docs/interop/vhost-user.txt

Vhost-user Protocol
===================

Copyright (c) 2014 Virtual Open Systems Sarl.

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

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 intended to
be a software Ethernet switch running in user space, such as Snabbswitch.

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

Message Specification
---------------------

Note that 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 |
------------------------------------

 * 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 VHOST_USER_PROTOCOL_F_REPLY_ACK for
     details.
 * Size - 32-bit size of the 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: a 8-bit value:
    - 0: No access
    - 1: Read access
    - 2: Write access
    - 3: Read/Write access
   Type: a 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

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;
    };
} 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)

[ Also see 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

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.

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, 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
----------------------

Multiple queue 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 queue pairs 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. One queue pair
is enabled initially. More queues are enabled dynamically, by sending
message VHOST_USER_SET_VRING_ENABLE.

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.

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.

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

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
      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
      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

      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

      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.

 * 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 a kick.

 * 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.

 * 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.

 * 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 endianess 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 endianesses
      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

      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

      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.

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

     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_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.)