Vhost-user Protocol — QEMU 5.1.0 (v5.1.0-11824-g8699890d91-dirty) documentation

# Vhost-user Protocol

Copyright

  2014 Virtual Open Systems Sarl.

Copyright

  2019 Intel Corporation

Licence

  This work is licensed under the terms of the GNU GPL, version 2 or later. See the COPYING file in the top-level directory.

Table of Contents

- Vhost-user Protocol

  - Introduction

  - Message Specification

    - Header

    - Payload

      - A single 64-bit integer

      - A vring state description

      - A vring address description

      - Memory regions description

      - Log description

      - An IOTLB message

      - Virtio device config space

      - Vring area description

      - Inflight description

    - C structure

  - Communication

    - Starting and stopping rings

    - Multiple queue support

    - Migration

    - Memory access

    - IOMMU support

    - Slave communication

    - Inflight I/O tracking

    - In-band notifications

    - Protocol features

    - Master message types

    - Slave message types

    - VHOST_USER_PROTOCOL_F_REPLY_ACK

  - Backend program conventions

    - vhost-user-input

    - vhost-user-gpu

    - vhost-user-blk

## Introduction

This protocol is aiming to complement the `ioctl` interface used to control the vhost implementation in the Linux kernel. It implements the control plane needed to establish virtqueue sharing with a user space process on the same host. It uses communication over a Unix domain socket to share file descriptors in the ancillary data of the message.

The protocol defines 2 sides of the communication, master and slave. Master is the application that shares its virtqueues, in our case QEMU. Slave is the consumer of the virtqueues.

In the current implementation QEMU is the master, and the slave is the external process consuming the virtio queues, for example a software Ethernet switch running in user space, such as Snabbswitch, or a block device backend processing read & write to a virtual disk. In order to facilitate interoperability between various backend implementations, it is recommended to follow the Backend program conventions.

Master and slave can be either a client (i.e. connecting) or server (listening) in the socket communication.

## Message Specification

Note

All numbers are in the machine native byte order.

A vhost-user message consists of 3 header fields and a payload.

request | flags | size | payload

### Header

request

  32-bit type of the request

flags

  32-bit bit field

- Lower 2 bits are the version (currently 0x01)

- Bit 2 is the reply flag - needs to be sent on each reply from the slave

- Bit 3 is the need_reply flag - see REPLY_ACK for details.

size

  32-bit size of the payload

### Payload

Depending on the request type, payload can be:

#### A single 64-bit integer

u64

u64

  a 64-bit unsigned integer

#### A vring state description

index | num

index

  a 32-bit index

num

  a 32-bit number

#### A vring address description

index | flags | size | descriptor | used | available | log

index

  a 32-bit vring index

flags

  a 32-bit vring flags

descriptor

  a 64-bit ring address of the vring descriptor table

used

  a 64-bit ring address of the vring used ring

available

  a 64-bit ring address of the vring available ring

log

  a 64-bit guest address for logging

Note that a ring address is an IOVA if `VIRTIO_F_IOMMU_PLATFORM` has been negotiated. Otherwise it is a user address.

#### Memory regions description

num regions | padding | region0 | … | region7

num regions

  a 32-bit number of regions

padding

  32-bit

A region is:

guest address | size | user address | mmap offset

guest address

  a 64-bit guest address of the region

size

  a 64-bit size

user address

  a 64-bit user address

mmap offset

  64-bit offset where region starts in the mapped memory

#### Log description

log size | log offset

log size

  size of area used for logging

log offset

  offset from start of supplied file descriptor where logging starts (i.e. where guest address 0 would be logged)

#### An IOTLB message

iova | size | user address | permissions flags | type

iova

  a 64-bit I/O virtual address programmed by the guest

size

  a 64-bit size

user address

  a 64-bit user address

permissions flags

  an 8-bit value: - 0: No access - 1: Read access - 2: Write access - 3: Read/Write access

type

  an 8-bit IOTLB message type: - 1: IOTLB miss - 2: IOTLB update - 3: IOTLB invalidate - 4: IOTLB access fail

#### Virtio device config space

offset | size | flags | payload

offset

  a 32-bit offset of virtio device’s configuration space

size

  a 32-bit configuration space access size in bytes

flags

  a 32-bit value: - 0: Vhost master messages used for writeable fields - 1: Vhost master messages used for live migration

payload

  Size bytes array holding the contents of the virtio device’s configuration space

#### Vring area description

u64 | size | offset

u64

  a 64-bit integer contains vring index and flags

size

  a 64-bit size of this area

offset

  a 64-bit offset of this area from the start of the supplied file descriptor

#### Inflight description

mmap size | mmap offset | num queues | queue size

mmap size

  a 64-bit size of area to track inflight I/O

mmap offset

  a 64-bit offset of this area from the start of the supplied file descriptor

num queues

  a 16-bit number of virtqueues

queue size

  a 16-bit size of virtqueues

### C structure

In QEMU the vhost-user message is implemented with the following struct:

    typedef struct VhostUserMsg {
        VhostUserRequest request;
        uint32_t flags;
        uint32_t size;
        union {
            uint64_t u64;
            struct vhost_vring_state state;
            struct vhost_vring_addr addr;
            VhostUserMemory memory;
            VhostUserLog log;
            struct vhost_iotlb_msg iotlb;
            VhostUserConfig config;
            VhostUserVringArea area;
            VhostUserInflight inflight;
        };
    } QEMU_PACKED VhostUserMsg;

## Communication

The protocol for vhost-user is based on the existing implementation of vhost for the Linux Kernel. Most messages that can be sent via the Unix domain socket implementing vhost-user have an equivalent ioctl to the kernel implementation.

The communication consists of master sending message requests and slave sending message replies. Most of the requests don’t require replies. Here is a list of the ones that do:

- `VHOST_USER_GET_FEATURES`

- `VHOST_USER_GET_PROTOCOL_FEATURES`

- `VHOST_USER_GET_VRING_BASE`

- `VHOST_USER_SET_LOG_BASE` (if `VHOST_USER_PROTOCOL_F_LOG_SHMFD`)

- `VHOST_USER_GET_INFLIGHT_FD` (if `VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD`)

See also

REPLY_ACK

  The section on `REPLY_ACK` protocol extension.

