migration/colo: make colo_incoming_co() return void

Currently, it always returns 0, no need to check the return value at all.
In addition, enter colo coroutine only if migration_incoming_colo_enabled()
is true.
Once the destination side enters the COLO* state, the COLO process will
take over the remaining processes until COLO exits.

Cc: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Zhang Chen <chen.zhang@intel.com>
Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
[fixed mangled author email address]
Signed-off-by: Fabiano Rosas <farosas@suse.de>

include/migration/colo.h

@@ -49,7 +49,7 @@ void colo_checkpoint_delay_set(void);
  *
  * Called with BQL locked, may temporary release BQL.
  */
-int coroutine_fn colo_incoming_co(void);
+void coroutine_fn colo_incoming_co(void);
 
 void colo_shutdown(void);
 #endif

migration/colo-stubs.c

@@ -9,9 +9,8 @@ void colo_shutdown(void)
 {
 }
 
-int coroutine_fn colo_incoming_co(void)
+void coroutine_fn colo_incoming_co(void)
 {
-    return 0;
 }
 
 void colo_checkpoint_delay_set(void)

migration/colo.c

@@ -928,16 +928,13 @@ out:
     return NULL;
 }
 
-int coroutine_fn colo_incoming_co(void)
+void coroutine_fn colo_incoming_co(void)
 {
     MigrationIncomingState *mis = migration_incoming_get_current();
     QemuThread th;
 
     assert(bql_locked());
-
-    if (!migration_incoming_colo_enabled()) {
-        return 0;
-    }
+    assert(migration_incoming_colo_enabled());
 
     qemu_thread_create(&th, "COLO incoming", colo_process_incoming_thread,
                        mis, QEMU_THREAD_JOINABLE);
@@ -953,6 +950,4 @@ int coroutine_fn colo_incoming_co(void)
 
     /* We hold the global BQL, so it is safe here */
     colo_release_ram_cache();
-
-    return 0;
 }

migration/migration.c

@@ -776,9 +776,9 @@ process_incoming_migration_co(void *opaque)
         goto fail;
     }
 
-    if (colo_incoming_co() < 0) {
-        error_setg(&local_err, "colo incoming failed");
-        goto fail;
+    if (migration_incoming_colo_enabled()) {
+        /* yield until COLO exit */
+        colo_incoming_co();
     }
 
     migration_bh_schedule(process_incoming_migration_bh, mis);