# vfio-user Protocol Specification

## Introduction

vfio-user is a protocol that allows a device to be emulated in a separate process outside of a Virtual Machine Monitor (VMM). vfio-user devices consist of a generic VFIO device type, living inside the VMM, which we call the client, and the core device implementation, living outside the VMM, which we call the server.

The vfio-user specification is partly based on the Linux VFIO ioctl interface.

VFIO is a mature and stable API, backed by an extensively used framework. The existing VFIO client implementation in QEMU (`qemu/hw/vfio/`) can be largely re-used, though there is nothing in this specification that requires that particular implementation. None of the VFIO kernel modules are required for supporting the protocol, on either the client or server side. Some source definitions in VFIO are re-used for vfio-user.

The main idea is to allow a virtual device to function in a separate process in the same host over a UNIX domain socket. A UNIX domain socket (`AF_UNIX`) is chosen because file descriptors can be trivially sent over it, which in turn allows:

- Sharing of client memory for DMA with the server.

- Sharing of server memory with the client for fast MMIO.

- Efficient sharing of eventfd's for triggering interrupts.

Other socket types could be used which allow the server to run in a separate guest in the same host (`AF_VSOCK`) or remotely (`AF_INET`). Theoretically the underlying transport does not necessarily have to be a socket, however we do not examine such alternatives. In this protocol version we focus on using a UNIX domain socket and introduce basic support for the other two types of sockets without considering performance implications.

While passing of file descriptors is desirable for performance reasons, support is not necessary for either the client or the server in order to implement the protocol. There is always an in-band, message-passing fall back mechanism.

## Overview

VFIO is a framework that allows a physical device to be securely passed through to a user space process; the device-specific kernel driver does not drive the device at all. Typically, the user space process is a VMM and the device is passed through to it in order to achieve high performance. VFIO provides an API and the required functionality in the kernel. QEMU has adopted VFIO to allow a guest to directly access physical devices, instead of emulating them in software.

vfio-user reuses the core VFIO concepts defined in its API, but implements them as messages to be sent over a socket. It does not change the kernel-based VFIO in any way, in fact none of the VFIO kernel modules need to be loaded to use vfio-user. It is also possible for the client to concurrently use the current kernel-based VFIO for one device, and vfio-user for another device.

### VFIO Device Model

A device under VFIO presents a standard interface to the user process. Many of the VFIO operations in the existing interface use the `ioctl()` system call, and references to the existing interface are called the `ioctl()` implementation in this document.

The following sections describe the set of messages that implement the vfio-user interface over a socket. In many cases, the messages are analogous to data structures used in the `ioctl()` implementation. Messages derived from the `ioctl()` will have a name derived from the `ioctl()` command name. E.g., the `VFIO_DEVICE_GET_INFO` `ioctl()` command becomes a `VFIO_USER_DEVICE_GET_INFO` message. The purpose of this reuse is to share as much code as feasible with the `ioctl()` implementation``.

#### Connection Initiation

After the client connects to the server, the initial client message is `VFIO_USER_VERSION` to propose a protocol version and set of capabilities to apply to the session. The server replies with a compatible version and set of capabilities it supports, or closes the connection if it cannot support the advertised version.

#### Device Information

The client uses a `VFIO_USER_DEVICE_GET_INFO` message to query the server for information about the device. This information includes:

- The device type and whether it supports reset (`VFIO_DEVICE_FLAGS_`),

- the number of device regions, and

- the device presents to the client the number of interrupt types the device supports.

#### Region Information

The client uses `VFIO_USER_DEVICE_GET_REGION_INFO` messages to query the server for information about the device's regions. This information describes:

- Read and write permissions, whether it can be memory mapped, and whether it supports additional capabilities (`VFIO_REGION_INFO_CAP_`).

- Region index, size, and offset.

When a device region can be mapped by the client, the server provides a file descriptor which the client can `mmap()`. The server is responsible for polling for client updates to memory mapped regions.

##### Region Capabilities

Some regions have additional capabilities that cannot be described adequately by the region info data structure. These capabilities are returned in the region info reply in a list similar to PCI capabilities in a PCI device's configuration space.

##### Sparse Regions

A region can be memory-mappable in whole or in part. When only a subset of a region can be mapped by the client, a `VFIO_REGION_INFO_CAP_SPARSE_MMAP` capability is included in the region info reply. This capability describes which portions can be mapped by the client.

Note

For example, in a virtual NVMe controller, sparse regions can be used so that accesses to the NVMe registers (found in the beginning of BAR0) are trapped (an infrequent event), while allowing direct access to the doorbells (an extremely frequent event as every I/O submission requires a write to BAR0), found in the next page after the NVMe registers in BAR0.

##### Device-Specific Regions

A device can define regions additional to the standard ones (e.g. PCI indexes 0-8). This is achieved by including a `VFIO_REGION_INFO_CAP_TYPE` capability in the region info reply of a device-specific region. Such regions are reflected in `struct vfio_user_device_info.num_regions`. Thus, for PCI devices this value can be equal to, or higher than, `VFIO_PCI_NUM_REGIONS`.