There are several messages that the master sends with file descriptors passed in the ancillary data:

- `VHOST_USER_SET_MEM_TABLE`

- `VHOST_USER_SET_LOG_BASE` (if `VHOST_USER_PROTOCOL_F_LOG_SHMFD`)

- `VHOST_USER_SET_LOG_FD`

- `VHOST_USER_SET_VRING_KICK`

- `VHOST_USER_SET_VRING_CALL`

- `VHOST_USER_SET_VRING_ERR`

- `VHOST_USER_SET_SLAVE_REQ_FD`

- `VHOST_USER_SET_INFLIGHT_FD` (if `VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD`)

If master is unable to send the full message or receives a wrong reply it will close the connection. An optional reconnection mechanism can be implemented.

If slave detects some error such as incompatible features, it may also close the connection. This should only happen in exceptional circumstances.

Any protocol extensions are gated by protocol feature bits, which allows full backwards compatibility on both master and slave. As older slaves don’t support negotiating protocol features, a feature bit was dedicated for this purpose:

    #define VHOST_USER_F_PROTOCOL_FEATURES 30

### Starting and stopping rings

Client must only process each ring when it is started.

Client must only pass data between the ring and the backend, when the ring is enabled.

If ring is started but disabled, client must process the ring without talking to the backend.

For example, for a networking device, in the disabled state client must not supply any new RX packets, but must process and discard any TX packets.

If `VHOST_USER_F_PROTOCOL_FEATURES` has not been negotiated, the ring is initialized in an enabled state.

If `VHOST_USER_F_PROTOCOL_FEATURES` has been negotiated, the ring is initialized in a disabled state. Client must not pass data to/from the backend until ring is enabled by `VHOST_USER_SET_VRING_ENABLE` with parameter 1, or after it has been disabled by `VHOST_USER_SET_VRING_ENABLE` with parameter 0.

Each ring is initialized in a stopped state, client must not process it until ring is started, or after it has been stopped.

Client must start ring upon receiving a kick (that is, detecting that file descriptor is readable) on the descriptor specified by `VHOST_USER_SET_VRING_KICK` or receiving the in-band message `VHOST_USER_VRING_KICK` if negotiated, and stop ring upon receiving `VHOST_USER_GET_VRING_BASE`.

While processing the rings (whether they are enabled or not), client must support changing some configuration aspects on the fly.

### Multiple queue support

Many devices have a fixed number of virtqueues. In this case the master already knows the number of available virtqueues without communicating with the slave.

Some devices do not have a fixed number of virtqueues. Instead the maximum number of virtqueues is chosen by the slave. The number can depend on host resource availability or slave implementation details. Such devices are called multiple queue devices.

Multiple queue support allows the slave to advertise the maximum number of queues. This is treated as a protocol extension, hence the slave has to implement protocol features first. The multiple queues feature is supported only when the protocol feature `VHOST_USER_PROTOCOL_F_MQ` (bit 0) is set.

The max number of queues the slave supports can be queried with message `VHOST_USER_GET_QUEUE_NUM`. Master should stop when the number of requested queues is bigger than that.

As all queues share one connection, the master uses a unique index for each queue in the sent message to identify a specified queue.

The master enables queues by sending message `VHOST_USER_SET_VRING_ENABLE`. vhost-user-net has historically automatically enabled the first queue pair.

Slaves should always implement the `VHOST_USER_PROTOCOL_F_MQ` protocol feature, even for devices with a fixed number of virtqueues, since it is simple to implement and offers a degree of introspection.

Masters must not rely on the `VHOST_USER_PROTOCOL_F_MQ` protocol feature for devices with a fixed number of virtqueues. Only true multiqueue devices require this protocol feature.

### Migration

During live migration, the master may need to track the modifications the slave makes to the memory mapped regions. The client should mark the dirty pages in a log. Once it complies to this logging, it may declare the `VHOST_F_LOG_ALL` vhost feature.

To start/stop logging of data/used ring writes, server may send messages `VHOST_USER_SET_FEATURES` with `VHOST_F_LOG_ALL` and `VHOST_USER_SET_VRING_ADDR` with `VHOST_VRING_F_LOG` in ring’s flags set to 1/0, respectively.

All the modifications to memory pointed by vring “descriptor” should be marked. Modifications to “used” vring should be marked if `VHOST_VRING_F_LOG` is part of ring’s flags.

Dirty pages are of size:

    #define VHOST_LOG_PAGE 0x1000

The log memory fd is provided in the ancillary data of `VHOST_USER_SET_LOG_BASE` message when the slave has `VHOST_USER_PROTOCOL_F_LOG_SHMFD` protocol feature.

The size of the log is supplied as part of `VhostUserMsg` which should be large enough to cover all known guest addresses. Log starts at the supplied offset in the supplied file descriptor. The log covers from address 0 to the maximum of guest regions. In pseudo-code, to mark page at `addr` as dirty:

    page = addr / VHOST_LOG_PAGE
    log[page / 8] |= 1 << page % 8

Where `addr` is the guest physical address.

Use atomic operations, as the log may be concurrently manipulated.

Note that when logging modifications to the used ring (when `VHOST_VRING_F_LOG` is set for this ring), `log_guest_addr` should be used to calculate the log offset: the write to first byte of the used ring is logged at this offset from log start. Also note that this value might be outside the legal guest physical address range (i.e. does not have to be covered by the `VhostUserMemory` table), but the bit offset of the last byte of the ring must fall within the size supplied by `VhostUserLog`.

`VHOST_USER_SET_LOG_FD` is an optional message with an eventfd in ancillary data, it may be used to inform the master that the log has been modified.

Once the source has finished migration, rings will be stopped by the source. No further update must be done before rings are restarted.

In postcopy migration the slave is started before all the memory has been received from the source host, and care must be taken to avoid accessing pages that have yet to be received. The slave opens a ‘userfault’-fd and registers the memory with it; this fd is then passed back over to the master. The master services requests on the userfaultfd for pages that are accessed and when the page is available it performs WAKE ioctl’s on the userfaultfd to wake the stalled slave. The client indicates support for this via the `VHOST_USER_PROTOCOL_F_PAGEFAULT` feature.

### Memory access

The master sends a list of vhost memory regions to the slave using the `VHOST_USER_SET_MEM_TABLE` message. Each region has two base addresses: a guest address and a user address.

Messages contain guest addresses and/or user addresses to reference locations within the shared memory. The mapping of these addresses works as follows.

User addresses map to the vhost memory region containing that user address.

When the `VIRTIO_F_IOMMU_PLATFORM` feature has not been negotiated:

- Guest addresses map to the vhost memory region containing that guest address.

When the `VIRTIO_F_IOMMU_PLATFORM` feature has been negotiated:

- Guest addresses are also called I/O virtual addresses (IOVAs). They are translated to user addresses via the IOTLB.

- The vhost memory region guest address is not used.

### IOMMU support

When the `VIRTIO_F_IOMMU_PLATFORM` feature has been negotiated, the master sends IOTLB entries update & invalidation by sending `VHOST_USER_IOTLB_MSG` requests to the slave with a `struct vhost_iotlb_msg` as payload. For update events, the `iotlb` payload has to be filled with the update message type (2), the I/O virtual address, the size, the user virtual address, and the permissions flags. Addresses and size must be within vhost memory regions set via the `VHOST_USER_SET_MEM_TABLE` request. For invalidation events, the `iotlb` payload has to be filled with the invalidation message type (3), the I/O virtual address and the size. On success, the slave is expected to reply with a zero payload, non-zero otherwise.

The slave relies on the slave communcation channel (see Slave communication section below) to send IOTLB miss and access failure events, by sending `VHOST_USER_SLAVE_IOTLB_MSG` requests to the master with a `struct vhost_iotlb_msg` as payload. For miss events, the iotlb payload has to be filled with the miss message type (1), the I/O virtual address and the permissions flags. For access failure event, the iotlb payload has to be filled with the access failure message type (4), the I/O virtual address and the permissions flags. For synchronization purpose, the slave may rely on the reply-ack feature, so the master may send a reply when operation is completed if the reply-ack feature is negotiated and slaves requests a reply. For miss events, completed operation means either master sent an update message containing the IOTLB entry containing requested address and permission, or master sent nothing if the IOTLB miss message is invalid (invalid IOVA or permission).

The master isn’t expected to take the initiative to send IOTLB update messages, as the slave sends IOTLB miss messages for the guest virtual memory areas it needs to access.

### Slave communication

An optional communication channel is provided if the slave declares `VHOST_USER_PROTOCOL_F_SLAVE_REQ` protocol feature, to allow the slave to make requests to the master.

The fd is provided via `VHOST_USER_SET_SLAVE_REQ_FD` ancillary data.

A slave may then send `VHOST_USER_SLAVE_*` messages to the master using this fd communication channel.

If `VHOST_USER_PROTOCOL_F_SLAVE_SEND_FD` protocol feature is negotiated, slave can send file descriptors (at most 8 descriptors in each message) to master via ancillary data using this fd communication channel.

### Inflight I/O tracking

To support reconnecting after restart or crash, slave may need to resubmit inflight I/Os. If virtqueue is processed in order, we can easily achieve that by getting the inflight descriptors from descriptor table (split virtqueue) or descriptor ring (packed virtqueue). However, it can’t work when we process descriptors out-of-order because some entries which store the information of inflight descriptors in available ring (split virtqueue) or descriptor ring (packed virtqueue) might be overrided by new entries. To solve this problem, slave need to allocate an extra buffer to store this information of inflight descriptors and share it with master for persistent. `VHOST_USER_GET_INFLIGHT_FD` and `VHOST_USER_SET_INFLIGHT_FD` are used to transfer this buffer between master and slave. And the format of this buffer is described below:

queue0 region | queue1 region | … | queueN region

N is the number of available virtqueues. Slave could get it from num queues field of `VhostUserInflight`.

For split virtqueue, queue region can be implemented as:

    typedef struct DescStateSplit {
        /* Indicate whether this descriptor is inflight or not.
         * Only available for head-descriptor. */
        uint8_t inflight;

        /* Padding */
        uint8_t padding[5];

        /* Maintain a list for the last batch of used descriptors.
         * Only available when batching is used for submitting */
        uint16_t next;

        /* Used to preserve the order of fetching available descriptors.
         * Only available for head-descriptor. */
        uint64_t counter;
    } DescStateSplit;

    typedef struct QueueRegionSplit {
        /* The feature flags of this region. Now it's initialized to 0. */
        uint64_t features;

        /* The version of this region. It's 1 currently.
         * Zero value indicates an uninitialized buffer */
        uint16_t version;

        /* The size of DescStateSplit array. It's equal to the virtqueue
         * size. Slave could get it from queue size field of VhostUserInflight. */
        uint16_t desc_num;

        /* The head of list that track the last batch of used descriptors. */
        uint16_t last_batch_head;

        /* Store the idx value of used ring */
        uint16_t used_idx;

        /* Used to track the state of each descriptor in descriptor table */
        DescStateSplit desc[];
    } QueueRegionSplit;

To track inflight I/O, the queue region should be processed as follows:

When receiving available buffers from the driver:

1. Get the next available head-descriptor index from available ring, `i`

2. Set `desc[i].counter` to the value of global counter

3. Increase global counter by 1

4. Set `desc[i].inflight` to 1

When supplying used buffers to the driver:

1. Get corresponding used head-descriptor index, i

2. Set `desc[i].next` to `last_batch_head`

3. Set `last_batch_head` to `i`

4. Steps 1,2,3 may be performed repeatedly if batching is possible

5. Increase the `idx` value of used ring by the size of the batch

6. Set the `inflight` field of each `DescStateSplit` entry in the batch to 0

7. Set `used_idx` to the `idx` value of used ring

When reconnecting:

1. If the value of `used_idx` does not match the `idx` value of used ring (means the inflight field of `DescStateSplit` entries in last batch may be incorrect),

   1. Subtract the value of `used_idx` from the `idx` value of used ring to get last batch size of `DescStateSplit` entries

   2. Set the `inflight` field of each `DescStateSplit` entry to 0 in last batch list which starts from `last_batch_head`

   3. Set `used_idx` to the `idx` value of used ring

2. Resubmit inflight `DescStateSplit` entries in order of their counter value

For packed virtqueue, queue region can be implemented as:

    typedef struct DescStatePacked {
        /* Indicate whether this descriptor is inflight or not.
         * Only available for head-descriptor. */
        uint8_t inflight;

        /* Padding */
        uint8_t padding;

        /* Link to the next free entry */
        uint16_t next;

        /* Link to the last entry of descriptor list.
         * Only available for head-descriptor. */
        uint16_t last;

        /* The length of descriptor list.
         * Only available for head-descriptor. */
        uint16_t num;

        /* Used to preserve the order of fetching available descriptors.
         * Only available for head-descriptor. */
        uint64_t counter;

        /* The buffer id */
        uint16_t id;

        /* The descriptor flags */
        uint16_t flags;

        /* The buffer length */
        uint32_t len;

        /* The buffer address */
        uint64_t addr;
    } DescStatePacked;

    typedef struct QueueRegionPacked {
        /* The feature flags of this region. Now it's initialized to 0. */
        uint64_t features;

        /* The version of this region. It's 1 currently.
         * Zero value indicates an uninitialized buffer */
        uint16_t version;

        /* The size of DescStatePacked array. It's equal to the virtqueue
         * size. Slave could get it from queue size field of VhostUserInflight. */
        uint16_t desc_num;

        /* The head of free DescStatePacked entry list */
        uint16_t free_head;

        /* The old head of free DescStatePacked entry list */
        uint16_t old_free_head;

        /* The used index of descriptor ring */
        uint16_t used_idx;

        /* The old used index of descriptor ring */
        uint16_t old_used_idx;

        /* Device ring wrap counter */
        uint8_t used_wrap_counter;

        /* The old device ring wrap counter */
        uint8_t old_used_wrap_counter;

        /* Padding */
        uint8_t padding[7];

        /* Used to track the state of each descriptor fetched from descriptor ring */
        DescStatePacked desc[];
    } QueueRegionPacked;

To track inflight I/O, the queue region should be processed as follows:

When receiving available buffers from the driver:

1. Get the next available descriptor entry from descriptor ring, `d`

2. If `d` is head descriptor,

   1. Set `desc[old_free_head].num` to 0

   2. Set `desc[old_free_head].counter` to the value of global counter

   3. Increase global counter by 1

   4. Set `desc[old_free_head].inflight` to 1

3. If `d` is last descriptor, set `desc[old_free_head].last` to `free_head`

4. Increase `desc[old_free_head].num` by 1

5. Set `desc[free_head].addr`, `desc[free_head].len`, `desc[free_head].flags`, `desc[free_head].id` to `d.addr`, `d.len`, `d.flags`, `d.id`

6. Set `free_head` to `desc[free_head].next`

7. If `d` is last descriptor, set `old_free_head` to `free_head`

When supplying used buffers to the driver:

1. Get corresponding used head-descriptor entry from descriptor ring, `d`

2. Get corresponding `DescStatePacked` entry, `e`

3. Set `desc[e.last].next` to `free_head`

4. Set `free_head` to the index of `e`

5. Steps 1,2,3,4 may be performed repeatedly if batching is possible

6. Increase `used_idx` by the size of the batch and update `used_wrap_counter` if needed

7. Update `d.flags`

8. Set the `inflight` field of each head `DescStatePacked` entry in the batch to 0

9. Set `old_free_head`, `old_used_idx`, `old_used_wrap_counter` to `free_head`, `used_idx`, `used_wrap_counter`

When reconnecting:

1. If `used_idx` does not match `old_used_idx` (means the `inflight` field of `DescStatePacked` entries in last batch may be incorrect),

   1. Get the next descriptor ring entry through `old_used_idx`, `d`

   2. Use `old_used_wrap_counter` to calculate the available flags

   3. If `d.flags` is not equal to the calculated flags value (means slave has submitted the buffer to guest driver before crash, so it has to commit the in-progres update), set `old_free_head`, `old_used_idx`, `old_used_wrap_counter` to `free_head`, `used_idx`, `used_wrap_counter`

2. Set `free_head`, `used_idx`, `used_wrap_counter` to `old_free_head`, `old_used_idx`, `old_used_wrap_counter` (roll back any in-progress update)

3. Set the `inflight` field of each `DescStatePacked` entry in free list to 0

4. Resubmit inflight `DescStatePacked` entries in order of their counter value

### In-band notifications

In some limited situations (e.g. for simulation) it is desirable to have the kick, call and error (if used) signals done via in-band messages instead of asynchronous eventfd notifications. This can be done by negotiating the `VHOST_USER_PROTOCOL_F_INBAND_NOTIFICATIONS` protocol feature.

Note that due to the fact that too many messages on the sockets can cause the sending application(s) to block, it is not advised to use this feature unless absolutely necessary. It is also considered an error to negotiate this feature without also negotiating `VHOST_USER_PROTOCOL_F_SLAVE_REQ` and `VHOST_USER_PROTOCOL_F_REPLY_ACK`, the former is necessary for getting a message channel from the slave to the master, while the latter needs to be used with the in-band notification messages to block until they are processed, both to avoid blocking later and for proper processing (at least in the simulation use case.) As it has no other way of signalling this error, the slave should close the connection as a response to a `VHOST_USER_SET_PROTOCOL_FEATURES` message that sets the in-band notifications feature flag without the other two.

### Protocol features

    #define VHOST_USER_PROTOCOL_F_MQ                    0
    #define VHOST_USER_PROTOCOL_F_LOG_SHMFD             1
    #define VHOST_USER_PROTOCOL_F_RARP                  2
    #define VHOST_USER_PROTOCOL_F_REPLY_ACK             3
    #define VHOST_USER_PROTOCOL_F_MTU                   4
    #define VHOST_USER_PROTOCOL_F_SLAVE_REQ             5
    #define VHOST_USER_PROTOCOL_F_CROSS_ENDIAN          6
    #define VHOST_USER_PROTOCOL_F_CRYPTO_SESSION        7
    #define VHOST_USER_PROTOCOL_F_PAGEFAULT             8
    #define VHOST_USER_PROTOCOL_F_CONFIG                9
    #define VHOST_USER_PROTOCOL_F_SLAVE_SEND_FD        10
    #define VHOST_USER_PROTOCOL_F_HOST_NOTIFIER        11
    #define VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD       12
    #define VHOST_USER_PROTOCOL_F_RESET_DEVICE         13
    #define VHOST_USER_PROTOCOL_F_INBAND_NOTIFICATIONS 14
    #define VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS  15
    #define VHOST_USER_PROTOCOL_F_STATUS               16

### Master message types

`VHOST_USER_GET_FEATURES`
  id

    1

  equivalent ioctl

    `VHOST_GET_FEATURES`

  master payload

    N/A

  slave payload

    `u64`

  Get from the underlying vhost implementation the features bitmask. Feature bit `VHOST_USER_F_PROTOCOL_FEATURES` signals slave support for `VHOST_USER_GET_PROTOCOL_FEATURES` and `VHOST_USER_SET_PROTOCOL_FEATURES`.

