Device

    The virtio block device is a simple virtual block device (ie. disk). Read and write requests (and other exotic requests) are placed in the queue, and serviced (probably out of order) by the device except where noted.

    5.2.1 Device ID

    2

    5.2.2 Virtqueues

    0
    requestq

    5.2.3 Feature bits

    VIRTIO_BLK_F_SIZE_MAX (1)
    Maximum size of any single segment is in size_max.
    VIRTIO_BLK_F_SEG_MAX (2)
    Maximum number of segments in a request is in seg_max.
    VIRTIO_BLK_F_GEOMETRY (4)
    Disk-style geometry specified in geometry.
    VIRTIO_BLK_F_RO (5)
    Device is read-only.
    VIRTIO_BLK_F_BLK_SIZE (6)
    Block size of disk is in blk_size.
    VIRTIO_BLK_F_FLUSH (9)
    Cache flush command support.
    VIRTIO_BLK_F_TOPOLOGY (10)
    Device exports information on optimal I/O alignment.
    VIRTIO_BLK_F_CONFIG_WCE (11)
    Device can toggle its cache between writeback and writethrough modes.
    VIRTIO_BLK_F_DISCARD (13)
    Device can support discard command, maximum discard sectors size in max_discard_sectors and maximum discard segment number in max_discard_seg.
    VIRTIO_BLK_F_WRITE_ZEROES (14)
    Device can support write zeroes command, maximum write zeroes sectors size in max_write_zeroes_sectors and maximum write zeroes segment number in max_write_zeroes_seg.
    5.2.3.1 Legacy Interface: Feature bits
    VIRTIO_BLK_F_BARRIER (0)
    Device supports request barriers.
    VIRTIO_BLK_F_SCSI (7)
    Device supports scsi packet commands.
    Note: In the legacy interface, VIRTIO_BLK_F_FLUSH was also called VIRTIO_BLK_F_WCE.

    5.2.4 Device configuration layout

    The capacity of the device (expressed in 512-byte sectors) is always present. The availability of the others all depend on various feature bits as indicated above.

    The parameters in the configuration space of the device max_discard_sectors discard_sector_alignment are expressed in 512-byte units if the VIRTIO_BLK_F_DISCARD feature bit is negotiated. The max_write_zeroes_sectors is expressed in 512-byte units if the VIRTIO_BLK_F_WRITE_ZEROES feature bit is negotiated.

    struct virtio_blk_config { 
            le64 capacity; 
            le32 size_max; 
            le32 seg_max; 
            struct virtio_blk_geometry { 
                    le16 cylinders; 
                    u8 heads; 
                    u8 sectors; 
            } geometry; 
            le32 blk_size; 
            struct virtio_blk_topology { 
                    // # of logical blocks per physical block (log2) 
                    u8 physical_block_exp; 
                    // offset of first aligned logical block 
                    u8 alignment_offset; 
                    // suggested minimum I/O size in blocks 
                    le16 min_io_size; 
                    // optimal (suggested maximum) I/O size in blocks 
                    le32 opt_io_size; 
            } topology; 
            u8 writeback; 
            u8 unused0[3]; 
            le32 max_discard_sectors; 
            le32 max_discard_seg; 
            le32 discard_sector_alignment; 
            le32 max_write_zeroes_sectors; 
            le32 max_write_zeroes_seg; 
            u8 write_zeroes_may_unmap; 
            u8 unused1[3]; 
    };
    5.2.4.1 Legacy Interface: Device configuration layout

    When using the legacy interface, transitional devices and drivers MUST format the fields in struct virtio_blk_config according to the native endian of the guest rather than (necessarily when not using the legacy interface) little-endian.

    5.2.5 Device Initialization

    1. The device size can be read from capacity.
    2. If the VIRTIO_BLK_F_BLK_SIZE feature is negotiated, blk_size can be read to determine the optimal sector size for the driver to use. This does not affect the units used in the protocol (always 512 bytes), but awareness of the correct value can affect performance.
    3. If the VIRTIO_BLK_F_RO feature is set by the device, any write requests will fail.
    4. If the VIRTIO_BLK_F_TOPOLOGY feature is negotiated, the fields in the topology struct can be read to determine the physical block size and optimal I/O lengths for the driver to use. This also does not affect the units in the protocol, only performance.
    5. If the VIRTIO_BLK_F_CONFIG_WCE feature is negotiated, the cache mode can be read or set through the writeback field. 0 corresponds to a writethrough cache, 1 to a writeback cache11. The cache mode after reset can be either writeback or writethrough. The actual mode can be determined by reading writeback after feature negotiation.
    6. If the VIRTIO_BLK_F_DISCARD feature is negotiated, max_discard_sectors and max_discard_seg can be read to determine the maximum discard sectors and maximum number of discard segments for the block driver to use. discard_sector_alignment can be used by OS when splitting a request based on alignment.
    7. if the VIRTIO_BLK_F_WRITE_ZEROES feature is negotiated, max_write_zeroes_sectors and max_write_zeroes_seg can be read to determine the maximum write zeroes sectors and maximum number of write zeroes segments for the block driver to use.
    5.2.5.1 Driver Requirements: Device Initialization

    Drivers SHOULD NOT negotiate VIRTIO_BLK_F_FLUSH if they are incapable of sending VIRTIO_BLK_T_FLUSH commands.

    If neither VIRTIO_BLK_F_CONFIG_WCE nor VIRTIO_BLK_F_FLUSH are negotiated, the driver MAY deduce the presence of a writethrough cache. If VIRTIO_BLK_F_CONFIG_WCE was not negotiated but VIRTIO_BLK_F_FLUSH was, the driver SHOULD assume presence of a writeback cache.

    The driver MUST NOT read writeback before setting the FEATURES_OK device status bit.

    5.2.5.2 Device Requirements: Device Initialization

    Devices SHOULD always offer VIRTIO_BLK_F_FLUSH, and MUST offer it if they offer VIRTIO_BLK_F_CONFIG_WCE.

    If VIRTIO_BLK_F_CONFIG_WCE is negotiated but VIRTIO_BLK_F_FLUSH is not, the device MUST initialize writeback to 0.

    The device MUST initialize padding bytes unused0 and unused1 to 0.

    5.2.5.3 Legacy Interface: Device Initialization

    Because legacy devices do not have FEATURES_OK, transitional devices MUST implement slightly different behavior around feature negotiation when used through the legacy interface. In particular, when using the legacy interface:

    5.2.6 Device Operation

    The driver queues requests to the virtqueue, and they are used by the device (not necessarily in order). Each request is of form:

    struct virtio_blk_req { 
            le32 type; 
            le32 reserved; 
            le64 sector; 
            u8 data[]; 
            u8 status; 
    };

    The type of the request is either a read (VIRTIO_BLK_T_IN), a write (VIRTIO_BLK_T_OUT), a discard (VIRTIO_BLK_T_DISCARD), a write zeroes (VIRTIO_BLK_T_WRITE_ZEROES) or a flush (VIRTIO_BLK_T_FLUSH).

    #define VIRTIO_BLK_T_IN           0 
    #define VIRTIO_BLK_T_OUT          1 
    #define VIRTIO_BLK_T_FLUSH        4 
    #define VIRTIO_BLK_T_DISCARD      11 
    #define VIRTIO_BLK_T_WRITE_ZEROES 13

    The sector number indicates the offset (multiplied by 512) where the read or write is to occur. This field is unused and set to 0 for commands other than read or write.

    VIRTIO_BLK_T_IN requests populate data with the contents of sectors read from the block device (in multiples of 512 bytes). VIRTIO_BLK_T_OUT requests write the contents of data to the block device (in multiples of 512 bytes).

    The data used for discard or write zeroes commands consists of one or more segments. The maximum number of segments is max_discard_seg for discard commands and max_write_zeroes_seg for write zeroes commands. Each segment is of form:

    struct virtio_blk_discard_write_zeroes { 
           le64 sector; 
           le32 num_sectors; 
           struct { 
                   le32 unmap:1; 
                   le32 reserved:31; 
           } flags; 
    };

    sector indicates the starting offset (in 512-byte units) of the segment, while num_sectors indicates the number of sectors in each discarded range. unmap is only used in write zeroes commands and allows the device to discard the specified range, provided that following reads return zeroes.

    The final status byte is written by the device: either VIRTIO_BLK_S_OK for success, VIRTIO_BLK_S_IOERR for device or driver error or VIRTIO_BLK_S_UNSUPP for a request unsupported by device:

    #define VIRTIO_BLK_S_OK        0 
    #define VIRTIO_BLK_S_IOERR     1 
    #define VIRTIO_BLK_S_UNSUPP    2

    The status of individual segments is indeterminate when a discard or write zero command produces VIRTIO_BLK_S_IOERR. A segment may have completed successfully, failed, or not been processed by the device.

    5.2.6.1 Driver Requirements: Device Operation

    A driver MUST NOT submit a request which would cause a read or write beyond capacity.

