467 lines
16 KiB
Plaintext
467 lines
16 KiB
Plaintext
Vhost-user Protocol
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===================
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Copyright (c) 2014 Virtual Open Systems Sarl.
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This work is licensed under the terms of the GNU GPL, version 2 or later.
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See the COPYING file in the top-level directory.
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===================
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This protocol is aiming to complement the ioctl interface used to control the
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vhost implementation in the Linux kernel. It implements the control plane needed
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to establish virtqueue sharing with a user space process on the same host. It
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uses communication over a Unix domain socket to share file descriptors in the
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ancillary data of the message.
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The protocol defines 2 sides of the communication, master and slave. Master is
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the application that shares its virtqueues, in our case QEMU. Slave is the
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consumer of the virtqueues.
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In the current implementation QEMU is the Master, and the Slave is intended to
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be a software Ethernet switch running in user space, such as Snabbswitch.
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Master and slave can be either a client (i.e. connecting) or server (listening)
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in the socket communication.
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Message Specification
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---------------------
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Note that all numbers are in the machine native byte order. A vhost-user message
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consists of 3 header fields and a payload:
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------------------------------------
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| request | flags | size | payload |
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------------------------------------
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* Request: 32-bit type of the request
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* Flags: 32-bit bit field:
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- Lower 2 bits are the version (currently 0x01)
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- Bit 2 is the reply flag - needs to be sent on each reply from the slave
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* Size - 32-bit size of the payload
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Depending on the request type, payload can be:
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* A single 64-bit integer
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-------
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| u64 |
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-------
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u64: a 64-bit unsigned integer
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* A vring state description
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---------------
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| index | num |
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---------------
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Index: a 32-bit index
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Num: a 32-bit number
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* A vring address description
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--------------------------------------------------------------
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| index | flags | size | descriptor | used | available | log |
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--------------------------------------------------------------
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Index: a 32-bit vring index
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Flags: a 32-bit vring flags
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Descriptor: a 64-bit user address of the vring descriptor table
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Used: a 64-bit user address of the vring used ring
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Available: a 64-bit user address of the vring available ring
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Log: a 64-bit guest address for logging
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* Memory regions description
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---------------------------------------------------
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| num regions | padding | region0 | ... | region7 |
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---------------------------------------------------
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Num regions: a 32-bit number of regions
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Padding: 32-bit
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A region is:
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-----------------------------------------------------
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| guest address | size | user address | mmap offset |
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-----------------------------------------------------
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Guest address: a 64-bit guest address of the region
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Size: a 64-bit size
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User address: a 64-bit user address
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mmap offset: 64-bit offset where region starts in the mapped memory
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* Log description
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---------------------------
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| log size | log offset |
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---------------------------
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log size: size of area used for logging
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log offset: offset from start of supplied file descriptor
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where logging starts (i.e. where guest address 0 would be logged)
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In QEMU the vhost-user message is implemented with the following struct:
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typedef struct VhostUserMsg {
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VhostUserRequest request;
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uint32_t flags;
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uint32_t size;
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union {
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uint64_t u64;
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struct vhost_vring_state state;
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struct vhost_vring_addr addr;
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VhostUserMemory memory;
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VhostUserLog log;
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};
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} QEMU_PACKED VhostUserMsg;
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Communication
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-------------
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The protocol for vhost-user is based on the existing implementation of vhost
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for the Linux Kernel. Most messages that can be sent via the Unix domain socket
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implementing vhost-user have an equivalent ioctl to the kernel implementation.
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The communication consists of master sending message requests and slave sending
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message replies. Most of the requests don't require replies. Here is a list of
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the ones that do:
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* VHOST_GET_FEATURES
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* VHOST_GET_PROTOCOL_FEATURES
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* VHOST_GET_VRING_BASE
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* VHOST_SET_LOG_BASE (if VHOST_USER_PROTOCOL_F_LOG_SHMFD)
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There are several messages that the master sends with file descriptors passed
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in the ancillary data:
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* VHOST_SET_MEM_TABLE
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* VHOST_SET_LOG_BASE (if VHOST_USER_PROTOCOL_F_LOG_SHMFD)
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* VHOST_SET_LOG_FD
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* VHOST_SET_VRING_KICK
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* VHOST_SET_VRING_CALL
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* VHOST_SET_VRING_ERR
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If Master is unable to send the full message or receives a wrong reply it will
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close the connection. An optional reconnection mechanism can be implemented.
