vhost-user daemons in contrib — QEMU documentation

# vhost-user daemons in contrib

QEMU provides a number of vhost-user back ends 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-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 QEMU Storage Daemon instead which supports a wide variety of storage modes and exports a number of interfaces including vhost-user.

## vhost-user-gpu - gpu device

vhost-user-gpu presents a paravirtualized GPU and display controller. You probably want to use the internal 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-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 shared memory.

    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 - SCSI controller

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

## 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 \
          ...