    A driver SHOULD accept the VIRTIO_BLK_F_RO feature if offered.

    A driver MUST set sector to 0 for a VIRTIO_BLK_T_FLUSH request. A driver SHOULD NOT include any data in a VIRTIO_BLK_T_FLUSH request.

    The length of data MUST be a multiple of 512 bytes for VIRTIO_BLK_T_IN and VIRTIO_BLK_T_OUT requests.

    The length of data MUST be a multiple of the size of struct virtio_blk_discard_write_zeroes for VIRTIO_BLK_T_DISCARD and VIRTIO_BLK_T_WRITE_ZEROES requests.

    VIRTIO_BLK_T_DISCARD requests MUST NOT contain more than max_discard_seg struct virtio_blk_discard_write_zeroes segments in data.

    VIRTIO_BLK_T_WRITE_ZEROES requests MUST NOT contain more than max_write_zeroes_seg struct virtio_blk_discard_write_zeroes segments in data.

    If the VIRTIO_BLK_F_CONFIG_WCE feature is negotiated, the driver MAY switch to writethrough or writeback mode by writing respectively 0 and 1 to the writeback field. After writing a 0 to writeback, the driver MUST NOT assume that any volatile writes have been committed to persistent device backend storage.

    The unmap bit MUST be zero for discard commands. The driver MUST NOT assume anything about the data returned by read requests after a range of sectors has been discarded.

    A driver MUST NOT assume that individual segments in a multi-segment VIRTIO_BLK_T_DISCARD or VIRTIO_BLK_T_WRITE_ZEROES request completed successfully, failed, or were processed by the device at all if the request failed with VIRTIO_BLK_S_IOERR.

    5.2.6.2 Device Requirements: Device Operation

    A device MUST set the status byte to VIRTIO_BLK_S_IOERR for a write request if the VIRTIO_BLK_F_RO feature if offered, and MUST NOT write any data.

    The device MUST set the status byte to VIRTIO_BLK_S_UNSUPP for discard and write zeroes commands if any unknown flag is set. Furthermore, the device MUST set the status byte to VIRTIO_BLK_S_UNSUPP for discard commands if the unmap flag is set.

    For discard commands, the device MAY deallocate the specified range of sectors in the device backend storage.

    For write zeroes commands, if the unmap is set, the device MAY deallocate the specified range of sectors in the device backend storage, as if the discard command had been sent. After a write zeroes command is completed, reads of the specified ranges of sectors MUST return zeroes. This is true independent of whether unmap was set or clear.

    The device SHOULD clear the write_zeroes_may_unmap field of the virtio configuration space if and only if a write zeroes request cannot result in deallocating one or more sectors. The device MAY change the content of the field during operation of the device; when this happens, the device SHOULD trigger a configuration change notification.

    A write is considered volatile when it is submitted; the contents of sectors covered by a volatile write are undefined in persistent device backend storage until the write becomes stable. A write becomes stable once it is completed and one or more of the following conditions is true:

    1. neither VIRTIO_BLK_F_CONFIG_WCE nor VIRTIO_BLK_F_FLUSH feature were negotiated, but VIRTIO_BLK_F_FLUSH was offered by the device;
    2. the VIRTIO_BLK_F_CONFIG_WCE feature was negotiated and the writeback field in configuration space was 0 all the time between the submission of the write and its completion;
    3. a VIRTIO_BLK_T_FLUSH request is sent after the write is completed and is completed itself.

    If the device is backed by persistent storage, the device MUST ensure that stable writes are committed to it, before reporting completion of the write (cases 1 and 2) or the flush (case 3). Failure to do so can cause data loss in case of a crash.

    If the driver changes writeback between the submission of the write and its completion, the write could be either volatile or stable when its completion is reported; in other words, the exact behavior is undefined.

    If VIRTIO_BLK_F_FLUSH was not offered by the device12, the device MAY also commit writes to persistent device backend storage before reporting their completion. Unlike case 1, however, this is not an absolute requirement of the specification. Note: An implementation that does not offer VIRTIO_BLK_F_FLUSH and does not commit completed writes will not be resilient to data loss in case of crashes. Not offering VIRTIO_BLK_F_FLUSH is an absolute requirement for implementations that do not wish to be safe against such data losses.

    5.2.6.3 Legacy Interface: Device Operation

    When using the legacy interface, transitional devices and drivers MUST format the fields in struct virtio_blk_req according to the native endian of the guest rather than (necessarily when not using the legacy interface) little-endian.

    When using the legacy interface, transitional drivers SHOULD ignore the used length values. Note: Historically, some devices put the total descriptor length, or the total length of device-writable buffers there, even when only the status byte was actually written.

    The reserved field was previously called ioprio. ioprio is a hint about the relative priorities of requests to the device: higher numbers indicate more important requests.

    #define VIRTIO_BLK_T_FLUSH_OUT    5

    The command VIRTIO_BLK_T_FLUSH_OUT was a synonym for VIRTIO_BLK_T_FLUSH; a driver MUST treat it as a VIRTIO_BLK_T_FLUSH command.

    #define VIRTIO_BLK_T_BARRIER     0x80000000

    If the device has VIRTIO_BLK_F_BARRIER feature the high bit (VIRTIO_BLK_T_BARRIER) indicates that this request acts as a barrier and that all preceding requests SHOULD be complete before this one, and all following requests SHOULD NOT be started until this is complete. Note: A barrier does not flush caches in the underlying backend device in host, and thus does not serve as data consistency guarantee. Only a VIRTIO_BLK_T_FLUSH request does that.

    Some older legacy devices did not commit completed writes to persistent device backend storage when VIRTIO_BLK_F_FLUSH was offered but not negotiated. In order to work around this, the driver MAY set the writeback to 0 (if available) or it MAY send an explicit flush request after every completed write.

    If the device has VIRTIO_BLK_F_SCSI feature, it can also support scsi packet command requests, each of these requests is of form:

    /* All fields are in guests native endian. */ 
    struct virtio_scsi_pc_req { 
            u32 type; 
            u32 ioprio; 
            u64 sector; 
            u8 cmd[]; 
            u8 data[][512]; 
    #define SCSI_SENSE_BUFFERSIZE   96 
            u8 sense[SCSI_SENSE_BUFFERSIZE]; 
            u32 errors; 
            u32 data_len; 
            u32 sense_len; 
            u32 residual; 
            u8 status; 
    };

    A request type can also be a scsi packet command (VIRTIO_BLK_T_SCSI_CMD or VIRTIO_BLK_T_SCSI_CMD_OUT). The two types are equivalent, the device does not distinguish between them:

    #define VIRTIO_BLK_T_SCSI_CMD     2 
    #define VIRTIO_BLK_T_SCSI_CMD_OUT 3

    The cmd field is only present for scsi packet command requests, and indicates the command to perform. This field MUST reside in a single, separate device-readable buffer; command length can be derived from the length of this buffer.

    Note that these first three (four for scsi packet commands) fields are always device-readable: data is either device-readable or device-writable, depending on the request. The size of the read or write can be derived from the total size of the request buffers.

    sense is only present for scsi packet command requests, and indicates the buffer for scsi sense data.

    data_len is only present for scsi packet command requests, this field is deprecated, and SHOULD be ignored by the driver. Historically, devices copied data length there.

    sense_len is only present for scsi packet command requests and indicates the number of bytes actually written to the sense buffer.

    residual field is only present for scsi packet command requests and indicates the residual size, calculated as data length - number of bytes actually transferred.

    5.2.6.4 Legacy Interface: Framing Requirements

    When using legacy interfaces, transitional drivers which have not negotiated VIRTIO_F_ANY_LAYOUT:

    See 2.6.4.

    5.3 Console Device

    The virtio console device is a simple device for data input and output. A device MAY have one or more ports. Each port has a pair of input and output virtqueues. Moreover, a device has a pair of control IO virtqueues. The control virtqueues are used to communicate information between the device and the driver about ports being opened and closed on either side of the connection, indication from the device about whether a particular port is a console port, adding new ports, port hot-plug/unplug, etc., and indication from the driver about whether a port or a device was successfully added, port open/close, etc. For data IO, one or more empty buffers are placed in the receive queue for incoming data and outgoing characters are placed in the transmit queue.

    5.3.1 Device ID

    3

    5.3.2 Virtqueues

    0
    receiveq(port0)
    1
    transmitq(port0)
    2
    control receiveq
    3
    control transmitq
    4
    receiveq(port1)
    5
    transmitq(port1)

    The port 0 receive and transmit queues always exist: other queues only exist if VIRTIO_CONSOLE_F_MULTIPORT is set.

    5.3.3 Feature bits

    VIRTIO_CONSOLE_F_SIZE (0)
    Configuration cols and rows are valid.
    VIRTIO_CONSOLE_F_MULTIPORT (1)
    Device has support for multiple ports; max_nr_ports is valid and control virtqueues will be used.
    VIRTIO_CONSOLE_F_EMERG_WRITE (2)
    Device has support for emergency write. Configuration field emerg_wr is valid.

