docs/system: add a basic enumeration of vhost-user devices

Make it clear the vhost-user-device is intended for expert use only.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240104210945.1223134-8-alex.bennee@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

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

@@ -1,3 +1,5 @@
+.. _vhost_user_rng:
+
 QEMU vhost-user-rng - RNG emulation
 ===================================
 

docs/system/devices/vhost-user.rst

@@ -8,13 +8,81 @@ outside of QEMU itself. To do this there are a number of things
 required.
 
 vhost-user device
-===================
+=================
 
 These are simple stub devices that ensure the VirtIO device is visible
 to the guest. The code is mostly boilerplate although each device has
 a ``chardev`` option which specifies the ID of the ``--chardev``
 device that connects via a socket to the vhost-user *daemon*.
 
+Each device will have an virtio-mmio and virtio-pci variant. See your
+platform details for what sort of virtio bus to use.
+
+.. list-table:: vhost-user devices
+  :widths: 20 20 60
+  :header-rows: 1
+
+  * - Device
+    - Type
+    - Notes
+  * - vhost-user-blk
+    - Block storage
+    - See contrib/vhost-user-blk
+  * - vhost-user-fs
+    - File based storage driver
+    - See https://gitlab.com/virtio-fs/virtiofsd
+  * - vhost-user-gpio
+    - Proxy gpio pins to host
+    - See https://github.com/rust-vmm/vhost-device
+  * - vhost-user-gpu
+    - GPU driver
+    - See contrib/vhost-user-gpu
+  * - vhost-user-i2c
+    - Proxy i2c devices to host
+    - See https://github.com/rust-vmm/vhost-device
+  * - vhost-user-input
+    - Generic input driver
+    - See contrib/vhost-user-input
+  * - vhost-user-rng
+    - Entropy driver
+    - :ref:`vhost_user_rng`
+  * - vhost-user-scmi
+    - System Control and Management Interface
+    - See https://github.com/rust-vmm/vhost-device
+  * - vhost-user-snd
+    - Audio device
+    - See https://github.com/rust-vmm/vhost-device/staging
+  * - vhost-user-scsi
+    - SCSI based storage
+    - See contrib/vhost-user-scsi
+  * - vhost-user-vsock
+    - Socket based communication
+    - See https://github.com/rust-vmm/vhost-device
+
+The referenced *daemons* are not exhaustive, any conforming backend
+implementing the device and using the vhost-user protocol should work.
+
+vhost-user-device
+^^^^^^^^^^^^^^^^^
+
+The vhost-user-device is a generic development device intended for
+expert use while developing new backends. The user needs to specify
+all the required parameters including:
+
+  - Device ``virtio-id``
+  - The ``num_vqs`` it needs and their ``vq_size``
+  - The ``config_size`` if needed
+
+.. note::
+  To prevent user confusion you cannot currently instantiate
+  vhost-user-device without first patching out::
+
+    /* Reason: stop inexperienced users confusing themselves */
+    dc->user_creatable = false;
+
+  in ``vhost-user-device.c`` and ``vhost-user-device-pci.c`` file and
+  rebuilding.
+
 vhost-user daemon
 =================