docs/system: Add vhost-user-input documentation

This adds basic documentation for vhost-user-input.

Signed-off-by: Leo Yan <leo.yan@linaro.org>
Message-Id: <20231120043721.50555-3-leo.yan@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240104210945.1223134-10-alex.bennee@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

MAINTAINERS

@@ -2289,6 +2289,7 @@ L: virtio-fs@lists.linux.dev
 virtio-input
 M: Gerd Hoffmann <kraxel@redhat.com>
 S: Odd Fixes
+F: docs/system/devices/vhost-user-input.rst
 F: hw/input/vhost-user-input.c
 F: hw/input/virtio-input*.c
 F: include/hw/virtio/virtio-input.h

docs/system/device-emulation.rst

@@ -94,6 +94,7 @@ Emulated Devices
    devices/virtio-gpu.rst
    devices/virtio-pmem.rst
    devices/virtio-snd.rst
+   devices/vhost-user-input.rst
    devices/vhost-user-rng.rst
    devices/canokey.rst
    devices/usb-u2f.rst

docs/system/devices/vhost-user-input.rst

@@ -0,0 +1,45 @@
+.. _vhost_user_input:
+
+QEMU vhost-user-input - Input emulation
+=======================================
+
+This document describes the setup and usage of the Virtio input device.
+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							\
+      ...

docs/system/devices/vhost-user.rst

@@ -42,7 +42,7 @@ platform details for what sort of virtio bus to use.
     - See https://github.com/rust-vmm/vhost-device
   * - vhost-user-input
     - Generic input driver
-    - See contrib/vhost-user-input
+    - :ref:`vhost_user_input`
   * - vhost-user-rng
     - Entropy driver
     - :ref:`vhost_user_rng`
@@ -91,6 +91,8 @@ following the :ref:`vhost_user_proto`. There are a number of daemons
 that can be built when enabled by the project although any daemon that
 meets the specification for a given device can be used.
 
+.. _shared_memory_object:
+
 Shared memory object
 ====================