    5.3.4 Device configuration layout

    The size of the console is supplied in the configuration space if the VIRTIO_CONSOLE_F_SIZE feature is set. Furthermore, if the VIRTIO_CONSOLE_F_MULTIPORT feature is set, the maximum number of ports supported by the device can be fetched.

    If VIRTIO_CONSOLE_F_EMERG_WRITE is set then the driver can use emergency write to output a single character without initializing virtio queues, or even acknowledging the feature.

    struct virtio_console_config { 
            le16 cols; 
            le16 rows; 
            le32 max_nr_ports; 
            le32 emerg_wr; 
    };
    5.3.4.1 Legacy Interface: Device configuration layout

    When using the legacy interface, transitional devices and drivers MUST format the fields in struct virtio_console_config according to the native endian of the guest rather than (necessarily when not using the legacy interface) little-endian.

    5.3.5 Device Initialization

    1. If the VIRTIO_CONSOLE_F_EMERG_WRITE feature is offered, emerg_wr field of the configuration can be written at any time. Thus it works for very early boot debugging output as well as catastophic OS failures (eg. virtio ring corruption).
    2. If the VIRTIO_CONSOLE_F_SIZE feature is negotiated, the driver can read the console dimensions from cols and rows.
    3. If the VIRTIO_CONSOLE_F_MULTIPORT feature is negotiated, the driver can spawn multiple ports, not all of which are necessarily attached to a console. Some could be generic ports. In this case, the control virtqueues are enabled and according to max_nr_ports, the appropriate number of virtqueues are created. A control message indicating the driver is ready is sent to the device. The device can then send control messages for adding new ports to the device. After creating and initializing each port, a VIRTIO_CONSOLE_PORT_READY control message is sent to the device for that port so the device can let the driver know of any additional configuration options set for that port.
    4. The receiveq for each port is populated with one or more receive buffers.
    5.3.5.1 Device Requirements: Device Initialization

    The device MUST allow a write to emerg_wr, even on an unconfigured device.

    The device SHOULD transmit the lower byte written to emerg_wr to an appropriate log or output method.

    5.3.6 Device Operation

    1. For output, a buffer containing the characters is placed in the port’s transmitq13.
    2. When a buffer is used in the receiveq (signalled by a used buffer notification), the contents is the input to the port associated with the virtqueue for which the notification was received.
    3. If the driver negotiated the VIRTIO_CONSOLE_F_SIZE feature, a configuration change notification indicates that the updated size can be read from the configuration fields. This size applies to port 0 only.
    4. If the driver negotiated the VIRTIO_CONSOLE_F_MULTIPORT feature, active ports are announced by the device using the VIRTIO_CONSOLE_PORT_ADD control message. The same message is used for port hot-plug as well.
    5.3.6.1 Driver Requirements: Device Operation

    The driver MUST NOT put a device-readable in a receiveq. The driver MUST NOT put a device-writable buffer in a transmitq.

    5.3.6.2 Multiport Device Operation

    If the driver negotiated the VIRTIO_CONSOLE_F_MULTIPORT, the two control queues are used to manipulate the different console ports: the control receiveq for messages from the device to the driver, and the control sendq for driver-to-device messages. The layout of the control messages is:

    struct virtio_console_control { 
            le32 id;    /* Port number */ 
            le16 event; /* The kind of control event */ 
            le16 value; /* Extra information for the event */ 
    };

    The values for event are:

    VIRTIO_CONSOLE_DEVICE_READY (0)
    Sent by the driver at initialization to indicate that it is ready to receive control messages. A value of 1 indicates success, and 0 indicates failure. The port number id is unused.
    VIRTIO_CONSOLE_DEVICE_ADD (1)
    Sent by the device, to create a new port. value is unused.
    VIRTIO_CONSOLE_DEVICE_REMOVE (2)
    Sent by the device, to remove an existing port. value is unused.
    VIRTIO_CONSOLE_PORT_READY (3)
    Sent by the driver in response to the device’s VIRTIO_CONSOLE_PORT_ADD message, to indicate that the port is ready to be used. A value of 1 indicates success, and 0 indicates failure.
    VIRTIO_CONSOLE_CONSOLE_PORT (4)
    Sent by the device to nominate a port as a console port. There MAY be more than one console port.
    VIRTIO_CONSOLE_RESIZE (5)
    Sent by the device to indicate a console size change. value is unused. The buffer is followed by the number of columns and rows:
    struct virtio_console_resize { 
            le16 cols; 
            le16 rows; 
    };
    VIRTIO_CONSOLE_PORT_OPEN (6)
    This message is sent by both the device and the driver. value indicates the state: 0 (port closed) or 1 (port open). This allows for ports to be used directly by guest and host processes to communicate in an application-defined manner.
    VIRTIO_CONSOLE_PORT_NAME (7)
    Sent by the device to give a tag to the port. This control command is immediately followed by the UTF-8 name of the port for identification within the guest (without a NUL terminator).
    5.3.6.2.1 Device Requirements: Multiport Device Operation
    The device MUST NOT specify a port which exists in a VIRTIO_CONSOLE_DEVICE_ADD message, nor a port which is equal or greater than max_nr_ports.

    The device MUST NOT specify a port in VIRTIO_CONSOLE_DEVICE_REMOVE which has not been created with a previous VIRTIO_CONSOLE_DEVICE_ADD.

    5.3.6.2.2 Driver Requirements: Multiport Device Operation
    The driver MUST send a VIRTIO_CONSOLE_DEVICE_READY message if VIRTIO_CONSOLE_F_MULTIPORT is negotiated.

    Upon receipt of a VIRTIO_CONSOLE_CONSOLE_PORT message, the driver SHOULD treat the port in a manner suitable for text console access and MUST respond with a VIRTIO_CONSOLE_PORT_OPEN message, which MUST have value set to 1.

    5.3.6.3 Legacy Interface: Device Operation

    When using the legacy interface, transitional devices and drivers MUST format the fields in struct virtio_console_control according to the native endian of the guest rather than (necessarily when not using the legacy interface) little-endian.

    When using the legacy interface, the driver SHOULD ignore the used length values for the transmit queues and the control transmitq. Note: Historically, some devices put the total descriptor length there, even though no data was actually written.

    5.3.6.4 Legacy Interface: Framing Requirements

    When using legacy interfaces, transitional drivers which have not negotiated VIRTIO_F_ANY_LAYOUT MUST use only a single descriptor for all buffers in the control receiveq and control transmitq.

    5.4 Entropy Device

    The virtio entropy device supplies high-quality randomness for guest use.

    5.4.1 Device ID

    4

    5.4.2 Virtqueues

    0
    requestq

    5.4.3 Feature bits

    None currently defined

    5.4.4 Device configuration layout

    None currently defined.

    5.4.5 Device Initialization

    1. The virtqueue is initialized

    5.4.6 Device Operation

    When the driver requires random bytes, it places the descriptor of one or more buffers in the queue. It will be completely filled by random data by the device.

    5.4.6.1 Driver Requirements: Device Operation

    The driver MUST NOT place driver-readable buffers into the queue.

    The driver MUST examine the length written by the device to determine how many random bytes were received.

    5.4.6.2 Device Requirements: Device Operation

    The device MUST place one or more random bytes into the buffer, but it MAY use less than the entire buffer length.

    5.5 Traditional Memory Balloon Device

    This is the traditional balloon device. The device number 13 is reserved for a new memory balloon interface, with different semantics, which is expected in a future version of the standard.

    The traditional virtio memory balloon device is a primitive device for managing guest memory: the device asks for a certain amount of memory, and the driver supplies it (or withdraws it, if the device has more than it asks for). This allows the guest to adapt to changes in allowance of underlying physical memory. If the feature is negotiated, the device can also be used to communicate guest memory statistics to the host.

    5.5.1 Device ID

    5

    5.5.2 Virtqueues

    0
    inflateq
    1
    deflateq
    2
    statsq.

    Virtqueue 2 only exists if VIRTIO_BALLON_F_STATS_VQ set.

    5.5.3 Feature bits

    VIRTIO_BALLOON_F_MUST_TELL_HOST (0)
    Host has to be told before pages from the balloon are used.
    VIRTIO_BALLOON_F_STATS_VQ (1)
    A virtqueue for reporting guest memory statistics is present.
    VIRTIO_BALLOON_F_DEFLATE_ON_OOM (2)
    Deflate balloon on guest out of memory condition.
    5.5.3.1 Driver Requirements: Feature bits

    The driver SHOULD accept the VIRTIO_BALLOON_F_MUST_TELL_HOST feature if offered by the device.

    5.5.3.2 Device Requirements: Feature bits

    If the device offers the VIRTIO_BALLOON_F_MUST_TELL_HOST feature bit, and if the driver did not accept this feature bit, the device MAY signal failure by failing to set FEATURES_OK device status bit when the driver writes it.

