Re: [PATCH v2 1/1] docs/devel: Add VFIO device migration documentation

[Dr. David Alan Gilbert]

* Tian, Kevin (kevin.tian@intel.com) wrote:
> > From: Qemu-devel <qemu-devel-bounces+kevin.tian=intel.com@nongnu.org>
> > On Behalf Of Dr. David Alan Gilbert
> > 
> > * Daniel P. Berrangé (berrange@redhat.com) wrote:
> > > On Thu, Mar 11, 2021 at 12:50:09AM +0530, Tarun Gupta wrote:
> > > > Document interfaces used for VFIO device migration. Added flow of state
> > changes
> > > > during live migration with VFIO device. Tested by building docs with the
> > new
> > > > vfio-migration.rst file.
> > > >
> > > > v2:
> > > > - Included the new vfio-migration.rst file in index.rst
> > > > - Updated dirty page tracking section, also added details about
> > > >   'pre-copy-dirty-page-tracking' opt-out option.
> > > > - Incorporated comments around wording of doc.
> > > >
> > > > Signed-off-by: Tarun Gupta <targupta@nvidia.com>
> > > > Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
> > > > ---
> > > >  MAINTAINERS                   |   1 +
> > > >  docs/devel/index.rst          |   1 +
> > > >  docs/devel/vfio-migration.rst | 135
> > ++++++++++++++++++++++++++++++++++
> > > >  3 files changed, 137 insertions(+)
> > > >  create mode 100644 docs/devel/vfio-migration.rst
> > >
> > >
> > > > +Postcopy
> > > > +========
> > > > +
> > > > +Postcopy migration is not supported for VFIO devices.
> > >
> > > What is the problem here and is there any plan for how to address it ?
> > 
> > There's no equivalent to userfaultfd for accesses to RAM made by a
> > device.
> > There's some potential for this to be doable with an IOMMU or the like,
> > but:
> >   a) IOMMUs and devices aren't currently happy at recovering from
> > failures
> >   b) the fragementation you get during a postcopy probably isn't pretty
> > when you get to build IOMMU tables.
> 
> To overcome such limitations one may adopt a prefault-and-pull scheme if 
> the vendor driver has the capability to track pending DMA buffers in the
> migration process (with additional uAPI changes in VFIO or userfaultfd), 
> as discussed here:
> 
> https://static.sched.com/hosted_files/kvmforum2019/7a/kvm-forum-postcopy-final.pdf

Did that get any further?

I can imagine that might be tricikier for a GPU than a network card; the
shaders in a GPU are pretty random as to what they go off and access, so
I can't see how you could prefault.

Dave

> > 
> > > Postcopy is essentially the only migration mechanism that can reliably
> > > complete, so it really should be considered the default approach to
> > > migration for all mgmt apps wanting to do migration, except in special
> > > cases.   IOW, if we want VFIO migration to be viable, we need postcopy
> > > support.
> > 
> > There's lots of other things postcopy doesn't work with; so hmm.
> > 
> 
> Agree. Also given the amount of work even for pre-copy migration, it makes 
> more sense to do things step-by-step.
> 
> Thanks
> Kevin
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK