pcie: Add some SR/IOV API documentation in docs/pcie_sriov.txt

Add a small intro + minimal documentation for how to
implement SR/IOV support for an emulated device.

Signed-off-by: Knut Omang <knuto@ifi.uio.no>
Message-Id: <20220217174504.1051716-3-lukasz.maniak@linux.intel.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

docs/pcie_sriov.txt

+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. Yet no fully implemented devices exists in Qemu, but a
+proof-of-concept hack of the Intel igb can be found here:
+
+git://github.com/knuto/qemu.git sriov_patches_v5
+
+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, 1);
+      ...
+
+      /* 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, 1);
+      ...
+      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