`VHOST_USER_SET_FEATURES`
  id

    2

  equivalent ioctl

    `VHOST_SET_FEATURES`

  master payload

    `u64`

  Enable features in the underlying vhost implementation using a bitmask. Feature bit `VHOST_USER_F_PROTOCOL_FEATURES` signals slave support for `VHOST_USER_GET_PROTOCOL_FEATURES` and `VHOST_USER_SET_PROTOCOL_FEATURES`.

`VHOST_USER_GET_PROTOCOL_FEATURES`
  id

    15

  equivalent ioctl

    `VHOST_GET_FEATURES`

  master payload

    N/A

  slave payload

    `u64`

  Get the protocol feature bitmask from the underlying vhost implementation. Only legal if feature bit `VHOST_USER_F_PROTOCOL_FEATURES` is present in `VHOST_USER_GET_FEATURES`.

Note

Slave that reported `VHOST_USER_F_PROTOCOL_FEATURES` must support this message even before `VHOST_USER_SET_FEATURES` was called.

`VHOST_USER_SET_PROTOCOL_FEATURES`
  id

    16

  equivalent ioctl

    `VHOST_SET_FEATURES`

  master payload

    `u64`

  Enable protocol features in the underlying vhost implementation.

  Only legal if feature bit `VHOST_USER_F_PROTOCOL_FEATURES` is present in `VHOST_USER_GET_FEATURES`.

Note

Slave that reported `VHOST_USER_F_PROTOCOL_FEATURES` must support this message even before `VHOST_USER_SET_FEATURES` was called.

`VHOST_USER_SET_OWNER`
  id

    3

  equivalent ioctl

    `VHOST_SET_OWNER`

  master payload

    N/A

  Issued when a new connection is established. It sets the current master as an owner of the session. This can be used on the slave as a “session start” flag.

`VHOST_USER_RESET_OWNER`
  id

    4

  master payload

    N/A

Deprecated

This is no longer used. Used to be sent to request disabling all rings, but some clients interpreted it to also discard connection state (this interpretation would lead to bugs). It is recommended that clients either ignore this message, or use it to disable all rings.

`VHOST_USER_SET_MEM_TABLE`
  id

    5

  equivalent ioctl

    `VHOST_SET_MEM_TABLE`

  master payload

    memory regions description

  slave payload

    (postcopy only) memory regions description

  Sets the memory map regions on the slave so it can translate the vring addresses. In the ancillary data there is an array of file descriptors for each memory mapped region. The size and ordering of the fds matches the number and ordering of memory regions.

  When `VHOST_USER_POSTCOPY_LISTEN` has been received, `SET_MEM_TABLE` replies with the bases of the memory mapped regions to the master. The slave must have mmap’d the regions but not yet accessed them and should not yet generate a userfault event.

Note

`NEED_REPLY_MASK` is not set in this case. QEMU will then reply back to the list of mappings with an empty `VHOST_USER_SET_MEM_TABLE` as an acknowledgement; only upon reception of this message may the guest start accessing the memory and generating faults.

`VHOST_USER_SET_LOG_BASE`
  id

    6

  equivalent ioctl

    `VHOST_SET_LOG_BASE`

  master payload

    u64

  slave payload

    N/A

  Sets logging shared memory space.

  When slave has `VHOST_USER_PROTOCOL_F_LOG_SHMFD` protocol feature, the log memory fd is provided in the ancillary data of `VHOST_USER_SET_LOG_BASE` message, the size and offset of shared memory area provided in the message.

`VHOST_USER_SET_LOG_FD`
  id

    7

  equivalent ioctl

    `VHOST_SET_LOG_FD`

  master payload

    N/A

  Sets the logging file descriptor, which is passed as ancillary data.

`VHOST_USER_SET_VRING_NUM`
  id

    8

  equivalent ioctl

    `VHOST_SET_VRING_NUM`

  master payload

    vring state description

  Set the size of the queue.

`VHOST_USER_SET_VRING_ADDR`
  id

    9

  equivalent ioctl

    `VHOST_SET_VRING_ADDR`

  master payload

    vring address description

  slave payload

    N/A

  Sets the addresses of the different aspects of the vring.

`VHOST_USER_SET_VRING_BASE`
  id

    10

  equivalent ioctl

    `VHOST_SET_VRING_BASE`

  master payload

    vring state description

  Sets the base offset in the available vring.

`VHOST_USER_GET_VRING_BASE`
  id

    11

  equivalent ioctl

    `VHOST_USER_GET_VRING_BASE`

  master payload

    vring state description

  slave payload

    vring state description

  Get the available vring base offset.

`VHOST_USER_SET_VRING_KICK`
  id

    12

  equivalent ioctl

    `VHOST_SET_VRING_KICK`

  master payload

    `u64`

  Set the event file descriptor for adding buffers to the vring. It is passed in the ancillary data.

  Bits (0-7) of the payload contain the vring index. Bit 8 is the invalid FD flag. This flag is set when there is no file descriptor in the ancillary data. This signals that polling should be used instead of waiting for the kick. Note that if the protocol feature `VHOST_USER_PROTOCOL_F_INBAND_NOTIFICATIONS` has been negotiated this message isn’t necessary as the ring is also started on the `VHOST_USER_VRING_KICK` message, it may however still be used to set an event file descriptor (which will be preferred over the message) or to enable polling.

`VHOST_USER_SET_VRING_CALL`
  id

    13

  equivalent ioctl

    `VHOST_SET_VRING_CALL`

  master payload

    `u64`

  Set the event file descriptor to signal when buffers are used. It is passed in the ancillary data.

