vhost-user daemons in contrib

QEMU provides a number of :ref:`vhost_user` daemons in the contrib
directory. They were often written when vhost-user was initially added
to the code base. You should also consider if other vhost-user daemons
such as those from the rust-vmm `vhost-device repository`_ are better
suited for production use.

.. _vhost-device repository: https://github.com/rust-vmm/vhost-device

.. _vhost_user_block:

vhost-user-block - block device
===============================

vhost-user-block is a backend for exposing block devices. It can
present a flat file or block device as a simple block device to the
guest. You almost certainly want to use the :ref:`storage-daemon`
instead which supports a wide variety of storage modes and exports a
number of interfaces including vhost-user.

.. _vhost_user_gpu:

vhost-user-gpu - gpu device
===========================

vhost-user-gpu presents a paravirtualized GPU and display controller.
You probably want to use the internal :ref:`virtio_gpu` implementation
if you want the latest features. There is also a `vhost_device_gpu`_
daemon as part of the rust-vmm project.

.. _vhost_device_gpu: https://github.com/rust-vmm/vhost-device/tree/main/vhost-device-gpu

.. _vhost_user_input:

vhost-user-input - Input emulation
==================================

The Virtio input device is a paravirtualized device for input events.

Description
-----------

The vhost-user-input device implementation was designed to work with a daemon
polling on input devices and passes input events to the guest.

QEMU provides a backend implementation in contrib/vhost-user-input.

Linux kernel support
--------------------

Virtio input requires a guest Linux kernel built with the
``CONFIG_VIRTIO_INPUT`` option.

Examples
--------

The backend daemon should be started first:

::

  host# vhost-user-input --socket-path=input.sock	\
      --evdev-path=/dev/input/event17

The QEMU invocation needs to create a chardev socket to communicate with the
backend daemon and access the VirtIO queues with the guest over the
:ref:`shared memory <shared_memory_object>`.

::

  host# qemu-system								\
      -chardev socket,path=/tmp/input.sock,id=mouse0				\
      -device vhost-user-input-pci,chardev=mouse0				\
      -m 4096 									\
      -object memory-backend-file,id=mem,size=4G,mem-path=/dev/shm,share=on	\
      -numa node,memdev=mem							\
      ...


.. _vhost_user_scsi:

vhost-user-scsi - SCSI controller
=================================

The vhost-user-scsi daemon can proxy iSCSI devices onto a virtualized
SCSI controller.

.. _vhost_user_bridge:

vhost-user-bridge - Network bridge
==================================

The vhost-user-bridge daemon serves as a development tool for testing real
internet traffic by providing a networking backend, i.e. server, for the
vhost-user protocol.

Example
-------
For a single QEMU instance that both runs the user-mode net stack (slirp) and
serves as a vhost-user protocol frontend, i.e. client, simultaneously:

First, start vhost-user-bridge:

::

  $ vhost-user-bridge -u /tmp/vubr.sock \
                      -l 127.0.0.1:4444 \
                      -r 127.0.0.1:5555

Then, invoke QEMU:

::

  $ qemu-system-x86_64 \
        -m 4G \
        -object memory-backend-memfd,id=mem0,size=4G,share=on,prealloc=on \
        -numa node,memdev=mem0 \
        -chardev socket,id=char0,path=/tmp/vubr.sock \
        -netdev vhost-user,id=vhost0,chardev=char0,vhostforce=on \
        -device virtio-net-pci,netdev=vhost0 \
        -netdev socket,id=udp0,udp=localhost:4444,localaddr=localhost:5555 \
        -netdev user,id=user0 \
        -netdev hubport,id=hub0,hubid=0,netdev=udp0 \
        -netdev hubport,id=hub1,hubid=0,netdev=user0 \
        ...