xref: /openbmc/qemu/docs/pcie_sriov.txt (revision 5e25c93ccb8ddc8dda6845d6c09334ca44cbef17)

PCI SR/IOV EMULATION SUPPORT
============================

Description
===========
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.

QEMU now implements the basic common functionality to enable an emulated device
to support SR/IOV.

Implementation
==============
Implementing emulation of an SR/IOV capable device typically consists of
implementing support for two types of device classes; the "normal" physical device
(PF) and the virtual device (VF). From QEMU's perspective, the VFs are just
like other devices, except that some of their properties are derived from
the PF.

A virtual function is different from a physical function in that the BAR
space for all VFs are defined by the BAR registers in the PFs SR/IOV
capability. All VFs have the same BARs and BAR sizes.

Accesses to these virtual BARs then is computed as

   <VF BAR start> + <VF number> * <BAR sz> + <offset>

From our emulation perspective this means that there is a separate call for
setting up a BAR for a VF.

1) To enable SR/IOV support in the PF, it must be a PCI Express device so
   you would need to add a PCI Express capability in the normal PCI
   capability list. You might also want to add an ARI (Alternative
   Routing-ID Interpretation) capability to indicate that your device
   supports functions beyond it's "own" function space (0-7),
   which is necessary to support more than 7 functions, or
   if functions extends beyond offset 7 because they are placed at an
   offset > 1 or have stride > 1.

   ...
   #include "hw/pci/pcie.h"
   #include "hw/pci/pcie_sriov.h"

   pci_your_pf_dev_realize( ... )
   {
      ...
      int ret = pcie_endpoint_cap_init(d, 0x70);
      ...
      pcie_ari_init(d, 0x100);
      ...

      /* Add and initialize the SR/IOV capability */
      pcie_sriov_pf_init(d, 0x200, "your_virtual_dev",
                       vf_devid, initial_vfs, total_vfs,
                       fun_offset, stride);

      /* Set up individual VF BARs (parameters as for normal BARs) */
      pcie_sriov_pf_init_vf_bar( ... )
      ...
   }

   For cleanup, you simply call:

      pcie_sriov_pf_exit(device);

   which will delete all the virtual functions and associated resources.

2) Similarly in the implementation of the virtual function, you need to
   make it a PCI Express device and add a similar set of capabilities
   except for the SR/IOV capability. Then you need to set up the VF BARs as
   subregions of the PFs SR/IOV VF BARs by calling
   pcie_sriov_vf_register_bar() instead of the normal pci_register_bar() call:

   pci_your_vf_dev_realize( ... )
   {
      ...
      int ret = pcie_endpoint_cap_init(d, 0x60);
      ...
      pcie_ari_init(d, 0x100);
      ...
      memory_region_init(mr, ... )
      pcie_sriov_vf_register_bar(d, bar_nr, mr);
      ...
   }

Testing on Linux guest
======================
The easiest is if your device driver supports sysfs based SR/IOV
enabling. Support for this was added in kernel v.3.8, so not all drivers
support it yet.

To enable 4 VFs for a device at 01:00.0:

	modprobe yourdriver
	echo 4 > /sys/bus/pci/devices/0000:01:00.0/sriov_numvfs

You should now see 4 VFs with lspci.
To turn SR/IOV off again - the standard requires you to turn it off before you can enable
another VF count, and the emulation enforces this:

	echo 0 > /sys/bus/pci/devices/0000:01:00.0/sriov_numvfs

Older drivers typically provide a max_vfs module parameter
to enable it at load time:

	modprobe yourdriver max_vfs=4

To disable the VFs again then, you simply have to unload the driver:

	rmmod yourdriver