  Bits (0-7) of the payload contain the vring index. Bit 8 is the invalid FD flag. This flag is set when there is no file descriptor in the ancillary data. This signals that polling will be used instead of waiting for the call. Note that if the protocol features `VHOST_USER_PROTOCOL_F_INBAND_NOTIFICATIONS` and `VHOST_USER_PROTOCOL_F_SLAVE_REQ` have been negotiated this message isn’t necessary as the `VHOST_USER_SLAVE_VRING_CALL` message can be used, it may however still be used to set an event file descriptor or to enable polling.

`VHOST_USER_SET_VRING_ERR`
  id

    14

  equivalent ioctl

    `VHOST_SET_VRING_ERR`

  master payload

    `u64`

  Set the event file descriptor to signal when error occurs. It is passed in the ancillary data.

  Bits (0-7) of the payload contain the vring index. Bit 8 is the invalid FD flag. This flag is set when there is no file descriptor in the ancillary data. Note that if the protocol features `VHOST_USER_PROTOCOL_F_INBAND_NOTIFICATIONS` and `VHOST_USER_PROTOCOL_F_SLAVE_REQ` have been negotiated this message isn’t necessary as the `VHOST_USER_SLAVE_VRING_ERR` message can be used, it may however still be used to set an event file descriptor (which will be preferred over the message).

`VHOST_USER_GET_QUEUE_NUM`
  id

    17

  equivalent ioctl

    N/A

  master payload

    N/A

  slave payload

    u64

  Query how many queues the backend supports.

  This request should be sent only when `VHOST_USER_PROTOCOL_F_MQ` is set in queried protocol features by `VHOST_USER_GET_PROTOCOL_FEATURES`.

`VHOST_USER_SET_VRING_ENABLE`
  id

    18

  equivalent ioctl

    N/A

  master payload

    vring state description

  Signal slave to enable or disable corresponding vring.

  This request should be sent only when `VHOST_USER_F_PROTOCOL_FEATURES` has been negotiated.

`VHOST_USER_SEND_RARP`
  id

    19

  equivalent ioctl

    N/A

  master payload

    `u64`

  Ask vhost user backend to broadcast a fake RARP to notify the migration is terminated for guest that does not support GUEST_ANNOUNCE.

  Only legal if feature bit `VHOST_USER_F_PROTOCOL_FEATURES` is present in `VHOST_USER_GET_FEATURES` and protocol feature bit `VHOST_USER_PROTOCOL_F_RARP` is present in `VHOST_USER_GET_PROTOCOL_FEATURES`. The first 6 bytes of the payload contain the mac address of the guest to allow the vhost user backend to construct and broadcast the fake RARP.

`VHOST_USER_NET_SET_MTU`
  id

    20

  equivalent ioctl

    N/A

  master payload

    `u64`

  Set host MTU value exposed to the guest.

  This request should be sent only when `VIRTIO_NET_F_MTU` feature has been successfully negotiated, `VHOST_USER_F_PROTOCOL_FEATURES` is present in `VHOST_USER_GET_FEATURES` and protocol feature bit `VHOST_USER_PROTOCOL_F_NET_MTU` is present in `VHOST_USER_GET_PROTOCOL_FEATURES`.

  If `VHOST_USER_PROTOCOL_F_REPLY_ACK` is negotiated, slave must respond with zero in case the specified MTU is valid, or non-zero otherwise.

`VHOST_USER_SET_SLAVE_REQ_FD`
  id

    21

  equivalent ioctl

    N/A

  master payload

    N/A

  Set the socket file descriptor for slave initiated requests. It is passed in the ancillary data.

  This request should be sent only when `VHOST_USER_F_PROTOCOL_FEATURES` has been negotiated, and protocol feature bit `VHOST_USER_PROTOCOL_F_SLAVE_REQ` bit is present in `VHOST_USER_GET_PROTOCOL_FEATURES`. If `VHOST_USER_PROTOCOL_F_REPLY_ACK` is negotiated, slave must respond with zero for success, non-zero otherwise.

`VHOST_USER_IOTLB_MSG`
  id

    22

  equivalent ioctl

    N/A (equivalent to `VHOST_IOTLB_MSG` message type)

  master payload

    `struct vhost_iotlb_msg`

  slave payload

    `u64`

  Send IOTLB messages with `struct vhost_iotlb_msg` as payload.

  Master sends such requests to update and invalidate entries in the device IOTLB. The slave has to acknowledge the request with sending zero as `u64` payload for success, non-zero otherwise.

  This request should be send only when `VIRTIO_F_IOMMU_PLATFORM` feature has been successfully negotiated.

`VHOST_USER_SET_VRING_ENDIAN`
  id

    23

  equivalent ioctl

    `VHOST_SET_VRING_ENDIAN`

  master payload

    vring state description

  Set the endianness of a VQ for legacy devices. Little-endian is indicated with state.num set to 0 and big-endian is indicated with state.num set to 1. Other values are invalid.

  This request should be sent only when `VHOST_USER_PROTOCOL_F_CROSS_ENDIAN` has been negotiated. Backends that negotiated this feature should handle both endiannesses and expect this message once (per VQ) during device configuration (ie. before the master starts the VQ).

`VHOST_USER_GET_CONFIG`
  id

    24

  equivalent ioctl

    N/A

  master payload

    virtio device config space

  slave payload

    virtio device config space

  When `VHOST_USER_PROTOCOL_F_CONFIG` is negotiated, this message is submitted by the vhost-user master to fetch the contents of the virtio device configuration space, vhost-user slave’s payload size MUST match master’s request, vhost-user slave uses zero length of payload to indicate an error to vhost-user master. The vhost-user master may cache the contents to avoid repeated `VHOST_USER_GET_CONFIG` calls.

`VHOST_USER_SET_CONFIG`
  id

    25

  equivalent ioctl

    N/A

  master payload

    virtio device config space

  slave payload

    N/A

  When `VHOST_USER_PROTOCOL_F_CONFIG` is negotiated, this message is submitted by the vhost-user master when the Guest changes the virtio device configuration space and also can be used for live migration on the destination host. The vhost-user slave must check the flags field, and slaves MUST NOT accept SET_CONFIG for read-only configuration space fields unless the live migration bit is set.

`VHOST_USER_CREATE_CRYPTO_SESSION`
  id

    26

  equivalent ioctl

    N/A

  master payload

    crypto session description

  slave payload

    crypto session description

  Create a session for crypto operation. The server side must return the session id, 0 or positive for success, negative for failure. This request should be sent only when `VHOST_USER_PROTOCOL_F_CRYPTO_SESSION` feature has been successfully negotiated. It’s a required feature for crypto devices.

