migration/multifd: Unify RAM_SAVE_FLAG_MULTIFD_FLUSH messages

RAM_SAVE_FLAG_MULTIFD_FLUSH message should always be correlated to a sync
request on src.  Unify such message into one place, and conditionally send
the message only if necessary.

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20241206224755.1108686-5-peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>

migration/multifd-nocomp.c

@@ -20,6 +20,7 @@
 #include "qemu/cutils.h"
 #include "qemu/error-report.h"
 #include "trace.h"
+#include "qemu-file.h"
 
 static MultiFDSendData *multifd_ram_send;
 
@@ -343,9 +344,10 @@ retry:
     return true;
 }
 
-int multifd_ram_flush_and_sync(void)
+int multifd_ram_flush_and_sync(QEMUFile *f)
 {
     MultiFDSyncReq req;
+    int ret;
 
     if (!migrate_multifd()) {
         return 0;
@@ -361,7 +363,28 @@ int multifd_ram_flush_and_sync(void)
     /* File migrations only need to sync with threads */
     req = migrate_mapped_ram() ? MULTIFD_SYNC_LOCAL : MULTIFD_SYNC_ALL;
 
-    return multifd_send_sync_main(req);
+    ret = multifd_send_sync_main(req);
+    if (ret) {
+        return ret;
+    }
+
+    /* If we don't need to sync with remote at all, nothing else to do */
+    if (req == MULTIFD_SYNC_LOCAL) {
+        return 0;
+    }
+
+    /*
+     * Old QEMUs don't understand RAM_SAVE_FLAG_MULTIFD_FLUSH, it relies
+     * on RAM_SAVE_FLAG_EOS instead.
+     */
+    if (migrate_multifd_flush_after_each_section()) {
+        return 0;
+    }
+
+    qemu_put_be64(f, RAM_SAVE_FLAG_MULTIFD_FLUSH);
+    qemu_fflush(f);
+
+    return 0;
 }
 
 bool multifd_send_prepare_common(MultiFDSendParams *p)

migration/multifd.h

@@ -354,7 +354,7 @@ static inline uint32_t multifd_ram_page_count(void)
 
 void multifd_ram_save_setup(void);
 void multifd_ram_save_cleanup(void);
-int multifd_ram_flush_and_sync(void);
+int multifd_ram_flush_and_sync(QEMUFile *f);
 size_t multifd_ram_payload_size(void);
 void multifd_ram_fill_packet(MultiFDSendParams *p);
 int multifd_ram_unfill_packet(MultiFDRecvParams *p, Error **errp);

migration/ram.c

@@ -1306,15 +1306,10 @@ static int find_dirty_block(RAMState *rs, PageSearchStatus *pss)
                 (!migrate_multifd_flush_after_each_section() ||
                  migrate_mapped_ram())) {
                 QEMUFile *f = rs->pss[RAM_CHANNEL_PRECOPY].pss_channel;
-                int ret = multifd_ram_flush_and_sync();
+                int ret = multifd_ram_flush_and_sync(f);
                 if (ret < 0) {
                     return ret;
                 }
-
-                if (!migrate_mapped_ram()) {
-                    qemu_put_be64(f, RAM_SAVE_FLAG_MULTIFD_FLUSH);
-                    qemu_fflush(f);
-                }
             }
 
             /* Hit the end of the list */
@@ -3044,18 +3039,13 @@ static int ram_save_setup(QEMUFile *f, void *opaque, Error **errp)
     }
 
     bql_unlock();
-    ret = multifd_ram_flush_and_sync();
+    ret = multifd_ram_flush_and_sync(f);
     bql_lock();
     if (ret < 0) {
         error_setg(errp, "%s: multifd synchronization failed", __func__);
         return ret;
     }
 
-    if (migrate_multifd() && !migrate_multifd_flush_after_each_section()
-        && !migrate_mapped_ram()) {
-        qemu_put_be64(f, RAM_SAVE_FLAG_MULTIFD_FLUSH);
-    }
-
     qemu_put_be64(f, RAM_SAVE_FLAG_EOS);
     ret = qemu_fflush(f);
     if (ret < 0) {
@@ -3190,7 +3180,7 @@ out:
     if (ret >= 0 && migration_is_running()) {
         if (migrate_multifd() && migrate_multifd_flush_after_each_section() &&
             !migrate_mapped_ram()) {
-            ret = multifd_ram_flush_and_sync();
+            ret = multifd_ram_flush_and_sync(f);
             if (ret < 0) {
                 return ret;
             }
@@ -3268,7 +3258,7 @@ static int ram_save_complete(QEMUFile *f, void *opaque)
          * 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();
+        ret = multifd_ram_flush_and_sync(f);
         if (ret < 0) {
             return ret;
         }