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Any protocol extensions are gated by protocol feature bits,
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which allows full backwards compatibility on both master
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and slave.
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As older slaves don't support negotiating protocol features,
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a feature bit was dedicated for this purpose:
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#define VHOST_USER_F_PROTOCOL_FEATURES 30
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Starting and stopping rings
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----------------------
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Client must only process each ring when it is started.
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Client must only pass data between the ring and the
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backend, when the ring is enabled.
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If ring is started but disabled, client must process the
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ring without talking to the backend.
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For example, for a networking device, in the disabled state
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client must not supply any new RX packets, but must process
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and discard any TX packets.
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If VHOST_USER_F_PROTOCOL_FEATURES has not been negotiated, the ring is initialized
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in an enabled state.
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If VHOST_USER_F_PROTOCOL_FEATURES has been negotiated, the ring is initialized
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in a disabled state. Client must not pass data to/from the backend until ring is enabled by
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VHOST_USER_SET_VRING_ENABLE with parameter 1, or after it has been disabled by
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VHOST_USER_SET_VRING_ENABLE with parameter 0.
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Each ring is initialized in a stopped state, client must not process it until
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ring is started, or after it has been stopped.
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Client must start ring upon receiving a kick (that is, detecting that file
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descriptor is readable) on the descriptor specified by
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VHOST_USER_SET_VRING_KICK, and stop ring upon receiving
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VHOST_USER_GET_VRING_BASE.
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While processing the rings (whether they are enabled or not), client must
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support changing some configuration aspects on the fly.
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Multiple queue support
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----------------------
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Multiple queue is treated as a protocol extension, hence the slave has to
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implement protocol features first. The multiple queues feature is supported
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only when the protocol feature VHOST_USER_PROTOCOL_F_MQ (bit 0) is set.
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The max number of queues the slave supports can be queried with message
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VHOST_USER_GET_PROTOCOL_FEATURES. Master should stop when the number of
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requested queues is bigger than that.
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As all queues share one connection, the master uses a unique index for each
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queue in the sent message to identify a specified queue. One queue pair
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is enabled initially. More queues are enabled dynamically, by sending
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message VHOST_USER_SET_VRING_ENABLE.
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Migration
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---------
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During live migration, the master may need to track the modifications
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the slave makes to the memory mapped regions. The client should mark
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the dirty pages in a log. Once it complies to this logging, it may
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declare the VHOST_F_LOG_ALL vhost feature.
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To start/stop logging of data/used ring writes, server may send messages
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VHOST_USER_SET_FEATURES with VHOST_F_LOG_ALL and VHOST_USER_SET_VRING_ADDR with
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VHOST_VRING_F_LOG in ring's flags set to 1/0, respectively.
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All the modifications to memory pointed by vring "descriptor" should
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be marked. Modifications to "used" vring should be marked if
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VHOST_VRING_F_LOG is part of ring's flags.
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Dirty pages are of size:
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#define VHOST_LOG_PAGE 0x1000
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The log memory fd is provided in the ancillary data of
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VHOST_USER_SET_LOG_BASE message when the slave has
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VHOST_USER_PROTOCOL_F_LOG_SHMFD protocol feature.
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The size of the log is supplied as part of VhostUserMsg
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which should be large enough to cover all known guest
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addresses. Log starts at the supplied offset in the
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supplied file descriptor.
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The log covers from address 0 to the maximum of guest
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regions. In pseudo-code, to mark page at "addr" as dirty:
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page = addr / VHOST_LOG_PAGE
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log[page / 8] |= 1 << page % 8
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Where addr is the guest physical address.
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Use atomic operations, as the log may be concurrently manipulated.