`VHOST_USER_CLOSE_CRYPTO_SESSION`
  id

    27

  equivalent ioctl

    N/A

  master payload

    `u64`

  Close a session for crypto operation which was previously created by `VHOST_USER_CREATE_CRYPTO_SESSION`.

  This request should be sent only when `VHOST_USER_PROTOCOL_F_CRYPTO_SESSION` feature has been successfully negotiated. It’s a required feature for crypto devices.

`VHOST_USER_POSTCOPY_ADVISE`
  id

    28

  master payload

    N/A

  slave payload

    userfault fd

  When `VHOST_USER_PROTOCOL_F_PAGEFAULT` is supported, the master advises slave that a migration with postcopy enabled is underway, the slave must open a userfaultfd for later use. Note that at this stage the migration is still in precopy mode.

`VHOST_USER_POSTCOPY_LISTEN`
  id

    29

  master payload

    N/A

  Master advises slave that a transition to postcopy mode has happened. The slave must ensure that shared memory is registered with userfaultfd to cause faulting of non-present pages.

  This is always sent sometime after a `VHOST_USER_POSTCOPY_ADVISE`, and thus only when `VHOST_USER_PROTOCOL_F_PAGEFAULT` is supported.

`VHOST_USER_POSTCOPY_END`
  id

    30

  slave payload

    `u64`

  Master advises that postcopy migration has now completed. The slave must disable the userfaultfd. The response is an acknowledgement only.

  When `VHOST_USER_PROTOCOL_F_PAGEFAULT` is supported, this message is sent at the end of the migration, after `VHOST_USER_POSTCOPY_LISTEN` was previously sent.

  The value returned is an error indication; 0 is success.

`VHOST_USER_GET_INFLIGHT_FD`
  id

    31

  equivalent ioctl

    N/A

  master payload

    inflight description

  When `VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD` protocol feature has been successfully negotiated, this message is submitted by master to get a shared buffer from slave. The shared buffer will be used to track inflight I/O by slave. QEMU should retrieve a new one when vm reset.

`VHOST_USER_SET_INFLIGHT_FD`
  id

    32

  equivalent ioctl

    N/A

  master payload

    inflight description

  When `VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD` protocol feature has been successfully negotiated, this message is submitted by master to send the shared inflight buffer back to slave so that slave could get inflight I/O after a crash or restart.

`VHOST_USER_GPU_SET_SOCKET`
  id

    33

  equivalent ioctl

    N/A

  master payload

    N/A

  Sets the GPU protocol socket file descriptor, which is passed as ancillary data. The GPU protocol is used to inform the master of rendering state and updates. See vhost-user-gpu.rst for details.

`VHOST_USER_RESET_DEVICE`
  id

    34

  equivalent ioctl

    N/A

  master payload

    N/A

  slave payload

    N/A

  Ask the vhost user backend to disable all rings and reset all internal device state to the initial state, ready to be reinitialized. The backend retains ownership of the device throughout the reset operation.

  Only valid if the `VHOST_USER_PROTOCOL_F_RESET_DEVICE` protocol feature is set by the backend.

`VHOST_USER_VRING_KICK`
  id

    35

  equivalent ioctl

    N/A

  slave payload

    vring state description

  master payload

    N/A

  When the `VHOST_USER_PROTOCOL_F_INBAND_NOTIFICATIONS` protocol feature has been successfully negotiated, this message may be submitted by the master to indicate that a buffer was added to the vring instead of signalling it using the vring’s kick file descriptor or having the slave rely on polling.

  The state.num field is currently reserved and must be set to 0.

`VHOST_USER_GET_MAX_MEM_SLOTS`
  id

    36

  equivalent ioctl

    N/A

  slave payload

    u64

  When the `VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS` protocol feature has been successfully negotiated, this message is submitted by master to the slave. The slave should return the message with a u64 payload containing the maximum number of memory slots for QEMU to expose to the guest. The value returned by the backend will be capped at the maximum number of ram slots which can be supported by the target platform.

`VHOST_USER_ADD_MEM_REG`
  id

    37

  equivalent ioctl

    N/A

  slave payload

    memory region

  When the `VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS` protocol feature has been successfully negotiated, this message is submitted by the master to the slave. The message payload contains a memory region descriptor struct, describing a region of guest memory which the slave device must map in. When the `VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS` protocol feature has been successfully negotiated, along with the `VHOST_USER_REM_MEM_REG` message, this message is used to set and update the memory tables of the slave device.

`VHOST_USER_REM_MEM_REG`
  id

    38

  equivalent ioctl

    N/A

  slave payload

    memory region

  When the `VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS` protocol feature has been successfully negotiated, this message is submitted by the master to the slave. The message payload contains a memory region descriptor struct, describing a region of guest memory which the slave device must unmap. When the `VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS` protocol feature has been successfully negotiated, along with the `VHOST_USER_ADD_MEM_REG` message, this message is used to set and update the memory tables of the slave device.

`VHOST_USER_SET_STATUS`
  id

    39

  equivalent ioctl

    VHOST_VDPA_SET_STATUS

  slave payload

    N/A

  master payload

    `u64`

  When the `VHOST_USER_PROTOCOL_F_STATUS` protocol feature has been successfully negotiated, this message is submitted by the master to notify the backend with updated device status as defined in the Virtio specification.

`VHOST_USER_GET_STATUS`
  id

    40

  equivalent ioctl

    VHOST_VDPA_GET_STATUS

  slave payload

    `u64`

  master payload

    N/A

  When the `VHOST_USER_PROTOCOL_F_STATUS` protocol feature has been successfully negotiated, this message is submitted by the master to query the backend for its device status as defined in the Virtio specification.

### Slave message types

`VHOST_USER_SLAVE_IOTLB_MSG`
  id

    1

  equivalent ioctl

    N/A (equivalent to `VHOST_IOTLB_MSG` message type)

  slave payload

    `struct vhost_iotlb_msg`

  master payload

    N/A

  Send IOTLB messages with `struct vhost_iotlb_msg` as payload. Slave sends such requests to notify of an IOTLB miss, or an IOTLB access failure. If `VHOST_USER_PROTOCOL_F_REPLY_ACK` is negotiated, and slave set the `VHOST_USER_NEED_REPLY` flag, master must respond with zero when operation is successfully completed, or non-zero otherwise. This request should be send only when `VIRTIO_F_IOMMU_PLATFORM` feature has been successfully negotiated.

