> From: Qemu-devel <qemu-devel-bounces+kevin.tian=intel....@nongnu.org> > On Behalf Of Dr. David Alan Gilbert > > * Daniel P. Berrangé (berra...@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 <targu...@nvidia.com> > > > Signed-off-by: Kirti Wankhede <kwankh...@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 > > > 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