    5.5.3.2.0.1 Legacy Interface: Feature bits
    As the legacy interface does not have a way to gracefully report feature negotiation failure, when using the legacy interface, transitional devices MUST support guests which do not negotiate VIRTIO_BALLOON_F_MUST_TELL_HOST feature, and SHOULD allow guest to use memory before notifying host if VIRTIO_BALLOON_F_MUST_TELL_HOST is not negotiated.

    5.5.4 Device configuration layout

    Both fields of this configuration are always available.

    struct virtio_balloon_config { 
            le32 num_pages; 
            le32 actual; 
    };
    5.5.4.0.0.1 Legacy Interface: Device configuration layout
    When using the legacy interface, transitional devices and drivers MUST format the fields in struct virtio_balloon_config according to the little-endian format. Note: This is unlike the usual convention that legacy device fields are guest endian.

    5.5.5 Device Initialization

    The device initialization process is outlined below:

    1. The inflate and deflate virtqueues are identified.
    2. If the VIRTIO_BALLOON_F_STATS_VQ feature bit is negotiated:
      1. Identify the stats virtqueue.
      2. Add one empty buffer to the stats virtqueue.
      3. DRIVER_OK is set: device operation begins.
      4. Notify the device about the stats virtqueue buffer.

    5.5.6 Device Operation

    The device is driven either by the receipt of a configuration change notification, or by changing guest memory needs, such as performing memory compaction or responding to out of memory conditions.

    1. num_pages configuration field is examined. If this is greater than the actual number of pages, the balloon wants more memory from the guest. If it is less than actual, the balloon doesn’t need it all.
    2. To supply memory to the balloon (aka. inflate):
      1. The driver constructs an array of addresses of unused memory pages. These addresses are divided by 409614 and the descriptor describing the resulting 32-bit array is added to the inflateq.
    3. To remove memory from the balloon (aka. deflate):
      1. The driver constructs an array of addresses of memory pages it has previously given to the balloon, as described above. This descriptor is added to the deflateq.
      2. If the VIRTIO_BALLOON_F_MUST_TELL_HOST feature is negotiated, the guest informs the device of pages before it uses them.
      3. Otherwise, the guest is allowed to re-use pages previously given to the balloon before the device has acknowledged their withdrawal15.
    4. In either case, the device acknowledges inflate and deflate requests by using the descriptor.
    5. Once the device has acknowledged the inflation or deflation, the driver updates actual to reflect the new number of pages in the balloon.
    5.5.6.1 Driver Requirements: Device Operation

    The driver SHOULD supply pages to the balloon when num_pages is greater than the actual number of pages in the balloon.

    The driver MAY use pages from the balloon when num_pages is less than the actual number of pages in the balloon.

    The driver MAY supply pages to the balloon when num_pages is greater than or equal to the actual number of pages in the balloon.

    If VIRTIO_BALLOON_F_DEFLATE_ON_OOM has not been negotiated, the driver MUST NOT use pages from the balloon when num_pages is less than or equal to the actual number of pages in the balloon.

    If VIRTIO_BALLOON_F_DEFLATE_ON_OOM has been negotiated, the driver MAY use pages from the balloon when num_pages is less than or equal to the actual number of pages in the balloon if this is required for system stability (e.g. if memory is required by applications running within the guest).

    The driver MUST use the deflateq to inform the device of pages that it wants to use from the balloon.

    If the VIRTIO_BALLOON_F_MUST_TELL_HOST feature is negotiated, the driver MUST NOT use pages from the balloon until the device has acknowledged the deflate request.

    Otherwise, if the VIRTIO_BALLOON_F_MUST_TELL_HOST feature is not negotiated, the driver MAY begin to re-use pages previously given to the balloon before the device has acknowledged the deflate request.

    In any case, the driver MUST NOT use pages from the balloon after adding the pages to the balloon, but before the device has acknowledged the inflate request.

    The driver MUST NOT request deflation of pages in the balloon before the device has acknowledged the inflate request.

    The driver MUST update actual after changing the number of pages in the balloon.

    The driver MAY update actual once after multiple inflate and deflate operations.

    5.5.6.2 Device Requirements: Device Operation

    The device MAY modify the contents of a page in the balloon after detecting its physical number in an inflate request and before acknowledging the inflate request by using the inflateq descriptor.

    If the VIRTIO_BALLOON_F_MUST_TELL_HOST feature is negotiated, the device MAY modify the contents of a page in the balloon after detecting its physical number in an inflate request and before detecting its physical number in a deflate request and acknowledging the deflate request.

    5.5.6.2.1 Legacy Interface: Device Operation
    When using the legacy interface, the driver SHOULD ignore the used length values. Note: Historically, some devices put the total descriptor length there, even though no data was actually written.

    When using the legacy interface, the driver MUST write out all 4 bytes each time it updates the actual value in the configuration space, using a single atomic operation.

    When using the legacy interface, the device SHOULD NOT use the actual value written by the driver in the configuration space, until the last, most-significant byte of the value has been written. Note: Historically, devices used the actual value, even though when using Virtio Over PCI Bus the device-specific configuration space was not guaranteed to be atomic. Using intermediate values during update by driver is best avoided, except for debugging.

    Historically, drivers using Virtio Over PCI Bus wrote the actual value by using multiple single-byte writes in order, from the least-significant to the most-significant value.

    5.5.6.3 Memory Statistics

    The stats virtqueue is atypical because communication is driven by the device (not the driver). The channel becomes active at driver initialization time when the driver adds an empty buffer and notifies the device. A request for memory statistics proceeds as follows:

    1. The device uses the buffer and sends a used buffer notification.
    2. The driver pops the used buffer and discards it.
    3. The driver collects memory statistics and writes them into a new buffer.
    4. The driver adds the buffer to the virtqueue and notifies the device.
    5. The device pops the buffer (retaining it to initiate a subsequent request) and consumes the statistics.

    Within the buffer, statistics are an array of 6-byte entries. Each statistic consists of a 16 bit tag and a 64 bit value. All statistics are optional and the driver chooses which ones to supply. To guarantee backwards compatibility, devices omit unsupported statistics.

    struct virtio_balloon_stat { 
    #define VIRTIO_BALLOON_S_SWAP_IN  0 
    #define VIRTIO_BALLOON_S_SWAP_OUT 1 
    #define VIRTIO_BALLOON_S_MAJFLT   2 
    #define VIRTIO_BALLOON_S_MINFLT   3 
    #define VIRTIO_BALLOON_S_MEMFREE  4 
    #define VIRTIO_BALLOON_S_MEMTOT   5 
    #define VIRTIO_BALLOON_S_AVAIL    6 
    #define VIRTIO_BALLOON_S_CACHES   7 
    #define VIRTIO_BALLOON_S_HTLB_PGALLOC 8 
    #define VIRTIO_BALLOON_S_HTLB_PGFAIL  9 
            le16 tag; 
            le64 val; 
    } __attribute__((packed));
    5.5.6.3.1 Driver Requirements: Memory Statistics
    Normative statements in this section apply if and only if the VIRTIO_BALLOON_F_STATS_VQ feature has been negotiated.

    The driver MUST make at most one buffer available to the device in the statsq, at all times.

    After initializing the device, the driver MUST make an output buffer available in the statsq.

    Upon detecting that device has used a buffer in the statsq, the driver MUST make an output buffer available in the statsq.

    Before making an output buffer available in the statsq, the driver MUST initialize it, including one struct virtio_balloon_stat entry for each statistic that it supports.

    Driver MUST use an output buffer size which is a multiple of 6 bytes for all buffers submitted to the statsq.

    Driver MAY supply struct virtio_balloon_stat entries in the output buffer submitted to the statsq in any order, without regard to tag values.

    Driver MAY supply a subset of all statistics in the output buffer submitted to the statsq.

    Driver MUST supply the same subset of statistics in all buffers submitted to the statsq.

    5.5.6.3.2 Device Requirements: Memory Statistics
    Normative statements in this section apply if and only if the VIRTIO_BALLOON_F_STATS_VQ feature has been negotiated.

    Within an output buffer submitted to the statsq, the device MUST ignore entries with tag values that it does not recognize.

    Within an output buffer submitted to the statsq, the device MUST accept struct virtio_balloon_stat entries in any order without regard to tag values.

    5.5.6.3.3 Legacy Interface: Memory Statistics
    When using the legacy interface, transitional devices and drivers MUST format the fields in struct virtio_balloon_stat according to the native endian of the guest rather than (necessarily when not using the legacy interface) little-endian.

    When using the legacy interface, the device SHOULD ignore all values in the first buffer in the statsq supplied by the driver after device initialization. Note: Historically, drivers supplied an uninitialized buffer in the first buffer.