### Region I/O via file descriptors

For unmapped regions, region I/O from the client is done via `VFIO_USER_REGION_READ/WRITE`. As an optimization, ioeventfds or ioregionfds may be configured for sub-regions of some regions. A client may request information on these sub-regions via `VFIO_USER_DEVICE_GET_REGION_IO_FDS`; by configuring the returned file descriptors as ioeventfds or ioregionfds, the server can be directly notified of I/O (for example, by KVM) without taking a trip through the client.

#### Interrupts

The client uses `VFIO_USER_DEVICE_GET_IRQ_INFO` messages to query the server for the device's interrupt types. The interrupt types are specific to the bus the device is attached to, and the client is expected to know the capabilities of each interrupt type. The server can signal an interrupt by directly injecting interrupts into the guest via an event file descriptor. The client configures how the server signals an interrupt with `VFIO_USER_SET_IRQS` messages.

#### Device Read and Write

When the guest executes load or store operations to an unmapped device region, the client forwards these operations to the server with `VFIO_USER_REGION_READ` or `VFIO_USER_REGION_WRITE` messages. The server will reply with data from the device on read operations or an acknowledgement on write operations. See Read and Write Operations.

### Client memory access

The client uses `VFIO_USER_DMA_MAP` and `VFIO_USER_DMA_UNMAP` messages to inform the server of the valid DMA ranges that the server can access on behalf of a device (typically, VM guest memory). DMA memory may be accessed by the server via `VFIO_USER_DMA_READ` and `VFIO_USER_DMA_WRITE` messages over the socket. In this case, the "DMA" part of the naming is a misnomer.

Actual direct memory access of client memory from the server is possible if the client provides file descriptors the server can `mmap()`. Note that `mmap()` privileges cannot be revoked by the client, therefore file descriptors should only be exported in environments where the client trusts the server not to corrupt guest memory.

See Read and Write Operations.

## Client/server interactions

### Socket

A server can serve:

1. one or more clients, and/or

2. one or more virtual devices, belonging to one or more clients.

The current protocol specification requires a dedicated socket per client/server connection. It is a server-side implementation detail whether a single server handles multiple virtual devices from the same or multiple clients. The location of the socket is implementation-specific. Multiplexing clients, devices, and servers over the same socket is not supported in this version of the protocol.

### Authentication

For `AF_UNIX`, we rely on OS mandatory access controls on the socket files, therefore it is up to the management layer to set up the socket as required. Socket types that span guests or hosts will require a proper authentication mechanism. Defining that mechanism is deferred to a future version of the protocol.

### Command Concurrency

A client may pipeline multiple commands without waiting for previous command replies. The server will process commands in the order they are received. A consequence of this is if a client issues a command with the No_reply bit, then subsequently issues a command without No_reply, the older command will have been processed before the reply to the younger command is sent by the server. The client must be aware of the device's capability to process concurrent commands if pipelining is used. For example, pipelining allows multiple client threads to concurrently access device regions; the client must ensure these accesses obey device semantics.

An example is a frame buffer device, where the device may allow concurrent access to different areas of video memory, but may have indeterminate behavior if concurrent accesses are performed to command or status registers.

Note that unrelated messages sent from the server to the client can appear in between a client to server request/reply and vice versa.

Implementers should be prepared for certain commands to exhibit potentially unbounded latencies. For example, `VFIO_USER_DEVICE_RESET` may take an arbitrarily long time to complete; clients should take care not to block unnecessarily.

### Socket Disconnection Behavior

The server and the client can disconnect from each other, either intentionally or unexpectedly. Both the client and the server need to know how to handle such events.

#### Server Disconnection

A server disconnecting from the client may indicate that:

1. A virtual device has been restarted, either intentionally (e.g. because of a device update) or unintentionally (e.g. because of a crash).

2. A virtual device has been shut down with no intention to be restarted.

It is impossible for the client to know whether or not a failure is intermittent or innocuous and should be retried, therefore the client should reset the VFIO device when it detects the socket has been disconnected. Error recovery will be driven by the guest's device error handling behavior.

#### Client Disconnection

The client disconnecting from the server primarily means that the client has exited. Currently, this means that the guest is shut down so the device is no longer needed therefore the server can automatically exit. However, there can be cases where a client disconnection should not result in a server exit:

1. A single server serving multiple clients.

2. A multi-process QEMU upgrading itself step by step, which is not yet implemented.

Therefore in order for the protocol to be forward compatible, the server should respond to a client disconnection as follows:

> - all client memory regions are unmapped and cleaned up (including closing any passed file descriptors)

> - all IRQ file descriptors passed from the old client are closed

> - the device state should otherwise be retained

The expectation is that when a client reconnects, it will re-establish IRQ and client memory mappings.

If anything happens to the client (such as qemu really did exit), the control stack will know about it and can clean up resources accordingly.

### Security Considerations

Speaking generally, vfio-user clients should not trust servers, and vice versa. Standard tools and mechanisms should be used on both sides to validate input and prevent against denial of service scenarios, buffer overflow, etc.

### Request Retry and Response Timeout

A failed command is a command that has been successfully sent and has been responded to with an error code. Failure to send the command in the first place (e.g. because the socket is disconnected) is a different type of error examined earlier in the disconnect section.

Note

QEMU's VFIO retries certain operations if they fail. While this makes sense for real HW, we don't know for sure whether it makes sense for virtual devices.

Defining a retry and timeout scheme is deferred to a future version of the protocol.