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Note that when logging modifications to the used ring (when VHOST_VRING_F_LOG
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is set for this ring), log_guest_addr should be used to calculate the log
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offset: the write to first byte of the used ring is logged at this offset from
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log start. Also note that this value might be outside the legal guest physical
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address range (i.e. does not have to be covered by the VhostUserMemory table),
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but the bit offset of the last byte of the ring must fall within
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the size supplied by VhostUserLog.
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VHOST_USER_SET_LOG_FD is an optional message with an eventfd in
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ancillary data, it may be used to inform the master that the log has
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been modified.
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Once the source has finished migration, rings will be stopped by
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the source. No further update must be done before rings are
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restarted.
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Protocol features
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-----------------
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#define VHOST_USER_PROTOCOL_F_MQ 0
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#define VHOST_USER_PROTOCOL_F_LOG_SHMFD 1
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#define VHOST_USER_PROTOCOL_F_RARP 2
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Message types
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-------------
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* VHOST_USER_GET_FEATURES
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Id: 1
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Equivalent ioctl: VHOST_GET_FEATURES
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Master payload: N/A
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Slave payload: u64
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Get from the underlying vhost implementation the features bitmask.
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Feature bit VHOST_USER_F_PROTOCOL_FEATURES signals slave support for
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VHOST_USER_GET_PROTOCOL_FEATURES and VHOST_USER_SET_PROTOCOL_FEATURES.
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* VHOST_USER_SET_FEATURES
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Id: 2
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Ioctl: VHOST_SET_FEATURES
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Master payload: u64
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Enable features in the underlying vhost implementation using a bitmask.
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Feature bit VHOST_USER_F_PROTOCOL_FEATURES signals slave support for
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VHOST_USER_GET_PROTOCOL_FEATURES and VHOST_USER_SET_PROTOCOL_FEATURES.
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* VHOST_USER_GET_PROTOCOL_FEATURES
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Id: 15
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Equivalent ioctl: VHOST_GET_FEATURES
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Master payload: N/A
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Slave payload: u64
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Get the protocol feature bitmask from the underlying vhost implementation.
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Only legal if feature bit VHOST_USER_F_PROTOCOL_FEATURES is present in
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VHOST_USER_GET_FEATURES.
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Note: slave that reported VHOST_USER_F_PROTOCOL_FEATURES must support
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this message even before VHOST_USER_SET_FEATURES was called.
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* VHOST_USER_SET_PROTOCOL_FEATURES
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Id: 16
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Ioctl: VHOST_SET_FEATURES
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Master payload: u64
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Enable protocol features in the underlying vhost implementation.
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Only legal if feature bit VHOST_USER_F_PROTOCOL_FEATURES is present in
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VHOST_USER_GET_FEATURES.
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Note: slave that reported VHOST_USER_F_PROTOCOL_FEATURES must support
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this message even before VHOST_USER_SET_FEATURES was called.
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* VHOST_USER_SET_OWNER
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Id: 3
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Equivalent ioctl: VHOST_SET_OWNER
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Master payload: N/A
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Issued when a new connection is established. It sets the current Master
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as an owner of the session. This can be used on the Slave as a
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"session start" flag.
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* VHOST_USER_RESET_OWNER
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Id: 4
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Master payload: N/A
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This is no longer used. Used to be sent to request disabling
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all rings, but some clients interpreted it to also discard
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connection state (this interpretation would lead to bugs).
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It is recommended that clients either ignore this message,
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or use it to disable all rings.
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* VHOST_USER_SET_MEM_TABLE
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Id: 5
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Equivalent ioctl: VHOST_SET_MEM_TABLE
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Master payload: memory regions description
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Sets the memory map regions on the slave so it can translate the vring
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addresses. In the ancillary data there is an array of file descriptors
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for each memory mapped region. The size and ordering of the fds matches
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the number and ordering of memory regions.
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* VHOST_USER_SET_LOG_BASE
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Id: 6
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Equivalent ioctl: VHOST_SET_LOG_BASE
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Master payload: u64
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Slave payload: N/A
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Sets logging shared memory space.
