[PATCH 4/5] migration: Activate block devices if VM is paused when migrating

[Fabiano Rosas]

Currently a VM that has been target of a migration using
late-block-activate will crash at the end of a new migration (with it
as source) when releasing ownership of the disks due to the VM having
never taken ownership of the disks in the first place.

The issue is that late-block-activate expects a qmp_continue command
to be issued at some point on the destination VM after the migration
finishes. If the user decides to never continue the VM, but instead
issue a new migration, then bdrv_activate_all() will never be called
and the assert will be reached:

bdrv_inactivate_recurse: Assertion `!(bs->open_flags &
BDRV_O_INACTIVE)' failed.

Fix the issue by checking at the start of migration if the VM is
paused and call bdrv_activate_all() before migrating. Even if the
late-block-activate capability is not in play or if the VM has been
paused manually, there is no harm calling that function again.

Signed-off-by: Fabiano Rosas <farosas@suse.de>
---
 migration/migration.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/migration/migration.c b/migration/migration.c
index aedf7f0751..26af30137b 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -2029,6 +2029,25 @@ static bool migrate_prepare(MigrationState *s, bool 
resume, Error **errp)
         return false;
     }
 
+    /*
+     * The VM might have been target of a previous migration. If it
+     * was in the paused state then nothing will have required the
+     * block layer to be activated. Do it now to ensure this QEMU
+     * instance owns the disk locks.
+     */
+    if (!resume && runstate_check(RUN_STATE_PAUSED)) {
+        Error *local_err = NULL;
+
+        g_assert(bql_locked());
+
+        bdrv_activate_all(&local_err);
+        if (local_err) {
+            error_propagate(errp, local_err);
+            return false;
+        }
+        s->block_inactive = false;
+    }
+
     return true;
 }
 
-- 
2.35.3