    5.5.6.4 Memory Statistics Tags
    VIRTIO_BALLOON_S_SWAP_IN (0)
    The amount of memory that has been swapped in (in bytes).
    VIRTIO_BALLOON_S_SWAP_OUT (1)
    The amount of memory that has been swapped out to disk (in bytes).
    VIRTIO_BALLOON_S_MAJFLT (2)
    The number of major page faults that have occurred.
    VIRTIO_BALLOON_S_MINFLT (3)
    The number of minor page faults that have occurred.
    VIRTIO_BALLOON_S_MEMFREE (4)
    The amount of memory not being used for any purpose (in bytes).
    VIRTIO_BALLOON_S_MEMTOT (5)
    The total amount of memory available (in bytes).
    VIRTIO_BALLOON_S_AVAIL (6)
    An estimate of how much memory is available (in bytes) for starting new applications, without pushing the system to swap.
    VIRTIO_BALLOON_S_CACHES (7)
    The amount of memory, in bytes, that can be quickly reclaimed without additional I/O. Typically these pages are used for caching files from disk.
    VIRTIO_BALLOON_S_HTLB_PGALLOC (8)
    The number of successful hugetlb page allocations in the guest.
    VIRTIO_BALLOON_S_HTLB_PGFAIL (9)
    The number of failed hugetlb page allocations in the guest.

    5.6 SCSI Host Device

    The virtio SCSI host device groups together one or more virtual logical units (such as disks), and allows communicating to them using the SCSI protocol. An instance of the device represents a SCSI host to which many targets and LUNs are attached.

    The virtio SCSI device services two kinds of requests:

    The device is also able to send out notifications about added and removed logical units. Together, these capabilities provide a SCSI transport protocol that uses virtqueues as the transfer medium. In the transport protocol, the virtio driver acts as the initiator, while the virtio SCSI host provides one or more targets that receive and process the requests.

    This section relies on definitions from SAM.

    5.6.1 Device ID

    8

    5.6.2 Virtqueues

    0
    controlq
    1
    eventq
    2…n
    request queues

    5.6.3 Feature bits

    VIRTIO_SCSI_F_INOUT (0)
    A single request can include both device-readable and device-writable data buffers.
    VIRTIO_SCSI_F_HOTPLUG (1)
    The host SHOULD enable reporting of hot-plug and hot-unplug events for LUNs and targets on the SCSI bus. The guest SHOULD handle hot-plug and hot-unplug events.
    VIRTIO_SCSI_F_CHANGE (2)
    The host will report changes to LUN parameters via a VIRTIO_SCSI_T_PARAM_CHANGE event; the guest SHOULD handle them.
    VIRTIO_SCSI_F_T10_PI (3)
    The extended fields for T10 protection information (DIF/DIX) are included in the SCSI request header.

    5.6.4 Device configuration layout

    All fields of this configuration are always available.

    struct virtio_scsi_config { 
            le32 num_queues; 
            le32 seg_max; 
            le32 max_sectors; 
            le32 cmd_per_lun; 
            le32 event_info_size; 
            le32 sense_size; 
            le32 cdb_size; 
            le16 max_channel; 
            le16 max_target; 
            le32 max_lun; 
    };
    num_queues
    is the total number of request virtqueues exposed by the device. The driver MAY use only one request queue, or it can use more to achieve better performance.
    seg_max
    is the maximum number of segments that can be in a command. A bidirectional command can include seg_max input segments and seg_max output segments.
    max_sectors
    is a hint to the driver about the maximum transfer size to use.
    cmd_per_lun
    tells the driver the maximum number of linked commands it can send to one LUN.
    event_info_size
    is the maximum size that the device will fill for buffers that the driver places in the eventq. It is written by the device depending on the set of negotiated features.
    sense_size
    is the maximum size of the sense data that the device will write. The default value is written by the device and MUST be 96, but the driver can modify it. It is restored to the default when the device is reset.
    cdb_size
    is the maximum size of the CDB that the driver will write. The default value is written by the device and MUST be 32, but the driver can likewise modify it. It is restored to the default when the device is reset.
    max_channel, max_target and max_lun
    can be used by the driver as hints to constrain scanning the logical units on the host to channel/target/logical unit numbers that are less than or equal to the value of the fields. max_channel SHOULD be zero. max_target SHOULD be less than or equal to 255. max_lun SHOULD be less than or equal to 16383.
    5.6.4.1 Driver Requirements: Device configuration layout

    The driver MUST NOT write to device configuration fields other than sense_size and cdb_size.

    The driver MUST NOT send more than cmd_per_lun linked commands to one LUN, and MUST NOT send more than the virtqueue size number of linked commands to one LUN.

    5.6.4.2 Device Requirements: Device configuration layout

    On reset, the device MUST set sense_size to 96 and cdb_size to 32.

    5.6.4.3 Legacy Interface: Device configuration layout

    When using the legacy interface, transitional devices and drivers MUST format the fields in struct virtio_scsi_config according to the native endian of the guest rather than (necessarily when not using the legacy interface) little-endian.

    5.6.5 Device Requirements: Device Initialization

    On initialization the driver SHOULD first discover the device’s virtqueues.

    If the driver uses the eventq, the driver SHOULD place at least one buffer in the eventq.

    The driver MAY immediately issue requests16 or task management functions17.

    5.6.6 Device Operation

    Device operation consists of operating request queues, the control queue and the event queue.

    5.6.6.0.1 Legacy Interface: Device Operation
    When using the legacy interface, the driver SHOULD ignore the used length values. Note: Historically, devices put the total descriptor length, or the total length of device-writable buffers there, even when only part of the buffers were actually written.
    5.6.6.1 Device Operation: Request Queues

    The driver queues requests to an arbitrary request queue, and they are used by the device on that same queue. It is the responsibility of the driver to ensure strict request ordering for commands placed on different queues, because they will be consumed with no order constraints.

    Requests have the following format:

    struct virtio_scsi_req_cmd { 
            // Device-readable part 
            u8 lun[8]; 
            le64 id; 
            u8 task_attr; 
            u8 prio; 
            u8 crn; 
            u8 cdb[cdb_size]; 
            // The next three fields are only present if VIRTIO_SCSI_F_T10_PI 
            // is negotiated. 
            le32 pi_bytesout; 
            le32 pi_bytesin; 
            u8 pi_out[pi_bytesout]; 
            u8 dataout[]; 
     
            // Device-writable part 
            le32 sense_len; 
            le32 residual; 
            le16 status_qualifier; 
            u8 status; 
            u8 response; 
            u8 sense[sense_size]; 
            // The next field is only present if VIRTIO_SCSI_F_T10_PI 
            // is negotiated 
            u8 pi_in[pi_bytesin]; 
            u8 datain[]; 
    }; 
     
     
    /* command-specific response values */ 
    #define VIRTIO_SCSI_S_OK                0 
    #define VIRTIO_SCSI_S_OVERRUN           1 
    #define VIRTIO_SCSI_S_ABORTED           2 
    #define VIRTIO_SCSI_S_BAD_TARGET        3 
    #define VIRTIO_SCSI_S_RESET             4 
    #define VIRTIO_SCSI_S_BUSY              5 
    #define VIRTIO_SCSI_S_TRANSPORT_FAILURE 6 
    #define VIRTIO_SCSI_S_TARGET_FAILURE    7 
    #define VIRTIO_SCSI_S_NEXUS_FAILURE     8 
    #define VIRTIO_SCSI_S_FAILURE           9 
     
    /* task_attr */ 
    #define VIRTIO_SCSI_S_SIMPLE            0 
    #define VIRTIO_SCSI_S_ORDERED           1 
    #define VIRTIO_SCSI_S_HEAD              2 
    #define VIRTIO_SCSI_S_ACA               3

    lun addresses the REPORT LUNS well-known logical unit, or a target and logical unit in the virtio-scsi device’s SCSI domain. When used to address the REPORT LUNS logical unit, lun is 0xC1, 0x01 and six zero bytes. The virtio-scsi device SHOULD implement the REPORT LUNS well-known logical unit.

    When used to address a target and logical unit, the only supported format for lun is: first byte set to 1, second byte set to target, third and fourth byte representing a single level LUN structure, followed by four zero bytes. With this representation, a virtio-scsi device can serve up to 256 targets and 16384 LUNs per target. The device MAY also support having a well-known logical units in the third and fourth byte.

    id is the command identifier (“tag”).

    task_attr defines the task attribute as in the table above, but all task attributes MAY be mapped to SIMPLE by the device. Some commands are defined by SCSI standards as "implicit head of queue"; for such commands, all task attributes MAY also be mapped to HEAD OF QUEUE. Drivers and applications SHOULD NOT send a command with the ORDERED task attribute if the command has an implicit HEAD OF QUEUE attribute, because whether the ORDERED task attribute is honored is vendor-specific.

    crn may also be provided by clients, but is generally expected to be 0. The maximum CRN value defined by the protocol is 255, since CRN is stored in an 8-bit integer.

