vl: Drain before (block) job cancel when quitting

If the main loop cancels all block jobs while the block layer is not
drained, this cancelling may not happen instantaneously.  We can start a
drained section before vm_shutdown(), which entails another
bdrv_drain_all(); this nested bdrv_drain_all() will thus be a no-op,
basically.

We do not have to end the drained section, because we actually do not
want any requests to happen from this point on.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 11 insertions, 0 deletions

diff --git a/vl.c b/vl.c
index a5808f9..53335a5 100644
--- a/vl.c
+++ b/vl.c
@@ -4480,6 +4480,17 @@ int main(int argc, char **argv, char **envp)
      */
     migration_shutdown();
 
+    /*
+     * We must cancel all block jobs while the block layer is drained,
+     * or cancelling will be affected by throttling and thus may block
+     * for an extended period of time.
+     * vm_shutdown() will bdrv_drain_all(), so we may as well include
+     * it in the drained section.
+     * We do not need to end this section, because we do not want any
+     * requests happening from here on anyway.
+     */
+    bdrv_drain_all_begin();
+
     /* No more vcpu or device emulation activity beyond this point */
     vm_shutdown();