docs: Document composable SR-IOV device

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Message-Id: <20250314-sriov-v9-8-57dae8ae3ab5@daynix.com>
Tested-by: Yui Washizu <yui.washidu@gmail.com>
Tested-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

MAINTAINERS

@@ -2058,6 +2058,7 @@ F: hw/pci-bridge/*
 F: qapi/pci.json
 F: docs/pci*
 F: docs/specs/*pci*
+F: docs/system/sriov.rst
 
 PCIE DOE
 M: Huai-Cheng Kuo <hchkuo@avery-design.com.tw>

docs/system/index.rst

@@ -39,3 +39,4 @@ or Hypervisor.Framework.
    multi-process
    confidential-guest-support
    vm-templating
+   sriov

docs/system/sriov.rst

@@ -0,0 +1,36 @@
+.. SPDX-License-Identifier: GPL-2.0-or-later
+
+Compsable SR-IOV device
+=======================
+
+SR-IOV (Single Root I/O Virtualization) is an optional extended capability of a
+PCI Express device. It allows a single physical function (PF) to appear as
+multiple virtual functions (VFs) for the main purpose of eliminating software
+overhead in I/O from virtual machines.
+
+There are devices with predefined SR-IOV configurations, but it is also possible
+to compose an SR-IOV device yourself. Composing an SR-IOV device is currently
+only supported by virtio-net-pci.
+
+Users can configure an SR-IOV-capable virtio-net device by adding
+virtio-net-pci functions to a bus. Below is a command line example:
+
+.. code-block:: shell
+
+    -netdev user,id=n -netdev user,id=o
+    -netdev user,id=p -netdev user,id=q
+    -device pcie-root-port,id=b
+    -device virtio-net-pci,bus=b,addr=0x0.0x3,netdev=q,sriov-pf=f
+    -device virtio-net-pci,bus=b,addr=0x0.0x2,netdev=p,sriov-pf=f
+    -device virtio-net-pci,bus=b,addr=0x0.0x1,netdev=o,sriov-pf=f
+    -device virtio-net-pci,bus=b,addr=0x0.0x0,netdev=n,id=f
+
+The VFs specify the paired PF with ``sriov-pf`` property. The PF must be
+added after all VFs. It is the user's responsibility to ensure that VFs have
+function numbers larger than one of the PF, and that the function numbers
+have a consistent stride.
+
+You may also need to perform additional steps to activate the SR-IOV feature on
+your guest. For Linux, refer to [1]_.
+
+.. [1] https://docs.kernel.org/PCI/pci-iov-howto.html