# Composable 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:

    -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. Both the PF and VFs are ARI-capable so you can have 255 VFs at maximum.

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