### Message sizes

Some requests have an `argsz` field. In a request, it defines the maximum expected reply payload size, which should be at least the size of the fixed reply payload headers defined here. The request payload size is defined by the usual `msg_size` field in the header, not the `argsz` field.

In a reply, the server sets `argsz` field to the size needed for a full payload size. This may be less than the requested maximum size. This may be larger than the requested maximum size: in that case, the full payload is not included in the reply, but the `argsz` field in the reply indicates the needed size, allowing a client to allocate a larger buffer for holding the reply before trying again.

In addition, during negotiation (see Version), the client and server may each specify a `max_data_xfer_size` value; this defines the maximum data that may be read or written via one of the `VFIO_USER_DMA/REGION_READ/WRITE` messages; see Read and Write Operations.

## Protocol Specification

To distinguish from the base VFIO symbols, all vfio-user symbols are prefixed with `vfio_user` or `VFIO_USER`. In this revision, all data is in the endianness of the host system, although this may be relaxed in future revisions in cases where the client and server run on different hosts with different endianness.

Unless otherwise specified, all sizes should be presumed to be in bytes.

### Commands

The following table lists the VFIO message command IDs, and whether the message command is sent from the client or the server.

Name | Command | Request Direction
--- | --- | ---
`VFIO_USER_VERSION` | 1 | client -> server
`VFIO_USER_DMA_MAP` | 2 | client -> server
`VFIO_USER_DMA_UNMAP` | 3 | client -> server
`VFIO_USER_DEVICE_GET_INFO` | 4 | client -> server
`VFIO_USER_DEVICE_GET_REGION_INFO` | 5 | client -> server
`VFIO_USER_DEVICE_GET_REGION_IO_FDS` | 6 | client -> server
`VFIO_USER_DEVICE_GET_IRQ_INFO` | 7 | client -> server
`VFIO_USER_DEVICE_SET_IRQS` | 8 | client -> server
`VFIO_USER_REGION_READ` | 9 | client -> server
`VFIO_USER_REGION_WRITE` | 10 | client -> server
`VFIO_USER_DMA_READ` | 11 | server -> client
`VFIO_USER_DMA_WRITE` | 12 | server -> client
`VFIO_USER_DEVICE_RESET` | 13 | client -> server
`VFIO_USER_REGION_WRITE_MULTI` | 15 | client -> server

### Header

All messages, both command messages and reply messages, are preceded by a 16-byte header that contains basic information about the message. The header is followed by message-specific data described in the sections below.

Name | Offset | Size
--- | --- | ---
Message ID | 0 | 2
Command | 2 | 2
Message size | 4 | 4
Flags | 8 | 4

Bit | Definition
--- | ---
0-3 | Type
4 | No_reply
5 | Error

Error | 12 | 4
<message data> | 16 | variable

- Message ID identifies the message, and is echoed in the command's reply message. Message IDs belong entirely to the sender, can be re-used (even concurrently) and the receiver must not make any assumptions about their uniqueness.

- Command specifies the command to be executed, listed in Commands. It is also set in the reply header.

- Message size contains the size of the entire message, including the header.

- Flags contains attributes of the message:

  - The Type bits indicate the message type.

    - Command (value 0x0) indicates a command message.

    - Reply (value 0x1) indicates a reply message acknowledging a previous command with the same message ID.

  - No_reply in a command message indicates that no reply is needed for this command. This is commonly used when multiple commands are sent, and only the last needs acknowledgement.

  - Error in a reply message indicates the command being acknowledged had an error. In this case, the Error field will be valid.

- Error in a reply message is an optional UNIX errno value. It may be zero even if the Error bit is set in Flags. It is reserved in a command message.

Each command message in Commands must be replied to with a reply message, unless the message sets the No_Reply bit. The reply consists of the header with the Reply bit set, plus any additional data.

If an error occurs, the reply message must only include the reply header.

As the header is standard in both requests and replies, it is not included in the command-specific specifications below; each message definition should be appended to the standard header, and the offsets are given from the end of the standard header.

### `VFIO_USER_VERSION`

This is the initial message sent by the client after the socket connection is established; the same format is used for the server's reply.

Upon establishing a connection, the client must send a `VFIO_USER_VERSION` message proposing a protocol version and a set of capabilities. The server compares these with the versions and capabilities it supports and sends a `VFIO_USER_VERSION` reply according to the following rules.

- The major version in the reply must be the same as proposed. If the client does not support the proposed major, it closes the connection.

- The minor version in the reply must be equal to or less than the minor version proposed.

- The capability list must be a subset of those proposed. If the server requires a capability the client did not include, it closes the connection.

The protocol major version will only change when incompatible protocol changes are made, such as changing the message format. The minor version may change when compatible changes are made, such as adding new messages or capabilities, Both the client and server must support all minor versions less than the maximum minor version it supports. E.g., an implementation that supports version 1.3 must also support 1.0 through 1.2.

When making a change to this specification, the protocol version number must be included in the form "added in version X.Y"

#### Request

Name | Offset | Size
--- | --- | ---
version major | 0 | 2
version minor | 2 | 2
version data | 4 | variable (including terminating NUL). Optional.

The version data is an optional UTF-8 encoded JSON byte array with the following format:

Name | Type | Description
--- | --- | ---
capabilities | object | Contains common capabilities that the sender supports. Optional.

