Add some basic documentation on vfio-user usage.

Signed-off-by: John Levon <john.levon@nutanix.com>
---
 docs/system/device-emulation.rst  |  1 +
 docs/system/devices/vfio-user.rst | 24 ++++++++++++++++++++++++
 2 files changed, 25 insertions(+)
 create mode 100644 docs/system/devices/vfio-user.rst

diff --git a/docs/system/device-emulation.rst b/docs/system/device-emulation.rst
index a1b0d7997e..911381643f 100644
--- a/docs/system/device-emulation.rst
+++ b/docs/system/device-emulation.rst
@@ -85,6 +85,7 @@ Emulated Devices
    devices/can.rst
    devices/ccid.rst
    devices/cxl.rst
+   devices/vfio-user.rst
    devices/ivshmem.rst
    devices/ivshmem-flat.rst
    devices/keyboard.rst
diff --git a/docs/system/devices/vfio-user.rst b/docs/system/devices/vfio-user.rst
new file mode 100644
index 0000000000..e33e49d283
--- /dev/null
+++ b/docs/system/devices/vfio-user.rst
@@ -0,0 +1,24 @@
+=========
+vfio-user
+=========
+
+QEMU includes a ``vfio-user`` client. The ``vfio-user`` specification allows for
+implementing (PCI) devices in userspace outside of QEMU; it is similar to
+``vhost-user`` in this respect (see :doc:`vhost-user`), but can emulate arbitrary
+PCI devices, not just ``virtio``. Whereas ``vfio`` is handled by the host
+kernel, ``vfio-user``, while similar in implementation, is handled entirely in
+userspace.
+
+For example, SPDK includes a virtual PCI NVMe controller implementation; by
+setting up a ``vfio-user`` UNIX socket between QEMU and SPDK, a VM can send NVMe
+I/O to the SPDK process.
+
+Presuming a suitable ``vfio-user`` server has opened a socket at
+``/tmp/vfio-user.sock``, a device can be configured with for example:
+
+.. code-block:: console
+
+-device vfio-user-pci,socket=/tmp/vfio-user.sock,bus=pcie.0,addr=0x9
+
+See `libvfio-user <https://github.com/nutanix/libvfio-user/>`_ for further
+information.
-- 
2.43.0