* 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. > 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. Dave > > Regards, > Daniel > -- > |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| > |: https://libvirt.org -o- https://fstop138.berrange.com :| > |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :| -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
