Re: [PATCH v3 0/6] vfio/migration: Block VFIO migration with postcopy and background snapshot

["Cédric Le Goater" <clg@redhat.com>]

On 9/6/23 17:08, Avihai Horon wrote:
> 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.

Applied to vfio-next.

Thanks,

C.