[PULL 32/42] migration: Adjust postcopy bandwidth during switchover

[Fabiano Rosas]

From: Peter Xu <peterx@redhat.com>

Precopy uses unlimited bandwidth always during switchover, it makes sense
because this is so critical and no one would like to throttle bandwidth
during the VM blackout.

OTOH, postcopy surprisingly didn't do that.  There's one line that in the
middle of the postcopy switchover it tries to switch to postcopy's
specified max-postcopy-bandwidth, but even so it's somewhere in the middle
which is strange.

This patch brings the two modes to always use unlimited bandwidth for
switchover, meanwhile only apply the postcopy max bandwidth after the
switchover is completed.

Signed-off-by: Peter Xu <peterx@redhat.com>
Tested-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Juraj Marcin <jmarcin@redhat.com>
Link: 20250114230746.3268797-8-peterx@redhat.com">https://lore.kernel.org/r/20250114230746.3268797-8-peterx@redhat.com
Signed-off-by: Fabiano Rosas <farosas@suse.de>
---
 migration/migration.c | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/migration/migration.c b/migration/migration.c
index 46e30a4814..03e3631d5b 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -2629,7 +2629,6 @@ static int postcopy_start(MigrationState *ms, Error 
**errp)
     int ret;
     QIOChannelBuffer *bioc;
     QEMUFile *fb;
-    uint64_t bandwidth = migrate_max_postcopy_bandwidth();
     int cur_state = MIGRATION_STATUS_ACTIVE;
 
     /*
@@ -2678,6 +2677,9 @@ static int postcopy_start(MigrationState *ms, Error 
**errp)
         goto fail;
     }
 
+    /* Switchover phase, switch to unlimited */
+    migration_rate_set(RATE_LIMIT_DISABLED);
+
     /*
      * Cause any non-postcopiable, but iterative devices to
      * send out their final data.
@@ -2694,12 +2696,6 @@ static int postcopy_start(MigrationState *ms, Error 
**errp)
         ram_postcopy_send_discard_bitmap(ms);
     }
 
-    /*
-     * send rest of state - note things that are doing postcopy
-     * will notice we're in POSTCOPY_ACTIVE and not actually
-     * wrap their state up here
-     */
-    migration_rate_set(bandwidth);
     if (migrate_postcopy_ram()) {
         /* Ping just for debugging, helps line traces up */
         qemu_savevm_send_ping(ms->to_dst_file, 2);
@@ -2783,6 +2779,12 @@ static int postcopy_start(MigrationState *ms, Error 
**errp)
     }
     trace_postcopy_preempt_enabled(migrate_postcopy_preempt());
 
+    /*
+     * Now postcopy officially started, switch to postcopy bandwidth that
+     * user specified.
+     */
+    migration_rate_set(migrate_max_postcopy_bandwidth());
+
     return ret;
 
 fail_closefb:
-- 
2.35.3