[PULL 12/13] vfio/migration: Block VFIO migration with background snapshot

["Cédric Le Goater" <clg@redhat.com>]

From: Avihai Horon <avihaih@nvidia.com>

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.

To prevent this, block VFIO migration with background snapshot.

Signed-off-by: Avihai Horon <avihaih@nvidia.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>
---
 hw/vfio/migration.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/hw/vfio/migration.c b/hw/vfio/migration.c
index 20994dc1d60b1606728415fec17c19cfd00c4dee..da43dcd2fe0734091960e3eb2ffa26750073be72 100644
--- a/hw/vfio/migration.c
+++ b/hw/vfio/migration.c
@@ -340,7 +340,8 @@ static int vfio_save_prepare(void *opaque, Error **errp)
     VFIODevice *vbasedev = opaque;
 
     /*
-     * Snapshot doesn't use postcopy, so allow snapshot even if postcopy is on.
+     * Snapshot doesn't use postcopy nor background snapshot, so allow snapshot
+     * even if they are on.
      */
     if (runstate_check(RUN_STATE_SAVE_VM)) {
         return 0;
@@ -353,6 +354,14 @@ static int vfio_save_prepare(void *opaque, Error **errp)
         return -EOPNOTSUPP;
     }
 
+    if (migrate_background_snapshot()) {
+        error_setg(
+            errp,
+            "%s: VFIO migration is not supported with background snapshot",
+            vbasedev->name);
+        return -EOPNOTSUPP;
+    }
+
     return 0;
 }
 
-- 
2.41.0