Capabilities:

Name | Type | Description
--- | --- | ---
max_msg_fds | number | Maximum number of file descriptors that can be received by the sender in one message. Optional. If not specified then the receiver must assume a value of `1`.
max_data_xfer_size | number | Maximum `count` for data transfer messages; see Read and Write Operations. Optional, with a default value of 1048576 bytes.
pgsizes | number | Page sizes supported in DMA map operations or'ed together. Optional, with a default value of supporting only 4k pages.
max_dma_maps | number | Maximum number DMA map windows that can be valid simultaneously. Optional, with a value of 65535 (64k-1).
migration | object | Migration capability parameters. If missing then migration is not supported by the sender.
write_multiple | boolean | `VFIO_USER_REGION_WRITE_MULTI` messages are supported if the value is `true`.

The migration capability contains the following name/value pairs:

Name | Type | Description
--- | --- | ---
pgsize | number | Page size of dirty pages bitmap. The smallest between the client and the server is used.
max_bitmap_size | number | Maximum bitmap size in `VFIO_USER_DIRTY_PAGES` and `VFIO_DMA_UNMAP` messages. Optional, with a default value of 256MB.

#### Reply

The same message format is used in the server's reply with the semantics described above.

### `VFIO_USER_DMA_MAP`

This command message is sent by the client to the server to inform it of the memory regions the server can access. It must be sent before the server can perform any DMA to the client. It is normally sent directly after the version handshake is completed, but may also occur when memory is added to the client, or if the client uses a vIOMMU.

#### Request

The request payload for this message is a structure of the following format:

Name | Offset | Size
--- | --- | ---
argsz | 0 | 4
flags | 4 | 4

Bit | Definition
--- | ---
0 | readable
1 | writeable

offset | 8 | 8
address | 16 | 8
size | 24 | 8

- argsz is the size of the above structure. Note there is no reply payload, so this field differs from other message types.

- flags contains the following region attributes:

  - readable indicates that the region can be read from.

  - writeable indicates that the region can be written to.

- offset is the file offset of the region with respect to the associated file descriptor, or zero if the region is not mappable

- address is the base DMA address of the region.

- size is the size of the region.

This structure is 32 bytes in size, so the message size is 16 + 32 bytes.

If the DMA region being added can be directly mapped by the server, a file descriptor must be sent as part of the message meta-data. The region can be mapped via the mmap() system call. On `AF_UNIX` sockets, the file descriptor must be passed as `SCM_RIGHTS` type ancillary data. Otherwise, if the DMA region cannot be directly mapped by the server, no file descriptor must be sent as part of the message meta-data and the DMA region can be accessed by the server using `VFIO_USER_DMA_READ` and `VFIO_USER_DMA_WRITE` messages, explained in Read and Write Operations. A command to map over an existing region must be failed by the server with `EEXIST` set in error field in the reply.

#### Reply

There is no payload in the reply message.

### `VFIO_USER_DMA_UNMAP`

This command message is sent by the client to the server to inform it that a DMA region, previously made available via a `VFIO_USER_DMA_MAP` command message, is no longer available for DMA. It typically occurs when memory is subtracted from the client or if the client uses a vIOMMU. The DMA region is described by the following structure:

#### Request

The request payload for this message is a structure of the following format:

Name | Offset | Size
--- | --- | ---
argsz | 0 | 4
flags | 4 | 4
address | 8 | 8
size | 16 | 8

- argsz is the maximum size of the reply payload.

- flags is unused in this version.

- address is the base DMA address of the DMA region.

- size is the size of the DMA region.

The address and size of the DMA region being unmapped must match exactly a previous mapping.

#### Reply

Upon receiving a `VFIO_USER_DMA_UNMAP` command, if the file descriptor is mapped then the server must release all references to that DMA region before replying, which potentially includes in-flight DMA transactions.

The server responds with the original DMA entry in the request.

### `VFIO_USER_DEVICE_GET_INFO`

This command message is sent by the client to the server to query for basic information about the device.

#### Request

Name | Offset | Size
--- | --- | ---
argsz | 0 | 4
flags | 4 | 4

Bit | Definition
--- | ---
0 | VFIO_DEVICE_FLAGS_RESET
1 | VFIO_DEVICE_FLAGS_PCI

num_regions | 8 | 4
num_irqs | 12 | 4

- argsz is the maximum size of the reply payload

- all other fields must be zero.

#### Reply

Name | Offset | Size
--- | --- | ---
argsz | 0 | 4
flags | 4 | 4

Bit | Definition
--- | ---
0 | VFIO_DEVICE_FLAGS_RESET
1 | VFIO_DEVICE_FLAGS_PCI

num_regions | 8 | 4
num_irqs | 12 | 4

- argsz is the size required for the full reply payload (16 bytes today)

- flags contains the following device attributes.

  - `VFIO_DEVICE_FLAGS_RESET` indicates that the device supports the `VFIO_USER_DEVICE_RESET` message.

  - `VFIO_DEVICE_FLAGS_PCI` indicates that the device is a PCI device.

- num_regions is the number of memory regions that the device exposes.

- num_irqs is the number of distinct interrupt types that the device supports.

This version of the protocol only supports PCI devices. Additional devices may be supported in future versions.

