migration: Inhibit virtio-balloon for the duration of background snapshot

The same thing as for incoming postcopy - we cannot deal with concurrent
RAM discards when using background snapshot feature in outgoing migration.

Fixes: 8518278a6a (migration: implementation
  of background snapshot thread)
Signed-off-by: Andrey Gruzdev <andrey.gruzdev@virtuozzo.com>
Reported-by: David Hildenbrand <david@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Message-Id: <20210401092226.102804-3-andrey.gruzdev@virtuozzo.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

hw/virtio/virtio-balloon.c

@@ -66,8 +66,12 @@ static bool virtio_balloon_pbp_matches(PartiallyBalloonedPage *pbp,
 
 static bool virtio_balloon_inhibited(void)
 {
-    /* Postcopy cannot deal with concurrent discards, so it's special. */
-    return ram_block_discard_is_disabled() || migration_in_incoming_postcopy();
+    /*
+     * Postcopy cannot deal with concurrent discards,
+     * so it's special, as well as background snapshots.
+     */
+    return ram_block_discard_is_disabled() || migration_in_incoming_postcopy() ||
+            migration_in_bg_snapshot();
 }
 
 static void balloon_inflate_page(VirtIOBalloon *balloon,

include/migration/misc.h

@@ -70,6 +70,8 @@ bool migration_in_postcopy_after_devices(MigrationState *);
 void migration_global_dump(Monitor *mon);
 /* True if incomming migration entered POSTCOPY_INCOMING_DISCARD */
 bool migration_in_incoming_postcopy(void);
+/* True if background snapshot is active */
+bool migration_in_bg_snapshot(void);
 
 /* migration/block-dirty-bitmap.c */
 void dirty_bitmap_mig_init(void);

migration/migration.c

@@ -1976,6 +1976,14 @@ bool migration_in_incoming_postcopy(void)
     return ps >= POSTCOPY_INCOMING_DISCARD && ps < POSTCOPY_INCOMING_END;
 }
 
+bool migration_in_bg_snapshot(void)
+{
+    MigrationState *s = migrate_get_current();
+
+    return migrate_background_snapshot() &&
+            migration_is_setup_or_active(s->state);
+}
+
 bool migration_is_idle(void)
 {
     MigrationState *s = current_migration;