Skip saving and loading any ram data in the packet in the case of a
SYNC. This fixes a shortcoming of the current code which requires a
reset of the MultiFDPages_t fields right after the previous
pending_job finishes, otherwise the very next job might be a SYNC and
multifd_send_fill_packet() will put the stale values in the packet.

By not calling multifd_ram_fill_packet(), we can stop resetting
MultiFDPages_t in the multifd core and leave that to the client code.

Actually moving the reset function is not yet done because
pages->num==0 is used by the client code to determine whether the
MultiFDPages_t needs to be flushed. The subsequent patches will
replace that with a generic flag that is not dependent on
MultiFDPages_t.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
---
 migration/multifd.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/migration/multifd.c b/migration/multifd.c
index d64fcdf4ac..3a164c124d 100644
--- a/migration/multifd.c
+++ b/migration/multifd.c
@@ -452,6 +452,7 @@ void multifd_send_fill_packet(MultiFDSendParams *p)
 {
     MultiFDPacket_t *packet = p->packet;
     uint64_t packet_num;
+    bool sync_packet = p->flags & MULTIFD_FLAG_SYNC;
 
     memset(packet, 0, p->packet_len);
 
@@ -466,7 +467,9 @@ void multifd_send_fill_packet(MultiFDSendParams *p)
 
     p->packets_sent++;
 
-    multifd_ram_fill_packet(p);
+    if (!sync_packet) {
+        multifd_ram_fill_packet(p);
+    }
 
     trace_multifd_send_fill(p->id, packet_num,
                             p->flags, p->next_packet_size);
@@ -574,7 +577,9 @@ static int multifd_recv_unfill_packet(MultiFDRecvParams *p, Error **errp)
     p->packet_num = be64_to_cpu(packet->packet_num);
     p->packets_recved++;
 
-    ret = multifd_ram_unfill_packet(p, errp);
+    if (!(p->flags & MULTIFD_FLAG_SYNC)) {
+        ret = multifd_ram_unfill_packet(p, errp);
+    }
 
     trace_multifd_recv_unfill(p->id, p->packet_num, p->flags,
                               p->next_packet_size);
@@ -1536,7 +1541,9 @@ static void *multifd_recv_thread(void *opaque)
             flags = p->flags;
             /* recv methods don't know how to handle the SYNC flag */
             p->flags &= ~MULTIFD_FLAG_SYNC;
-            has_data = p->normal_num || p->zero_num;
+            if (!(flags & MULTIFD_FLAG_SYNC)) {
+                has_data = p->normal_num || p->zero_num;
+            }
             qemu_mutex_unlock(&p->mutex);
         } else {
             /*
-- 
2.35.3