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>
---
 migration/multifd.c    | 17 +++++++++++++----
 migration/trace-events |  2 +-
 2 files changed, 14 insertions(+), 5 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) {
diff --git a/migration/trace-events b/migration/trace-events
index 298ad2b0dd..bf1a069632 100644
--- a/migration/trace-events
+++ b/migration/trace-events
@@ -132,7 +132,7 @@ multifd_recv(uint8_t id, uint64_t packet_num, uint32_t used, uint32_t flags, uin
 multifd_recv_new_channel(uint8_t id) "channel %u"
 multifd_recv_sync_main(long packet_num) "packet num %ld"
 multifd_recv_sync_main_signal(uint8_t id) "channel %u"
-multifd_recv_sync_main_wait(uint8_t id) "channel %u"
+multifd_recv_sync_main_wait(uint8_t id) "iter %u"
 multifd_recv_terminate_threads(bool error) "error %d"
 multifd_recv_thread_end(uint8_t id, uint64_t packets, uint64_t pages) "channel %u packets %" PRIu64 " pages %" PRIu64
 multifd_recv_thread_start(uint8_t id) "%u"
-- 
2.35.3