[PATCH 1/2] migration/multifd: Further remove the SYNC on complete

[Peter Xu <peterx@redhat.com>]

Commit 637280aeb2 ("migration/multifd: Avoid the final FLUSH in
complete()") removed the FLUSH operation on complete() which should avoid
one global sync on destination side, because it's not needed.

However that commit overlooked multifd_ram_flush_and_sync() part of things,
as that's always used together with the FLUSH message on the main channel.

For very old binaries (multifd_flush_after_each_section==true), that's
still needed because each EOS received on destination will enforce
all-channel sync once.

For new binaries (multifd_flush_after_each_section==false), the flush and
sync shouldn't be needed just like the FLUSH, with the same reasoning.

Currently, on new binaries we'll still send SYNC messages on multifd
channels, even if FLUSH is omitted at the end.  It'll make all recv threads
hang at SYNC message.

Multifd is still all working fine because luckily recv side cleanup
code (mostly multifd_recv_sync_main()) is smart enough to make sure even if
recv threads are stuck at SYNC it'll get kicked out.  And since this is the
completion phase of migration, nothing else will be sent after the SYNCs.

This may be needed for VFIO in the future because VFIO can have data to
push after ram_save_complete(), hence we don't want the recv thread to be
stuck in SYNC message. Remove this limitation will make src even slightly
faster too to avoid some more code.

Stable branches do not need this patch, as no real bug I can think of that
will go wrong there.. so not attaching Fixes to be clear on the backport
not needed.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 migration/ram.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/migration/ram.c b/migration/ram.c
index 05ff9eb328..7284c34bd8 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -3283,9 +3283,16 @@ static int ram_save_complete(QEMUFile *f, void *opaque)
         }
     }
 
-    ret = multifd_ram_flush_and_sync();
-    if (ret < 0) {
-        return ret;
+    if (migrate_multifd() &&
+        migrate_multifd_flush_after_each_section()) {
+        /*
+         * Only the old dest QEMU will need this sync, because each EOS
+         * will require one SYNC message on each channel.
+         */
+        ret = multifd_ram_flush_and_sync();
+        if (ret < 0) {
+            return ret;
+        }
     }
 
     if (migrate_mapped_ram()) {
-- 
2.47.0