Re: [Qemu-devel] [PATCH v5 3/4] pcie: Add some SR/IOV API documentation in docs/pcie_sriov.txt

[Knut Omang]

On Fri, 2015-10-16 at 11:36 +0300, Michael S. Tsirkin wrote:
> On Wed, Oct 14, 2015 at 05:51:17PM +0200, Knut Omang wrote:
> > Add a small intro + minimal documentation for how to
> > implement SR/IOV support for an emulated device.
> 
> I worry that we won't keep this up to date as
> code changes. Could some or all of this go into
> comments in relevant headers?

Hopefully the documented part is not going to change that much - it is
not accurate code anyway, just pseudo code to aid in developing new
devices using it.

Marcel's idea to write a small doc seemed like a good idea now. We
could always remove it when a full example driver has been written.

> 
> > Signed-off-by: Knut Omang <address@hidden>
> > ---
> >  docs/pcie_sriov.txt | 115
> > ++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 115 insertions(+)
> >  create mode 100644 docs/pcie_sriov.txt
> > 
> > diff --git a/docs/pcie_sriov.txt b/docs/pcie_sriov.txt
> > new file mode 100644
> > index 0000000..f5e891e
> > --- /dev/null
> > +++ b/docs/pcie_sriov.txt
> > @@ -0,0 +1,115 @@
> > +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
> 
> That branch does not seem to be there.
> I don't think we should put such short-lived links into
> repository.

Sorry, I just forgot to push it, it's there now.
I'll make sure it stays valid for as long as the reference in the doc
is there.

Hopefully this is a temporary way to be cleaned up once a working
example has been implemented,

But feel free to just skip this patch, the most important is to get the
generic SR/IOV code in there for others to use.

Thanks,
Knut

> > +
> > +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