    The CDB is included in cdb and its size, cdb_size, is taken from the configuration space.

    All of these fields are defined in SAM and are always device-readable.

    pi_bytesout determines the size of the pi_out field in bytes. If it is nonzero, the pi_out field contains outgoing protection information for write operations. pi_bytesin determines the size of the pi_in field in the device-writable section, in bytes. All three fields are only present if VIRTIO_SCSI_F_T10_PI has been negotiated.

    The remainder of the device-readable part is the data output buffer, dataout.

    sense and subsequent fields are always device-writable. sense_len indicates the number of bytes actually written to the sense buffer.

    residual indicates the residual size, calculated as “data_length - number_of_transferred_bytes”, for read or write operations. For bidirectional commands, the number_of_transferred_bytes includes both read and written bytes. A residual that is less than the size of datain means that dataout was processed entirely. A residual that exceeds the size of datain means that dataout was processed partially and datain was not processed at all.

    If the pi_bytesin is nonzero, the pi_in field contains incoming protection information for read operations. pi_in is only present if VIRTIO_SCSI_F_T10_PI has been negotiated18.

    The remainder of the device-writable part is the data input buffer, datain.

    5.6.6.1.1 Device Requirements: Device Operation: Request Queues
    The device MUST write the status byte as the status code as defined in SAM.

    The device MUST write the response byte as one of the following:

    VIRTIO_SCSI_S_OK
    when the request was completed and the status byte is filled with a SCSI status code (not necessarily “GOOD”).
    VIRTIO_SCSI_S_OVERRUN
    if the content of the CDB (such as the allocation length, parameter length or transfer size) requires more data than is available in the datain and dataout buffers.
    VIRTIO_SCSI_S_ABORTED
    if the request was cancelled due to an ABORT TASK or ABORT TASK SET task management function.
    VIRTIO_SCSI_S_BAD_TARGET
    if the request was never processed because the target indicated by lun does not exist.
    VIRTIO_SCSI_S_RESET
    if the request was cancelled due to a bus or device reset (including a task management function).
    VIRTIO_SCSI_S_TRANSPORT_FAILURE
    if the request failed due to a problem in the connection between the host and the target (severed link).
    VIRTIO_SCSI_S_TARGET_FAILURE
    if the target is suffering a failure and to tell the driver not to retry on other paths.
    VIRTIO_SCSI_S_NEXUS_FAILURE
    if the nexus is suffering a failure but retrying on other paths might yield a different result.
    VIRTIO_SCSI_S_BUSY
    if the request failed but retrying on the same path is likely to work.
    VIRTIO_SCSI_S_FAILURE
    for other host or driver error. In particular, if neither dataout nor datain is empty, and the VIRTIO_SCSI_F_INOUT feature has not been negotiated, the request will be immediately returned with a response equal to VIRTIO_SCSI_S_FAILURE.

    All commands must be completed before the virtio-scsi device is reset or unplugged. The device MAY choose to abort them, or if it does not do so MUST pick the VIRTIO_SCSI_S_FAILURE response.

    5.6.6.1.2 Driver Requirements: Device Operation: Request Queues
    task_attr, prio and crn SHOULD be zero.

    Upon receiving a VIRTIO_SCSI_S_TARGET_FAILURE response, the driver SHOULD NOT retry the request on other paths.

    5.6.6.1.3 Legacy Interface: Device Operation: Request Queues
    When using the legacy interface, transitional devices and drivers MUST format the fields in struct virtio_scsi_req_cmd according to the native endian of the guest rather than (necessarily when not using the legacy interface) little-endian.
    5.6.6.2 Device Operation: controlq

    The controlq is used for other SCSI transport operations. Requests have the following format:

    struct virtio_scsi_ctrl { 
            le32 type; 
     
            u8 response; 
    }; 
     
    /* response values valid for all commands */ 
    #define VIRTIO_SCSI_S_OK                       0 
    #define VIRTIO_SCSI_S_BAD_TARGET               3 
    #define VIRTIO_SCSI_S_BUSY                     5 
    #define VIRTIO_SCSI_S_TRANSPORT_FAILURE        6 
    #define VIRTIO_SCSI_S_TARGET_FAILURE           7 
    #define VIRTIO_SCSI_S_NEXUS_FAILURE            8 
    #define VIRTIO_SCSI_S_FAILURE                  9 
    #define VIRTIO_SCSI_S_INCORRECT_LUN            12

    The type identifies the remaining fields.

    The following commands are defined:

    5.6.6.2.1 Legacy Interface: Device Operation: controlq
    When using the legacy interface, transitional devices and drivers MUST format the fields in struct virtio_scsi_ctrl, struct virtio_scsi_ctrl_tmf, struct virtio_scsi_ctrl_an and struct virtio_scsi_ctrl_an according to the native endian of the guest rather than (necessarily when not using the legacy interface) little-endian.
    5.6.6.3 Device Operation: eventq

    The eventq is populated by the driver for the device to report information on logical units that are attached to it. In general, the device will not queue events to cope with an empty eventq, and will end up dropping events if it finds no buffer ready. However, when reporting events for many LUNs (e.g. when a whole target disappears), the device can throttle events to avoid dropping them. For this reason, placing 10-15 buffers on the event queue is sufficient.

    Buffers returned by the device on the eventq will be referred to as “events” in the rest of this section. Events have the following format:

    #define VIRTIO_SCSI_T_EVENTS_MISSED   0x80000000 
     
    struct virtio_scsi_event { 
            // Device-writable part 
            le32 event; 
            u8  lun[8]; 
            le32 reason; 
    };

    The devices sets bit 31 in event to report lost events due to missing buffers.

    The meaning of reason depends on the contents of event. The following events are defined:

    5.6.6.3.1 Driver Requirements: Device Operation: eventq
    The driver SHOULD keep the eventq populated with buffers. These buffers MUST be device-writable, and SHOULD be at least event_info_size bytes long, and MUST be at least the size of struct virtio_scsi_event.

    If event has bit 31 set, the driver SHOULD poll the logical units for unit attention conditions, and/or do whatever form of bus scan is appropriate for the guest operating system and SHOULD poll for asynchronous events manually using SCSI commands.

    When receiving a VIRTIO_SCSI_T_TRANSPORT_RESET message with reason set to VIRTIO_SCSI_EVT_RESET_REMOVED or VIRTIO_SCSI_EVT_RESET_RESCAN for LUN 0, the driver SHOULD ask the initiator to rescan the target, in order to detect the case when an entire target has appeared or disappeared.

    5.6.6.3.2 Device Requirements: Device Operation: eventq
    The device MUST set bit 31 in event if events were lost due to missing buffers, and it MAY use a VIRTIO_SCSI_T_NO_EVENT event to report this.

    The device MUST NOT send VIRTIO_SCSI_T_TRANSPORT_RESET messages with reason set to VIRTIO_SCSI_EVT_RESET_REMOVED or VIRTIO_SCSI_EVT_RESET_RESCAN unless VIRTIO_SCSI_F_HOTPLUG was negotiated.

    The device MUST NOT report VIRTIO_SCSI_T_PARAM_CHANGE for MMC devices.

    5.6.6.3.3 Legacy Interface: Device Operation: eventq
    When using the legacy interface, transitional devices and drivers MUST format the fields in struct virtio_scsi_event according to the native endian of the guest rather than (necessarily when not using the legacy interface) little-endian.
    5.6.6.4 Legacy Interface: Framing Requirements

    When using legacy interfaces, transitional drivers which have not negotiated VIRTIO_F_ANY_LAYOUT MUST use a single descriptor for the lun, id, task_attr, prio, crn and cdb fields, and MUST only use a single descriptor for the sense_len, residual, status_qualifier, status, response and sense fields.

    5.7 GPU Device

    virtio-gpu is a virtio based graphics adapter. It can operate in 2D mode and in 3D (virgl) mode. 3D mode will offload rendering ops to the host gpu and therefore requires a gpu with 3D support on the host machine.

    3D mode is not covered (yet) in this specification, even though it is mentioned here and there due to some details of the virtual hardware being designed with 3D mode in mind.

    In 2D mode the virtio-gpu device provides support for ARGB Hardware cursors and multiple scanouts (aka heads).

    5.7.1 Device ID

    16

    5.7.2 Virtqueues

    0
    controlq - queue for sending control commands
    1
    cursorq - queue for sending cursor updates

    Both queues have the same format. Each request and each response have a fixed header, followed by command specific data fields. The separate cursor queue is the "fast track" for cursor commands (VIRTIO_GPU_CMD_UPDATE_CURSOR and VIRTIO_GPU_CMD_MOVE_CURSOR), so they go though without being delayed by time-consuming commands in the control queue.

    5.7.3 Feature bits

    VIRTIO_GPU_F_VIRGL (0)
    virgl 3D mode is supported.
    VIRTIO_GPU_F_EDID (1)
    EDID is supported.

    5.7.4 Device configuration layout

    GPU device configuration uses the following layout structure and definitions:

    #define VIRTIO_GPU_EVENT_DISPLAY (1 << 0) 
     
    struct virtio_gpu_config { 
            le32 events_read; 
            le32 events_clear; 
            le32 num_scanouts; 
            le32 reserved; 
    };
    5.7.4.1 Device configuration fields
    events_read
    signals pending events to the driver. The driver MUST NOT write to this field.
    events_clear
    clears pending events in the device. Writing a ’1’ into a bit will clear the corresponding bit in events_read, mimicking write-to-clear behavior.
    num_scanouts
    specifies the maximum number of scanouts supported by the device. Minimum value is 1, maximum value is 16.
    5.7.4.2 Events
    VIRTIO_GPU_EVENT_DISPLAY
    Display configuration has changed. The driver SHOULD use the VIRTIO_GPU_CMD_GET_DISPLAY_INFO command to fetch the information from the device.

    5.7.5 Device Requirements: Device Initialization

    The driver SHOULD query the display information from the device using the VIRTIO_GPU_CMD_GET_DISPLAY_INFO command and use that information for the initial scanout setup. In case no information is available or all displays are disabled the driver MAY choose to use a fallback, such as 1024x768 at display 0.

    5.7.6 Device Operation

    The virtio-gpu is based around the concept of resources private to the host, the guest must DMA transfer into these resources. This is a design requirement in order to interface with future 3D rendering. In the unaccelerated 2D mode there is no support for DMA transfers from resources, just to them.

    Resources are initially simple 2D resources, consisting of a width, height and format along with an identifier. The guest must then attach backing store to the resources in order for DMA transfers to work. This is like a GART in a real GPU.

    5.7.6.1 Device Operation: Create a framebuffer and configure scanout
    5.7.6.2 Device Operation: Update a framebuffer and scanout
    5.7.6.3 Device Operation: Using pageflip

    It is possible to create multiple framebuffers, flip between them using VIRTIO_GPU_CMD_SET_SCANOUT and VIRTIO_GPU_CMD_RESOURCE_FLUSH, and update the invisible framebuffer using VIRTIO_GPU_CMD_TRANSFER_TO_HOST_2D.

    5.7.6.4 Device Operation: Multihead setup

    In case two or more displays are present there are different ways to configure things:

    5.7.6.5 Device Requirements: Device Operation: Command lifecycle and fencing

    The device MAY process controlq commands asyncronously and return them to the driver before the processing is complete. If the driver needs to know when the processing is finished it can set the VIRTIO_GPU_FLAG_FENCE flag in the request. The device MUST finish the processing before returning the command then.

    Note: current qemu implementation does asyncrounous processing only in 3d mode, when offloading the processing to the host gpu.

    5.7.6.6 Device Operation: Configure mouse cursor

    The mouse cursor image is a normal resource, except that it must be 64x64 in size. The driver MUST create and populate the resource (using the usual VIRTIO_GPU_CMD_RESOURCE_CREATE_2D, VIRTIO_GPU_CMD_RESOURCE_ATTACH_BACKING and VIRTIO_GPU_CMD_TRANSFER_TO_HOST_2D controlq commands) and make sure they are completed (using VIRTIO_GPU_FLAG_FENCE).

    Then VIRTIO_GPU_CMD_UPDATE_CURSOR can be sent to the cursorq to set the pointer shape and position. To move the pointer without updating the shape use VIRTIO_GPU_CMD_MOVE_CURSOR instead.

    5.7.6.7 Device Operation: Request header

    All requests and responses on the virt queues have a fixed header using the following layout structure and definitions:

    enum virtio_gpu_ctrl_type { 
     
            /* 2d commands */ 
            VIRTIO_GPU_CMD_GET_DISPLAY_INFO = 0x0100, 
            VIRTIO_GPU_CMD_RESOURCE_CREATE_2D, 
            VIRTIO_GPU_CMD_RESOURCE_UNREF, 
            VIRTIO_GPU_CMD_SET_SCANOUT, 
            VIRTIO_GPU_CMD_RESOURCE_FLUSH, 
            VIRTIO_GPU_CMD_TRANSFER_TO_HOST_2D, 
            VIRTIO_GPU_CMD_RESOURCE_ATTACH_BACKING, 
            VIRTIO_GPU_CMD_RESOURCE_DETACH_BACKING, 
            VIRTIO_GPU_CMD_GET_CAPSET_INFO, 
            VIRTIO_GPU_CMD_GET_CAPSET, 
            VIRTIO_GPU_CMD_GET_EDID, 
     
            /* cursor commands */ 
            VIRTIO_GPU_CMD_UPDATE_CURSOR = 0x0300, 
            VIRTIO_GPU_CMD_MOVE_CURSOR, 
     
            /* success responses */ 
            VIRTIO_GPU_RESP_OK_NODATA = 0x1100, 
            VIRTIO_GPU_RESP_OK_DISPLAY_INFO, 
            VIRTIO_GPU_RESP_OK_CAPSET_INFO, 
            VIRTIO_GPU_RESP_OK_CAPSET, 
            VIRTIO_GPU_RESP_OK_EDID, 
     
            /* error responses */ 
            VIRTIO_GPU_RESP_ERR_UNSPEC = 0x1200, 
            VIRTIO_GPU_RESP_ERR_OUT_OF_MEMORY, 
            VIRTIO_GPU_RESP_ERR_INVALID_SCANOUT_ID, 
            VIRTIO_GPU_RESP_ERR_INVALID_RESOURCE_ID, 
            VIRTIO_GPU_RESP_ERR_INVALID_CONTEXT_ID, 
            VIRTIO_GPU_RESP_ERR_INVALID_PARAMETER, 
    }; 
     
    #define VIRTIO_GPU_FLAG_FENCE (1 << 0) 
     
    struct virtio_gpu_ctrl_hdr { 
            le32 type; 
            le32 flags; 
            le64 fence_id; 
            le32 ctx_id; 
            le32 padding; 
    };

    The fixed header struct virtio_gpu_ctrl_hdr in each request includes the following fields:

    type
    specifies the type of the driver request (VIRTIO_GPU_CMD_*) or device response (VIRTIO_GPU_RESP_*).
    flags
    request / response flags.
    fence_id
    If the driver sets the VIRTIO_GPU_FLAG_FENCE bit in the request flags field the device MUST:
    • set VIRTIO_GPU_FLAG_FENCE bit in the response,
    • copy the content of the fence_id field from the request to the response, and
    • send the response only after command processing is complete.
    ctx_id
    Rendering context (used in 3D mode only).

    On success the device will return VIRTIO_GPU_RESP_OK_NODATA in case there is no payload. Otherwise the type field will indicate the kind of payload.

    On error the device will return one of the VIRTIO_GPU_RESP_ERR_* error codes.

    5.7.6.8 Device Operation: controlq

    For any coordinates given 0,0 is top left, larger x moves right, larger y moves down.

    VIRTIO_GPU_CMD_GET_DISPLAY_INFO
    Retrieve the current output configuration. No request data (just bare struct virtio_gpu_ctrl_hdr). Response type is VIRTIO_GPU_RESP_OK_DISPLAY_INFO, response data is struct virtio_gpu_resp_display_info.
    #define VIRTIO_GPU_MAX_SCANOUTS 16 
     
    struct virtio_gpu_rect { 
            le32 x; 
            le32 y; 
            le32 width; 
            le32 height; 
    }; 
     
    struct virtio_gpu_resp_display_info { 
            struct virtio_gpu_ctrl_hdr hdr; 
            struct virtio_gpu_display_one { 
                    struct virtio_gpu_rect r; 
                    le32 enabled; 
                    le32 flags; 
            } pmodes[VIRTIO_GPU_MAX_SCANOUTS]; 
    };

    The response contains a list of per-scanout information. The info contains whether the scanout is enabled and what its preferred position and size is.

    The size (fields width and height) is similar to the native panel resolution in EDID display information, except that in the virtual machine case the size can change when the host window representing the guest display is gets resized.

    The position (fields x and y) describe how the displays are arranged (i.e. which is – for example – the left display).

    The enabled field is set when the user enabled the display. It is roughly the same as the connected state of a phyiscal display connector.

    VIRTIO_GPU_CMD_GET_EDID
    Retrieve the EDID data for a given scanout. Request data is struct virtio_gpu_get_edid). Response type is VIRTIO_GPU_RESP_OK_EDID, response data is struct virtio_gpu_resp_edid. Support is optional and negotiated using the VIRTIO_GPU_F_EDID feature flag.
    struct virtio_gpu_get_edid { 
            struct virtio_gpu_ctrl_hdr hdr; 
            le32 scanout; 
            le32 padding; 
    }; 
     
    struct virtio_gpu_resp_edid { 
            struct virtio_gpu_ctrl_hdr hdr; 
            le32 size; 
            le32 padding; 
            u8 edid[1024]; 
    };

    The response contains the EDID display data blob (as specified by VESA) for the scanout.