### `VFIO_USER_DEVICE_GET_REGION_INFO`

This command message is sent by the client to the server to query for information about device regions. The VFIO region info structure is defined in `<linux/vfio.h>` (`struct vfio_region_info`).

#### Request

Name | Offset | Size
--- | --- | ---
argsz | 0 | 4
flags | 4 | 4
index | 8 | 4
cap_offset | 12 | 4
size | 16 | 8
offset | 24 | 8

- argsz the maximum size of the reply payload

- index is the index of memory region being queried, it is the only field that is required to be set in the command message.

- all other fields must be zero.

#### Reply

Name | Offset | Size
--- | --- | ---
argsz | 0 | 4
flags | 4 | 4

Bit | Definition
--- | ---
0 | VFIO_REGION_INFO_FLAG_READ
1 | VFIO_REGION_INFO_FLAG_WRITE
2 | VFIO_REGION_INFO_FLAG_MMAP
3 | VFIO_REGION_INFO_FLAG_CAPS

index | 8 | 4
cap_offset | 12 | 4
size | 16 | 8
offset | 24 | 8

- argsz is the size required for the full reply payload (region info structure plus the size of any region capabilities)

- flags are attributes of the region:

  - `VFIO_REGION_INFO_FLAG_READ` allows client read access to the region.

  - `VFIO_REGION_INFO_FLAG_WRITE` allows client write access to the region.

  - `VFIO_REGION_INFO_FLAG_MMAP` specifies the client can mmap() the region. When this flag is set, the reply will include a file descriptor in its meta-data. On `AF_UNIX` sockets, the file descriptors will be passed as `SCM_RIGHTS` type ancillary data.

  - `VFIO_REGION_INFO_FLAG_CAPS` indicates additional capabilities found in the reply.

- index is the index of memory region being queried, it is the only field that is required to be set in the command message.

- cap_offset describes where additional region capabilities can be found. cap_offset is relative to the beginning of the VFIO region info structure. The data structure it points is a VFIO cap header defined in `<linux/vfio.h>`.

- size is the size of the region.

- offset is the offset that should be given to the mmap() system call for regions with the MMAP attribute. It is also used as the base offset when mapping a VFIO sparse mmap area, described below.

##### VFIO region capabilities

The VFIO region information can also include a capabilities list. This list is similar to a PCI capability list - each entry has a common header that identifies a capability and where the next capability in the list can be found. The VFIO capability header format is defined in `<linux/vfio.h>` (`struct vfio_info_cap_header`).

##### VFIO cap header format

Name | Offset | Size
--- | --- | ---
id | 0 | 2
version | 2 | 2
next | 4 | 4

- id is the capability identity.

- version is a capability-specific version number.

- next specifies the offset of the next capability in the capability list. It is relative to the beginning of the VFIO region info structure.

##### VFIO sparse mmap cap header

Name | Value
--- | ---
id | VFIO_REGION_INFO_CAP_SPARSE_MMAP
version | 0x1
next | <next>
sparse mmap info | VFIO region info sparse mmap

This capability is defined when only a subrange of the region supports direct access by the client via mmap(). The VFIO sparse mmap area is defined in `<linux/vfio.h>` (`struct vfio_region_sparse_mmap_area` and `struct vfio_region_info_cap_sparse_mmap`).

##### VFIO region info cap sparse mmap

Name | Offset | Size
--- | --- | ---
nr_areas | 0 | 4
reserved | 4 | 4
offset | 8 | 8
size | 16 | 8
... |  |

- nr_areas is the number of sparse mmap areas in the region.

- offset and size describe a single area that can be mapped by the client. There will be nr_areas pairs of offset and size. The offset will be added to the base offset given in the `VFIO_USER_DEVICE_GET_REGION_INFO` to form the offset argument of the subsequent mmap() call.

The VFIO sparse mmap area is defined in `<linux/vfio.h>` (`struct vfio_region_info_cap_sparse_mmap`).

### `VFIO_USER_DEVICE_GET_REGION_IO_FDS`

Clients can access regions via `VFIO_USER_REGION_READ/WRITE` or, if provided, by `mmap()` of a file descriptor provided by the server.

`VFIO_USER_DEVICE_GET_REGION_IO_FDS` provides an alternative access mechanism via file descriptors. This is an optional feature intended for performance improvements where an underlying sub-system (such as KVM) supports communication across such file descriptors to the vfio-user server, without needing to round-trip through the client.

The server returns an array of sub-regions for the requested region. Each sub-region describes a span (offset and size) of a region, along with the requested file descriptor notification mechanism to use. Each sub-region in the response message may choose to use a different method, as defined below. The two mechanisms supported in this specification are ioeventfds and ioregionfds.

The server in addition returns a file descriptor in the ancillary data; clients are expected to configure each sub-region's file descriptor with the requested notification method. For example, a client could configure KVM with the requested ioeventfd via a `KVM_IOEVENTFD` `ioctl()`.

#### Request

Name | Offset | Size
--- | --- | ---
argsz | 0 | 4
flags | 4 | 4
index | 8 | 4
count | 12 | 4

- argsz the maximum size of the reply payload

- index is the index of memory region being queried

- all other fields must be zero

The client must set `flags` to zero and specify the region being queried in the `index`.

