Hello,
Recently added VFIO migration is not compatible with some of the
pre-existing migration features. This was overlooked and today these
combinations are not blocked by QEMU. This series fixes it.
Postcopy migration:
VFIO migration is not compatible with postcopy migration. A VFIO device
in the destination can't handle page faults for pages that have not been
sent yet. Doing such migration will cause the VM to crash in the
destination.
Background snapshot:
Background snapshot allows creating a snapshot of the VM while it's
running and keeping it small by not including dirty RAM pages.
The way it works is by first stopping the VM, saving the non-iterable
devices' state and then starting the VM and saving the RAM while write
protecting it with UFFD. The resulting snapshot represents the VM state
at snapshot start.
VFIO migration is not compatible with background snapshot.
First of all, VFIO device state is not even saved in background snapshot
because only non-iterable device state is saved. But even if it was
saved, after starting the VM, a VFIO device could dirty pages without it
being detected by UFFD write protection. This would corrupt the
snapshot, as the RAM in it would not represent the RAM at snapshot
start.
This series fixes it by blocking these combinations. This is done by
adding a .save_prepare() handler to struct SaveVMHandler. The
.save_prepare() handler is called early, even before migration starts,
and allows VFIO migration to check the migration capabilities and fail
migration if needed.
Note that this series is based on the P2P series [1] sent a few weeks
ago.
Comments and suggestions will be greatly appreciated.