    VIRTIO_GPU_CMD_RESOURCE_CREATE_2D
    Create a 2D resource on the host. Request data is struct virtio_gpu_resource_create_2d. Response type is VIRTIO_GPU_RESP_OK_NODATA.
    enum virtio_gpu_formats { 
            VIRTIO_GPU_FORMAT_B8G8R8A8_UNORM  = 1, 
            VIRTIO_GPU_FORMAT_B8G8R8X8_UNORM  = 2, 
            VIRTIO_GPU_FORMAT_A8R8G8B8_UNORM  = 3, 
            VIRTIO_GPU_FORMAT_X8R8G8B8_UNORM  = 4, 
     
            VIRTIO_GPU_FORMAT_R8G8B8A8_UNORM  = 67, 
            VIRTIO_GPU_FORMAT_X8B8G8R8_UNORM  = 68, 
     
            VIRTIO_GPU_FORMAT_A8B8G8R8_UNORM  = 121, 
            VIRTIO_GPU_FORMAT_R8G8B8X8_UNORM  = 134, 
    }; 
     
    struct virtio_gpu_resource_create_2d { 
            struct virtio_gpu_ctrl_hdr hdr; 
            le32 resource_id; 
            le32 format; 
            le32 width; 
            le32 height; 
    };

    This creates a 2D resource on the host with the specified width, height and format. The resource ids are generated by the guest.

    VIRTIO_GPU_CMD_RESOURCE_UNREF
    Destroy a resource. Request data is struct virtio_gpu_resource_unref. Response type is VIRTIO_GPU_RESP_OK_NODATA.
    struct virtio_gpu_resource_unref { 
            struct virtio_gpu_ctrl_hdr hdr; 
            le32 resource_id; 
            le32 padding; 
    };

    This informs the host that a resource is no longer required by the guest.

    VIRTIO_GPU_CMD_SET_SCANOUT
    Set the scanout parameters for a single output. Request data is struct virtio_gpu_set_scanout. Response type is VIRTIO_GPU_RESP_OK_NODATA.
    struct virtio_gpu_set_scanout { 
            struct virtio_gpu_ctrl_hdr hdr; 
            struct virtio_gpu_rect r; 
            le32 scanout_id; 
            le32 resource_id; 
    };

    This sets the scanout parameters for a single scanout. The resource_id is the resource to be scanned out from, along with a rectangle.

    Scanout rectangles must be completely covered by the underlying resource. Overlapping (or identical) scanouts are allowed, typical use case is screen mirroring.

    The driver can use resource_id = 0 to disable a scanout.

    VIRTIO_GPU_CMD_RESOURCE_FLUSH
    Flush a scanout resource Request data is struct virtio_gpu_resource_flush. Response type is VIRTIO_GPU_RESP_OK_NODATA.
    struct virtio_gpu_resource_flush { 
            struct virtio_gpu_ctrl_hdr hdr; 
            struct virtio_gpu_rect r; 
            le32 resource_id; 
            le32 padding; 
    };

    This flushes a resource to screen. It takes a rectangle and a resource id, and flushes any scanouts the resource is being used on.

    VIRTIO_GPU_CMD_TRANSFER_TO_HOST_2D
    Transfer from guest memory to host resource. Request data is struct virtio_gpu_transfer_to_host_2d. Response type is VIRTIO_GPU_RESP_OK_NODATA.
    struct virtio_gpu_transfer_to_host_2d { 
            struct virtio_gpu_ctrl_hdr hdr; 
            struct virtio_gpu_rect r; 
            le64 offset; 
            le32 resource_id; 
            le32 padding; 
    };

    This takes a resource id along with an destination offset into the resource, and a box to transfer to the host backing for the resource.

    VIRTIO_GPU_CMD_RESOURCE_ATTACH_BACKING
    Assign backing pages to a resource. Request data is struct virtio_gpu_resource_attach_backing, followed by struct virtio_gpu_mem_entry entries. Response type is VIRTIO_GPU_RESP_OK_NODATA.
    struct virtio_gpu_resource_attach_backing { 
            struct virtio_gpu_ctrl_hdr hdr; 
            le32 resource_id; 
            le32 nr_entries; 
    }; 
     
    struct virtio_gpu_mem_entry { 
            le64 addr; 
            le32 length; 
            le32 padding; 
    };

    This assign an array of guest pages as the backing store for a resource. These pages are then used for the transfer operations for that resource from that point on.

    VIRTIO_GPU_CMD_RESOURCE_DETACH_BACKING
    Detach backing pages from a resource. Request data is struct virtio_gpu_resource_detach_backing. Response type is VIRTIO_GPU_RESP_OK_NODATA.
    struct virtio_gpu_resource_detach_backing { 
            struct virtio_gpu_ctrl_hdr hdr; 
            le32 resource_id; 
            le32 padding; 
    };

    This detaches any backing pages from a resource, to be used in case of guest swapping or object destruction.

    5.7.6.9 Device Operation: cursorq

    Both cursorq commands use the same command struct.

    struct virtio_gpu_cursor_pos { 
            le32 scanout_id; 
            le32 x; 
            le32 y; 
            le32 padding; 
    }; 
     
    struct virtio_gpu_update_cursor { 
            struct virtio_gpu_ctrl_hdr hdr; 
            struct virtio_gpu_cursor_pos pos; 
            le32 resource_id; 
            le32 hot_x; 
            le32 hot_y; 
            le32 padding; 
    };
    VIRTIO_GPU_CMD_UPDATE_CURSOR
    Update cursor. Request data is struct virtio_gpu_update_cursor. Response type is VIRTIO_GPU_RESP_OK_NODATA.

    Full cursor update. Cursor will be loaded from the specified resource_id and will be moved to pos. The driver must transfer the cursor into the resource beforehand (using control queue commands) and make sure the commands to fill the resource are actually processed (using fencing).

    VIRTIO_GPU_CMD_MOVE_CURSOR
    Move cursor. Request data is struct virtio_gpu_update_cursor. Response type is VIRTIO_GPU_RESP_OK_NODATA.

    Move cursor to the place specified in pos. The other fields are not used and will be ignored by the device.

    5.7.7 VGA Compatibility

    Applies to Virtio Over PCI only. The GPU device can come with and without VGA compatibility. The PCI class should be DISPLAY_VGA if VGA compatibility is present and DISPLAY_OTHER otherwise.

    VGA compatibility: PCI region 0 has the linear framebuffer, standard vga registers are present. Configuring a scanout (VIRTIO_GPU_CMD_SET_SCANOUT) switches the device from vga compatibility mode into native virtio mode. A reset switches it back into vga compatibility mode.

    Note: qemu implementation also provides bochs dispi interface io ports and mmio bar at pci region 1 and is therefore fully compatible with the qemu stdvga (see docs/specs/standard-vga.txt in the qemu source tree).

    5.8 Input Device

    The virtio input device can be used to create virtual human interface devices such as keyboards, mice and tablets. An instance of the virtio device represents one such input device. Device behavior mirrors that of the evdev layer in Linux, making pass-through implementations on top of evdev easy.

    This specification defines how evdev events are transported over virtio and how the set of supported events is discovered by a driver. It does not, however, define the semantics of input events as this is dependent on the particular evdev implementation. For the list of events used by Linux input devices, see include/uapi/linux/input-event-codes.h in the Linux source tree.

    5.8.1 Device ID

    18

    5.8.2 Virtqueues

    0
    eventq
    1
    statusq

    5.8.3 Feature bits

    None.

    5.8.4 Device configuration layout

    Device configuration holds all information the guest needs to handle the device, most importantly the events which are supported.

    enum virtio_input_config_select { 
      VIRTIO_INPUT_CFG_UNSET      = 0x00, 
      VIRTIO_INPUT_CFG_ID_NAME    = 0x01, 
      VIRTIO_INPUT_CFG_ID_SERIAL  = 0x02, 
      VIRTIO_INPUT_CFG_ID_DEVIDS  = 0x03, 
      VIRTIO_INPUT_CFG_PROP_BITS  = 0x10, 
      VIRTIO_INPUT_CFG_EV_BITS    = 0x11, 
      VIRTIO_INPUT_CFG_ABS_INFO   = 0x12, 
    }; 
     
    struct virtio_input_absinfo { 
      le32  min; 
      le32  max; 
      le32  fuzz; 
      le32  flat; 
      le32  res; 
    }; 
     
    struct virtio_input_devids { 
      le16  bustype; 
      le16  vendor; 
      le16  product; 
      le16  version; 
    }; 
     
    struct virtio_input_config { 
      u8    select; 
      u8    subsel; 
      u8    size; 
      u8    reserved[5]; 
      union { 
        char string[128]; 
        u8   bitmap[128]; 
        struct virtio_input_absinfo abs; 
        struct virtio_input_devids ids; 
      } u; 
    };

    To query a specific piece