migration/multifd: Cleanup multifd_recv_sync_main

Some minor cleanups and documentation for multifd_recv_sync_main.

Use thread_count as done in other parts of the code. Remove p->id from
the multifd_recv_state sync, since that is global and not tied to a
channel. Add documentation for the sync steps.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20240229153017.2221-2-farosas@suse.de
Signed-off-by: Peter Xu <peterx@redhat.com>

1 files changed, 13 insertions, 4 deletions

diff --git a/migration/multifd.c b/migration/multifd.c
index 6c07f19af1..c7389bf833 100644
--- a/migration/multifd.c
+++ b/migration/multifd.c
@@ -1182,18 +1182,27 @@ void multifd_recv_cleanup(void)
 
 void multifd_recv_sync_main(void)
 {
+    int thread_count = migrate_multifd_channels();
     int i;
 
     if (!migrate_multifd()) {
         return;
     }
-    for (i = 0; i < migrate_multifd_channels(); i++) {
-        MultiFDRecvParams *p = &multifd_recv_state->params[i];
 
-        trace_multifd_recv_sync_main_wait(p->id);
+    /*
+     * Initiate the synchronization by waiting for all channels.
+     * For socket-based migration this means each channel has received
+     * the SYNC packet on the stream.
+     */
+    for (i = 0; i < thread_count; i++) {
+        trace_multifd_recv_sync_main_wait(i);
         qemu_sem_wait(&multifd_recv_state->sem_sync);
     }
-    for (i = 0; i < migrate_multifd_channels(); i++) {
+
+    /*
+     * Sync done. Release the channels for the next iteration.
+     */
+    for (i = 0; i < thread_count; i++) {
         MultiFDRecvParams *p = &multifd_recv_state->params[i];
 
         WITH_QEMU_LOCK_GUARD(&p->mutex) {