#### Reply

Name | Offset | Size
--- | --- | ---
argsz | 0 | 4
flags | 4 | 4
index | 8 | 4
count | 12 | 4
sub-regions | 16 | ...

- argsz is the size of the region IO FD info structure plus the total size of the sub-region array. Thus, each array entry "i" is at offset i * ((argsz - 32) / count). Note that currently this is 40 bytes for both IO FD types, but this is not to be relied on. As elsewhere, this indicates the full reply payload size needed.

- flags must be zero

- index is the index of memory region being queried

- count is the number of sub-regions in the array

- sub-regions is the array of Sub-Region IO FD info structures

The reply message will additionally include at least one file descriptor in the ancillary data. Note that more than one sub-region may share the same file descriptor.

Note that it is the client's responsibility to verify the requested values (for example, that the requested offset does not exceed the region's bounds).

Each sub-region given in the response has one of two possible structures, depending whether type is `VFIO_USER_IO_FD_TYPE_IOEVENTFD` or `VFIO_USER_IO_FD_TYPE_IOREGIONFD`:

##### Sub-Region IO FD info format (ioeventfd)

Name | Offset | Size
--- | --- | ---
offset | 0 | 8
size | 8 | 8
fd_index | 16 | 4
type | 20 | 4
flags | 24 | 4
padding | 28 | 4
datamatch | 32 | 8

- offset is the offset of the start of the sub-region within the region requested ("physical address offset" for the region)

- size is the length of the sub-region. This may be zero if the access size is not relevant, which may allow for optimizations

- fd_index is the index in the ancillary data of the FD to use for ioeventfd notification; it may be shared.

- type is `VFIO_USER_IO_FD_TYPE_IOEVENTFD`

- flags is any of:

  - `KVM_IOEVENTFD_FLAG_DATAMATCH`

  - `KVM_IOEVENTFD_FLAG_PIO`

  - `KVM_IOEVENTFD_FLAG_VIRTIO_CCW_NOTIFY` (FIXME: makes sense?)

- datamatch is the datamatch value if needed

See https://www.kernel.org/doc/Documentation/virtual/kvm/api.txt, 4.59 KVM_IOEVENTFD for further context on the ioeventfd-specific fields.

##### Sub-Region IO FD info format (ioregionfd)

Name | Offset | Size
--- | --- | ---
offset | 0 | 8
size | 8 | 8
fd_index | 16 | 4
type | 20 | 4
flags | 24 | 4
padding | 28 | 4
user_data | 32 | 8

- offset is the offset of the start of the sub-region within the region requested ("physical address offset" for the region)

- size is the length of the sub-region. This may be zero if the access size is not relevant, which may allow for optimizations; `KVM_IOREGION_POSTED_WRITES` must be set in flags in this case

- fd_index is the index in the ancillary data of the FD to use for ioregionfd messages; it may be shared

- type is `VFIO_USER_IO_FD_TYPE_IOREGIONFD`

- flags is any of:

  - `KVM_IOREGION_PIO`

  - `KVM_IOREGION_POSTED_WRITES`

- user_data is an opaque value passed back to the server via a message on the file descriptor

For further information on the ioregionfd-specific fields, see: https://lore.kernel.org/kvm/cover.1613828726.git.eafanasova@gmail.com/

(FIXME: update with final API docs.)

### `VFIO_USER_DEVICE_GET_IRQ_INFO`

This command message is sent by the client to the server to query for information about device interrupt types. The VFIO IRQ info structure is defined in `<linux/vfio.h>` (`struct vfio_irq_info`).

#### Request

Name | Offset | Size
--- | --- | ---
argsz | 0 | 4
flags | 4 | 4

Bit | Definition
--- | ---
0 | VFIO_IRQ_INFO_EVENTFD
1 | VFIO_IRQ_INFO_MASKABLE
2 | VFIO_IRQ_INFO_AUTOMASKED
3 | VFIO_IRQ_INFO_NORESIZE

index | 8 | 4
count | 12 | 4

- argsz is the maximum size of the reply payload (16 bytes today)

- index is the index of IRQ type being queried (e.g. `VFIO_PCI_MSIX_IRQ_INDEX`)

- all other fields must be zero

#### Reply

Name | Offset | Size
--- | --- | ---
argsz | 0 | 4
flags | 4 | 4

Bit | Definition
--- | ---
0 | VFIO_IRQ_INFO_EVENTFD
1 | VFIO_IRQ_INFO_MASKABLE
2 | VFIO_IRQ_INFO_AUTOMASKED
3 | VFIO_IRQ_INFO_NORESIZE

index | 8 | 4
count | 12 | 4

- argsz is the size required for the full reply payload (16 bytes today)

- flags defines IRQ attributes:

  - `VFIO_IRQ_INFO_EVENTFD` indicates the IRQ type can support server eventfd signalling.

  - `VFIO_IRQ_INFO_MASKABLE` indicates that the IRQ type supports the `MASK` and `UNMASK` actions in a `VFIO_USER_DEVICE_SET_IRQS` message.

  - `VFIO_IRQ_INFO_AUTOMASKED` indicates the IRQ type masks itself after being triggered, and the client must send an `UNMASK` action to receive new interrupts.

  - `VFIO_IRQ_INFO_NORESIZE` indicates `VFIO_USER_SET_IRQS` operations setup interrupts as a set, and new sub-indexes cannot be enabled without disabling the entire type.