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When slave has VHOST_USER_PROTOCOL_F_LOG_SHMFD protocol
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feature, the log memory fd is provided in the ancillary data of
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VHOST_USER_SET_LOG_BASE message, the size and offset of shared
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memory area provided in the message.
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* VHOST_USER_SET_LOG_FD
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Id: 7
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Equivalent ioctl: VHOST_SET_LOG_FD
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Master payload: N/A
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Sets the logging file descriptor, which is passed as ancillary data.
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* VHOST_USER_SET_VRING_NUM
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Id: 8
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Equivalent ioctl: VHOST_SET_VRING_NUM
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Master payload: vring state description
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Set the size of the queue.
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* VHOST_USER_SET_VRING_ADDR
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Id: 9
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Equivalent ioctl: VHOST_SET_VRING_ADDR
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Master payload: vring address description
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Slave payload: N/A
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Sets the addresses of the different aspects of the vring.
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* VHOST_USER_SET_VRING_BASE
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Id: 10
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Equivalent ioctl: VHOST_SET_VRING_BASE
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Master payload: vring state description
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Sets the base offset in the available vring.
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* VHOST_USER_GET_VRING_BASE
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Id: 11
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Equivalent ioctl: VHOST_USER_GET_VRING_BASE
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Master payload: vring state description
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Slave payload: vring state description
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Get the available vring base offset.
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* VHOST_USER_SET_VRING_KICK
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Id: 12
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Equivalent ioctl: VHOST_SET_VRING_KICK
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Master payload: u64
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Set the event file descriptor for adding buffers to the vring. It
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is passed in the ancillary data.
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Bits (0-7) of the payload contain the vring index. Bit 8 is the
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invalid FD flag. This flag is set when there is no file descriptor
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in the ancillary data. This signals that polling should be used
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instead of waiting for a kick.
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* VHOST_USER_SET_VRING_CALL
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Id: 13
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Equivalent ioctl: VHOST_SET_VRING_CALL
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Master payload: u64
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Set the event file descriptor to signal when buffers are used. It
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is passed in the ancillary data.
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Bits (0-7) of the payload contain the vring index. Bit 8 is the
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invalid FD flag. This flag is set when there is no file descriptor
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in the ancillary data. This signals that polling will be used
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instead of waiting for the call.
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* VHOST_USER_SET_VRING_ERR
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Id: 14
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Equivalent ioctl: VHOST_SET_VRING_ERR
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Master payload: u64
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Set the event file descriptor to signal when error occurs. It
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is passed in the ancillary data.
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Bits (0-7) of the payload contain the vring index. Bit 8 is the
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invalid FD flag. This flag is set when there is no file descriptor
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in the ancillary data.
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* VHOST_USER_GET_QUEUE_NUM
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Id: 17
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Equivalent ioctl: N/A
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Master payload: N/A
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Slave payload: u64
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Query how many queues the backend supports. This request should be
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sent only when VHOST_USER_PROTOCOL_F_MQ is set in queried protocol
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features by VHOST_USER_GET_PROTOCOL_FEATURES.
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* VHOST_USER_SET_VRING_ENABLE
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Id: 18
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Equivalent ioctl: N/A
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Master payload: vring state description
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Signal slave to enable or disable corresponding vring.
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This request should be sent only when VHOST_USER_F_PROTOCOL_FEATURES
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has been negotiated.
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* VHOST_USER_SEND_RARP
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Id: 19
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Equivalent ioctl: N/A
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Master payload: u64
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Ask vhost user backend to broadcast a fake RARP to notify the migration
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is terminated for guest that does not support GUEST_ANNOUNCE.
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Only legal if feature bit VHOST_USER_F_PROTOCOL_FEATURES is present in
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VHOST_USER_GET_FEATURES and protocol feature bit VHOST_USER_PROTOCOL_F_RARP
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is present in VHOST_USER_GET_PROTOCOL_FEATURES.
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The first 6 bytes of the payload contain the mac address of the guest to
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allow the vhost user backend to construct and broadcast the fake RARP.
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