migration: isolate return path on src

There are some places that binded "return path" with postcopy. Let's be
prepared for its usage even without postcopy. This patch mainly did this
on source side.
Signed-off-by: NPeter Xu <peterx@redhat.com>
Reviewed-by: NJuan Quintela <quintela@redhat.com>
Signed-off-by: NJuan Quintela <quintela@redhat.com>

migration/migration.c

@@ -1857,13 +1857,12 @@ static void migration_completion(MigrationState *s, int current_active_state,
      * cleaning everything else up (since if there are no failures
      * it will wait for the destination to send it's status in
      * a SHUT command).
-     * Postcopy opens rp if enabled (even if it's not avtivated)
      */
-    if (migrate_postcopy_ram()) {
+    if (s->rp_state.from_dst_file) {
         int rp_error;
-        trace_migration_completion_postcopy_end_before_rp();
+        trace_migration_return_path_end_before();
         rp_error = await_return_path_close_on_source(s);
-        trace_migration_completion_postcopy_end_after_rp(rp_error);
+        trace_migration_return_path_end_after(rp_error);
         if (rp_error) {
             goto fail_invalidate;
         }
@@ -1938,13 +1937,15 @@ static void *migration_thread(void *opaque)
 
     qemu_savevm_state_header(s->to_dst_file);
 
-    if (migrate_postcopy_ram()) {
+    if (s->to_dst_file) {
         /* Now tell the dest that it should open its end so it can reply */
         qemu_savevm_send_open_return_path(s->to_dst_file);
 
         /* And do a ping that will make stuff easier to debug */
         qemu_savevm_send_ping(s->to_dst_file, 1);
+    }
 
+    if (migrate_postcopy_ram()) {
         /*
          * Tell the destination that we *might* want to do postcopy later;
          * if the other end can't do postcopy it should fail now, nice and

migration/trace-events

@@ -88,8 +88,8 @@ migrate_send_rp_message(int msg_type, uint16_t len) "%d: len %d"
 migration_completion_file_err(void) ""
 migration_completion_postcopy_end(void) ""
 migration_completion_postcopy_end_after_complete(void) ""
-migration_completion_postcopy_end_before_rp(void) ""
-migration_completion_postcopy_end_after_rp(int rp_error) "%d"
+migration_return_path_end_before(void) ""
+migration_return_path_end_after(int rp_error) "%d"
 migration_thread_after_loop(void) ""
 migration_thread_file_err(void) ""
 migration_thread_setup_complete(void) ""