`VHOST_USER_SLAVE_CONFIG_CHANGE_MSG`
  id

    2

  equivalent ioctl

    N/A

  slave payload

    N/A

  master payload

    N/A

  When `VHOST_USER_PROTOCOL_F_CONFIG` is negotiated, vhost-user slave sends such messages to notify that the virtio device’s configuration space has changed, for those host devices which can support such feature, host driver can send `VHOST_USER_GET_CONFIG` message to slave to get the latest content. If `VHOST_USER_PROTOCOL_F_REPLY_ACK` is negotiated, and slave set the `VHOST_USER_NEED_REPLY` flag, master must respond with zero when operation is successfully completed, or non-zero otherwise.

`VHOST_USER_SLAVE_VRING_HOST_NOTIFIER_MSG`
  id

    3

  equivalent ioctl

    N/A

  slave payload

    vring area description

  master payload

    N/A

  Sets host notifier for a specified queue. The queue index is contained in the `u64` field of the vring area description. The host notifier is described by the file descriptor (typically it’s a VFIO device fd) which is passed as ancillary data and the size (which is mmap size and should be the same as host page size) and offset (which is mmap offset) carried in the vring area description. QEMU can mmap the file descriptor based on the size and offset to get a memory range. Registering a host notifier means mapping this memory range to the VM as the specified queue’s notify MMIO region. Slave sends this request to tell QEMU to de-register the existing notifier if any and register the new notifier if the request is sent with a file descriptor.

  This request should be sent only when `VHOST_USER_PROTOCOL_F_HOST_NOTIFIER` protocol feature has been successfully negotiated.

`VHOST_USER_SLAVE_VRING_CALL`
  id

    4

  equivalent ioctl

    N/A

  slave payload

    vring state description

  master payload

    N/A

  When the `VHOST_USER_PROTOCOL_F_INBAND_NOTIFICATIONS` protocol feature has been successfully negotiated, this message may be submitted by the slave to indicate that a buffer was used from the vring instead of signalling this using the vring’s call file descriptor or having the master relying on polling.

  The state.num field is currently reserved and must be set to 0.

`VHOST_USER_SLAVE_VRING_ERR`
  id

    5

  equivalent ioctl

    N/A

  slave payload

    vring state description

  master payload

    N/A

  When the `VHOST_USER_PROTOCOL_F_INBAND_NOTIFICATIONS` protocol feature has been successfully negotiated, this message may be submitted by the slave to indicate that an error occurred on the specific vring, instead of signalling the error file descriptor set by the master via `VHOST_USER_SET_VRING_ERR`.

  The state.num field is currently reserved and must be set to 0.

### VHOST_USER_PROTOCOL_F_REPLY_ACK

The original vhost-user specification only demands replies for certain commands. This differs from the vhost protocol implementation where commands are sent over an `ioctl()` call and block until the client has completed.

With this protocol extension negotiated, the sender (QEMU) can set the `need_reply` [Bit 3] flag to any command. This indicates that the client MUST respond with a Payload `VhostUserMsg` indicating success or failure. The payload should be set to zero on success or non-zero on failure, unless the message already has an explicit reply body.

The response payload gives QEMU a deterministic indication of the result of the command. Today, QEMU is expected to terminate the main vhost-user loop upon receiving such errors. In future, qemu could be taught to be more resilient for selective requests.

For the message types that already solicit a reply from the client, the presence of `VHOST_USER_PROTOCOL_F_REPLY_ACK` or need_reply bit being set brings no behavioural change. (See the Communication section for details.)

## Backend program conventions

vhost-user backends can provide various devices & services and may need to be configured manually depending on the use case. However, it is a good idea to follow the conventions listed here when possible. Users, QEMU or libvirt, can then rely on some common behaviour to avoid heterogenous configuration and management of the backend programs and facilitate interoperability.

Each backend installed on a host system should come with at least one JSON file that conforms to the vhost-user.json schema. Each file informs the management applications about the backend type, and binary location. In addition, it defines rules for management apps for picking the highest priority backend when multiple match the search criteria (see `@VhostUserBackend` documentation in the schema file).

If the backend is not capable of enabling a requested feature on the host (such as 3D acceleration with virgl), or the initialization failed, the backend should fail to start early and exit with a status != 0. It may also print a message to stderr for further details.

The backend program must not daemonize itself, but it may be daemonized by the management layer. It may also have a restricted access to the system.

File descriptors 0, 1 and 2 will exist, and have regular stdin/stdout/stderr usage (they may have been redirected to /dev/null by the management layer, or to a log handler).

The backend program must end (as quickly and cleanly as possible) when the SIGTERM signal is received. Eventually, it may receive SIGKILL by the management layer after a few seconds.

The following command line options have an expected behaviour. They are mandatory, unless explicitly said differently:

`--socket-path=PATH`

  This option specify the location of the vhost-user Unix domain socket. It is incompatible with –fd.

`--fd=FDNUM`

  When this argument is given, the backend program is started with the vhost-user socket as file descriptor FDNUM. It is incompatible with –socket-path.

`--print-capabilities`

  Output to stdout the backend capabilities in JSON format, and then exit successfully. Other options and arguments should be ignored, and the backend program should not perform its normal function. The capabilities can be reported dynamically depending on the host capabilities.

The JSON output is described in the `vhost-user.json` schema, by ``@VHostUserBackendCapabilities`. Example:

    {
      "type": "foo",
      "features": [
        "feature-a",
        "feature-b"
      ]
    }

### vhost-user-input

Command line options:

`--evdev-path=PATH`

  Specify the linux input device.

  (optional)

`--no-grab`

  Do no request exclusive access to the input device.

  (optional)

### vhost-user-gpu

Command line options:

`--render-node=PATH`

  Specify the GPU DRM render node.

  (optional)

`--virgl`

  Enable virgl rendering support.

  (optional)

### vhost-user-blk

Command line options:

`--blk-file=PATH`

  Specify block device or file path.

  (optional)

`--read-only`

  Enable read-only.

  (optional)