- index is the index of IRQ type being queried

- count describes the number of interrupts of the queried type.

### `VFIO_USER_DEVICE_SET_IRQS`

This command message is sent by the client to the server to set actions for device interrupt types. The VFIO IRQ set structure is defined in `<linux/vfio.h>` (`struct vfio_irq_set`).

#### Request

Name | Offset | Size
--- | --- | ---
argsz | 0 | 4
flags | 4 | 4

Bit | Definition
--- | ---
0 | VFIO_IRQ_SET_DATA_NONE
1 | VFIO_IRQ_SET_DATA_BOOL
2 | VFIO_IRQ_SET_DATA_EVENTFD
3 | VFIO_IRQ_SET_ACTION_MASK
4 | VFIO_IRQ_SET_ACTION_UNMASK
5 | VFIO_IRQ_SET_ACTION_TRIGGER

index | 8 | 4
start | 12 | 4
count | 16 | 4
data | 20 | variable

- argsz is the size of the VFIO IRQ set request payload, including any data field. Note there is no reply payload, so this field differs from other message types.

- flags defines the action performed on the interrupt range. The `DATA` flags describe the data field sent in the message; the `ACTION` flags describe the action to be performed. The flags are mutually exclusive for both sets.

  - `VFIO_IRQ_SET_DATA_NONE` indicates there is no data field in the command. The action is performed unconditionally.

  - `VFIO_IRQ_SET_DATA_BOOL` indicates the data field is an array of boolean bytes. The action is performed if the corresponding boolean is true.

  - `VFIO_IRQ_SET_DATA_EVENTFD` indicates an array of event file descriptors was sent in the message meta-data. These descriptors will be signalled when the action defined by the action flags occurs. In `AF_UNIX` sockets, the descriptors are sent as `SCM_RIGHTS` type ancillary data. If no file descriptors are provided, this de-assigns the specified previously configured interrupts.

  - `VFIO_IRQ_SET_ACTION_MASK` indicates a masking event. It can be used with `VFIO_IRQ_SET_DATA_BOOL` or `VFIO_IRQ_SET_DATA_NONE` to mask an interrupt, or with `VFIO_IRQ_SET_DATA_EVENTFD` to generate an event when the guest masks the interrupt.

  - `VFIO_IRQ_SET_ACTION_UNMASK` indicates an unmasking event. It can be used with `VFIO_IRQ_SET_DATA_BOOL` or `VFIO_IRQ_SET_DATA_NONE` to unmask an interrupt, or with `VFIO_IRQ_SET_DATA_EVENTFD` to generate an event when the guest unmasks the interrupt.

  - `VFIO_IRQ_SET_ACTION_TRIGGER` indicates a triggering event. It can be used with `VFIO_IRQ_SET_DATA_BOOL` or `VFIO_IRQ_SET_DATA_NONE` to trigger an interrupt, or with `VFIO_IRQ_SET_DATA_EVENTFD` to generate an event when the server triggers the interrupt.

- index is the index of IRQ type being setup.

- start is the start of the sub-index being set.

- count describes the number of sub-indexes being set. As a special case, a count (and start) of 0, with data flags of `VFIO_IRQ_SET_DATA_NONE` disables all interrupts of the index.

- data is an optional field included when the `VFIO_IRQ_SET_DATA_BOOL` flag is present. It contains an array of booleans that specify whether the action is to be performed on the corresponding index. It's used when the action is only performed on a subset of the range specified.

Not all interrupt types support every combination of data and action flags. The client must know the capabilities of the device and IRQ index before it sends a `VFIO_USER_DEVICE_SET_IRQ` message.

In typical operation, a specific IRQ may operate as follows:

1. The client sends a `VFIO_USER_DEVICE_SET_IRQ` message with `flags=(VFIO_IRQ_SET_DATA_EVENTFD|VFIO_IRQ_SET_ACTION_TRIGGER)` along with an eventfd. This associates the IRQ with a particular eventfd on the server side.

2. The client may send a `VFIO_USER_DEVICE_SET_IRQ` message with `flags=(VFIO_IRQ_SET_DATA_EVENTFD|VFIO_IRQ_SET_ACTION_MASK/UNMASK)` along with another eventfd. This associates the given eventfd with the mask/unmask state on the server side.

3. The server may trigger the IRQ by writing 1 to the eventfd.

4. The server may mask/unmask an IRQ which will write 1 to the corresponding mask/unmask eventfd, if there is one.

5. A client may trigger a device IRQ itself, by sending a `VFIO_USER_DEVICE_SET_IRQ` message with `flags=(VFIO_IRQ_SET_DATA_NONE/BOOL|VFIO_IRQ_SET_ACTION_TRIGGER)`.

6. A client may mask or unmask the IRQ, by sending a `VFIO_USER_DEVICE_SET_IRQ` message with `flags=(VFIO_IRQ_SET_DATA_NONE/BOOL|VFIO_IRQ_SET_ACTION_MASK/UNMASK)`.

#### Reply

There is no payload in the reply.

Note that all of these operations must be supported by the client and/or server, even if the corresponding memory or device region has been shared as mappable.

The `count` field must not exceed the value of `max_data_xfer_size` of the peer, for both reads and writes.

