RE: [PATCH] pci-iov: Add support for unmanaged SR-IOV

[Tian, Kevin]

> From: Alex Williamson [mailto:alex.williamson@xxxxxxxxxx]
> Sent: Saturday, March 3, 2018 2:14 AM
> 
> On Fri, 2 Mar 2018 06:54:17 +0000
> "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote:
> 
> > > From: Alex Williamson
> > > Sent: Friday, March 2, 2018 4:22 AM
> > > >
> > > > I am pretty sure that you are describing is true of some, but not for
> > > > all. I think the Amazon solutions and the virtio solution are doing
> > > > hard partitioning of the part. I will leave it to those guys to speak
> > > > for themselves since I don't know anything about the hardware design
> > > > of those parts.
> > >
> > > I think we'd need device specific knowledge and enablement to be able
> > > to take advantage of any hardware partitioning, otherwise we need to
> > > assume the pf is privileged, as implemented in other sriov devices.
> > >
> > > I'm also trying to imagine whether there's a solution via the new
> > > vfio/mdev interface, where the mdev vendor driver would bind to the
> pf
> > > and effectively present itself as the mdev device.  The vendor driver
> > > could provide sriov capabilities and bear the burden of ensuring that
> > > the pf is used cooperatively.  The only existing mdev vendor drivers are
> > > vGPUs and rely on on-device DMA translation and isolation, such as
> > > through GTTs, but there have been some thoughts on providing IOMMU
> > > based
> > > isolation of mdev/sriov mixed devices (assuming DMA is even required
> > > for userspace management of the pf in this use case).  [Cc Kirti]
> > > Thanks,
> > >
> >
> > Hope not distracting this thread, but above sounds like an interesting
> > idea. Actually we ever brainstormed similar thought for another
> > potential usage - supporting VF live migration. We are already working
> > on an generic extension to allow state save/restore of mdev instance.
> > If vendor driver could further wrap pf as a mdev instance, it could
> > leverage the same framework for a clean state migration on VF. based
> > on mmap callback the vendor driver can easily switch back-and-forth
> > between pass through and trap/emulation of the VF resources. Of
> > course doing so alone doesn't address all the demands of VF live
> > migration (e.g. dirty page tracking still requires other techniques),
> > but it does pave a way toward a general framework to support VF
> > live migration.
> >
> > If above is feasible, finally we could use one mdev framework to
> > manage both mdev and pf/vf assignment, while providing added
> > values which are difficult to achieve today. :-)
> 
> mdev drivers may be the first to support migration, but I wonder if a
> full mdev implementation is necessary for it.  Once the vfio api is
> define, device specific extensions to vfio-pci might be able to
> implement migration more directly.  Thanks,
> 
> Alex

yes technically a full mdev implementation is not necessary. If
device specific extensions will be placed within vfio module, it's
obviously straightforward. What I thought earlier was in case vfio
wants to stay device-agnostic then we probably want device
specific extensions in vendor driver which is loaded but in a 
dummy mode which simply do basic PCI initialization as vfio-pci
and then wrap vf as mdev (since vfio-pci is not the vf driver in
this scenario). It's especially useful for vendor drivers which aim
to support both mdev and sr-iov by sharing common state mgmt.
knowledge, but looks an overkill to other vendor drivers. Possibly
finally we'll allow both - simple device extensions in vfio-pci and 
complex device extensions in vendor drivers through vfio-mdev.

Thanks
Kevin