[RFC PATCH v3 19/30] migration/ram: Ignore multifd flush when doing fixed-ram migration

[Fabiano Rosas]

Some functionalities of multifd are incompatible with the 'fixed-ram'
migration format.

The MULTIFD_FLUSH flag in particular is not used because in fixed-ram
there is no sinchronicity between migration source and destination so
there is not need for a sync packet. In fact, fixed-ram disables
packets in multifd as a whole.

However, we still need to sync the migration thread with the multifd
channels at key moments:

- between iterations, to avoid a slow channel being overrun by a fast
channel in the subsequent iteration;

- at ram_save_complete, to make sure all data has been transferred
  before finishing migration;

Make sure RAM_SAVE_FLAG_MULTIFD_FLUSH is only emitted for fixed-ram at
those key moments.

Signed-off-by: Fabiano Rosas <faro...@suse.de>
---
 migration/ram.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/migration/ram.c b/migration/ram.c
index 08604222f2..ad6abd1761 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -1363,7 +1363,7 @@ static int find_dirty_block(RAMState *rs, 
PageSearchStatus *pss)
         pss->page = 0;
         pss->block = QLIST_NEXT_RCU(pss->block, next);
         if (!pss->block) {
-            if (migrate_multifd() &&
+            if (migrate_multifd() && !migrate_fixed_ram() &&
                 !migrate_multifd_flush_after_each_section()) {
                 QEMUFile *f = rs->pss[RAM_CHANNEL_PRECOPY].pss_channel;
                 int ret = multifd_send_sync_main(f);
@@ -3112,7 +3112,8 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
         return ret;
     }
 
-    if (migrate_multifd() && !migrate_multifd_flush_after_each_section()) {
+    if (migrate_multifd() && !migrate_multifd_flush_after_each_section()
+        && !migrate_fixed_ram()) {
         qemu_put_be64(f, RAM_SAVE_FLAG_MULTIFD_FLUSH);
     }
 
@@ -3242,8 +3243,11 @@ static int ram_save_iterate(QEMUFile *f, void *opaque)
 out:
     if (ret >= 0
         && migration_is_setup_or_active(migrate_get_current()->state)) {
-        if (migrate_multifd() && migrate_multifd_flush_after_each_section()) {
-            ret = 
multifd_send_sync_main(rs->pss[RAM_CHANNEL_PRECOPY].pss_channel);
+        if (migrate_multifd() &&
+            (migrate_multifd_flush_after_each_section() ||
+             migrate_fixed_ram())) {
+            ret = multifd_send_sync_main(
+                rs->pss[RAM_CHANNEL_PRECOPY].pss_channel);
             if (ret < 0) {
                 return ret;
             }
-- 
2.35.3