### `VFIO_USER_REGION_READ`

If a device region is not mappable, it's not directly accessible by the client via `mmap()` of the underlying file descriptor. In this case, a client can read from a device region with this message.

#### Request

Name | Offset | Size
--- | --- | ---
offset | 0 | 8
region | 8 | 4
count | 12 | 4

- offset into the region being accessed.

- region is the index of the region being accessed.

- count is the size of the data to be transferred.

#### Reply

Name | Offset | Size
--- | --- | ---
offset | 0 | 8
region | 8 | 4
count | 12 | 4
data | 16 | variable

- offset into the region accessed.

- region is the index of the region accessed.

- count is the size of the data transferred.

- data is the data that was read from the device region.

### `VFIO_USER_REGION_WRITE`

If a device region is not mappable, it's not directly accessible by the client via mmap() of the underlying fd. In this case, a client can write to a device region with this message.

#### Request

Name | Offset | Size
--- | --- | ---
offset | 0 | 8
region | 8 | 4
count | 12 | 4
data | 16 | variable

- offset into the region being accessed.

- region is the index of the region being accessed.

- count is the size of the data to be transferred.

- data is the data to write

#### Reply

Name | Offset | Size
--- | --- | ---
offset | 0 | 8
region | 8 | 4
count | 12 | 4

- offset into the region accessed.

- region is the index of the region accessed.

- count is the size of the data transferred.

### `VFIO_USER_DMA_READ`

If the client has not shared mappable memory, the server can use this message to read from guest memory.

#### Request

Name | Offset | Size
--- | --- | ---
address | 0 | 8
count | 8 | 8

- address is the client DMA memory address being accessed. This address must have been previously exported to the server with a `VFIO_USER_DMA_MAP` message.

- count is the size of the data to be transferred.

#### Reply

Name | Offset | Size
--- | --- | ---
address | 0 | 8
count | 8 | 8
data | 16 | variable

- address is the client DMA memory address being accessed.

- count is the size of the data transferred.

- data is the data read.

### `VFIO_USER_DMA_WRITE`

If the client has not shared mappable memory, the server can use this message to write to guest memory.

#### Request

Name | Offset | Size
--- | --- | ---
address | 0 | 8
count | 8 | 8
data | 16 | variable

- address is the client DMA memory address being accessed. This address must have been previously exported to the server with a `VFIO_USER_DMA_MAP` message.

- count is the size of the data to be transferred.

- data is the data to write

#### Reply

Name | Offset | Size
--- | --- | ---
address | 0 | 8
count | 8 | 4

- address is the client DMA memory address being accessed.

- count is the size of the data transferred.

### `VFIO_USER_DEVICE_RESET`

This command message is sent from the client to the server to reset the device. Neither the request or reply have a payload.

### `VFIO_USER_REGION_WRITE_MULTI`

This message can be used to coalesce multiple device write operations into a single messgage. It is only used as an optimization when the outgoing message queue is relatively full.

#### Request

Name | Offset | Size
--- | --- | ---
wr_cnt | 0 | 8
wrs | 8 | variable

- wr_cnt is the number of device writes coalesced in the message

- wrs is an array of device writes defined below

##### Single Device Write Format

Name | Offset | Size
--- | --- | ---
offset | 0 | 8
region | 8 | 4
count | 12 | 4
data | 16 | 8

- offset into the region being accessed.

- region is the index of the region being accessed.

- count is the size of the data to be transferred. This format can only describe writes of 8 bytes or less.

- data is the data to write.

#### Reply

Name | Offset | Size
--- | --- | ---
wr_cnt | 0 | 8

- wr_cnt is the number of device writes completed.

## Appendices

### Unused VFIO `ioctl()` commands

The following VFIO commands do not have an equivalent vfio-user command:

- `VFIO_GET_API_VERSION`

- `VFIO_CHECK_EXTENSION`

- `VFIO_SET_IOMMU`

- `VFIO_GROUP_GET_STATUS`

- `VFIO_GROUP_SET_CONTAINER`

- `VFIO_GROUP_UNSET_CONTAINER`

- `VFIO_GROUP_GET_DEVICE_FD`

- `VFIO_IOMMU_GET_INFO`

However, once support for live migration for VFIO devices is finalized some of the above commands may have to be handled by the client in their corresponding vfio-user form. This will be addressed in a future protocol version.

#### VFIO groups and containers

The current VFIO implementation includes group and container idioms that describe how a device relates to the host IOMMU. In the vfio-user implementation, the IOMMU is implemented in SW by the client, and is not visible to the server. The simplest idea would be that the client put each device into its own group and container.

### Backend Program Conventions

vfio-user backend program conventions are based on the vhost-user ones.

- The backend program must not daemonize itself.

- No assumptions must be made as to what access the backend program has on the system.

- File descriptors 0, 1 and 2 must exist, must have regular stdin/stdout/stderr semantics, and can be redirected.

- The backend program must honor the SIGTERM signal.

- The backend program must accept the following commands line options:

  - `--socket-path=PATH`: path to UNIX domain socket,

  - `--fd=FDNUM`: file descriptor for UNIX domain socket, incompatible with `--socket-path`

- The backend program must be accompanied with a JSON file stored under `/usr/share/vfio-user`.

TODO add schema similar to docs/interop/vhost-user.json.