migration: Recover behavior of preempt channel creation for pre-7.2

In 8.0 devel window we reworked preempt channel creation, so that there'll
be no race condition when the migration channel and preempt channel got
established in the wrong order in commit 5655aab079.

However no one noticed that the change will also be not compatible with
older qemus, majorly 7.1/7.2 versions where preempt mode started to be
supported.

Leverage the same pre-7.2 flag introduced in the previous patch to recover
the behavior hopefully before 8.0 releases, so we don't break migration
when we migrate from 8.0 to older qemu binaries.

Fixes: 5655aab079 ("migration: Postpone postcopy preempt channel to be after main")
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

diff --git a/migration/migration.c b/migration/migration.c
index 37fc4fb3e2d24d968ef7709e0e379e146727a895..bda478919333a1828958f61a88344b44d2911ba4 100644 (file)
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -4388,6 +4388,15 @@ void migrate_fd_connect(MigrationState *s, Error *error_in)
         }
     }
 
+    /*
+     * This needs to be done before resuming a postcopy.  Note: for newer
+     * QEMUs we will delay the channel creation until postcopy_start(), to
+     * avoid disorder of channel creations.
+     */
+    if (migrate_postcopy_preempt() && s->preempt_pre_7_2) {
+        postcopy_preempt_setup(s);
+    }
+
     if (resume) {
         /* Wakeup the main migration thread to do the recovery */
         migrate_set_state(&s->state, MIGRATION_STATUS_POSTCOPY_PAUSED,

diff --git a/migration/migration.h b/migration/migration.h
index 67baba21846f502d6ee807a0e3a72d1753f1ef0e..310ae8901b806fb7f41378548220db5d26867877 100644 (file)
--- a/migration/migration.h
+++ b/migration/migration.h
@@ -384,12 +384,19 @@ struct MigrationState {
      * - postcopy preempt src QEMU instance will generate an EOS message at
      *   the end of migration to shut the preempt channel on dest side.
      *
+     * - postcopy preempt channel will be created at the setup phase on src
+         QEMU.
+     *
      * When clear:
      *
      * - postcopy preempt src QEMU instance will _not_ generate an EOS
      *   message at the end of migration, the dest qemu will shutdown the
      *   channel itself.
      *
+     * - postcopy preempt channel will be created at the switching phase
+     *   from precopy -> postcopy (to avoid race condtion of misordered
+     *   creation of channels).
+     *
      * NOTE: See message-id <ZBoShWArKDPpX/D7@work-vm> on qemu-devel
      * mailing list for more information on the possible race.  Everyone
      * should probably just keep this value untouched after set by the

diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
index 263bab75ecfd53c0532d505bf332bdc1250fbaa0..93f39f8e06b87161b9963e4bdebc1db22246e325 100644 (file)
--- a/migration/postcopy-ram.c
+++ b/migration/postcopy-ram.c
@@ -1630,8 +1630,14 @@ int postcopy_preempt_establish_channel(MigrationState *s)
         return 0;
     }
 
-    /* Kick off async task to establish preempt channel */
-    postcopy_preempt_setup(s);
+    /*
+     * Kick off async task to establish preempt channel.  Only do so with
+     * 8.0+ machines, because 7.1/7.2 require the channel to be created in
+     * setup phase of migration (even if racy in an unreliable network).
+     */
+    if (!s->preempt_pre_7_2) {
+        postcopy_preempt_setup(s);
+    }
 
     /*
      * We need the postcopy preempt channel to be established before