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-rw-r--r--migration/migration.c16
-rw-r--r--migration/migration.h7
-rw-r--r--migration/postcopy-ram.c56
-rw-r--r--migration/postcopy-ram.h1
4 files changed, 68 insertions, 12 deletions
diff --git a/migration/migration.c b/migration/migration.c
index 3119bd2e4b..427d4de185 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -3053,6 +3053,12 @@ static int postcopy_start(MigrationState *ms)
     int64_t bandwidth = migrate_max_postcopy_bandwidth();
     bool restart_block = false;
     int cur_state = MIGRATION_STATUS_ACTIVE;
+
+    if (postcopy_preempt_wait_channel(ms)) {
+        migrate_set_state(&ms->state, ms->state, MIGRATION_STATUS_FAILED);
+        return -1;
+    }
+
     if (!migrate_pause_before_switchover()) {
         migrate_set_state(&ms->state, MIGRATION_STATUS_ACTIVE,
                           MIGRATION_STATUS_POSTCOPY_ACTIVE);
@@ -3534,6 +3540,14 @@ static MigThrError postcopy_pause(MigrationState *s)
         if (s->state == MIGRATION_STATUS_POSTCOPY_RECOVER) {
             /* Woken up by a recover procedure. Give it a shot */
 
+            if (postcopy_preempt_wait_channel(s)) {
+                /*
+                 * Preempt enabled, and new channel create failed; loop
+                 * back to wait for another recovery.
+                 */
+                continue;
+            }
+
             /*
              * Firstly, let's wake up the return path now, with a new
              * return path channel.
@@ -4398,6 +4412,7 @@ static void migration_instance_finalize(Object *obj)
     qemu_sem_destroy(&ms->postcopy_pause_sem);
     qemu_sem_destroy(&ms->postcopy_pause_rp_sem);
     qemu_sem_destroy(&ms->rp_state.rp_sem);
+    qemu_sem_destroy(&ms->postcopy_qemufile_src_sem);
     error_free(ms->error);
 }
 
@@ -4444,6 +4459,7 @@ static void migration_instance_init(Object *obj)
     qemu_sem_init(&ms->rp_state.rp_sem, 0);
     qemu_sem_init(&ms->rate_limit_sem, 0);
     qemu_sem_init(&ms->wait_unplug_sem, 0);
+    qemu_sem_init(&ms->postcopy_qemufile_src_sem, 0);
     qemu_mutex_init(&ms->qemu_file_lock);
 }
 
diff --git a/migration/migration.h b/migration/migration.h
index 9220cec6bd..ae4ffd3454 100644
--- a/migration/migration.h
+++ b/migration/migration.h
@@ -219,6 +219,13 @@ struct MigrationState {
     QEMUFile *to_dst_file;
     /* Postcopy specific transfer channel */
     QEMUFile *postcopy_qemufile_src;
+    /*
+     * It is posted when the preempt channel is established.  Note: this is
+     * used for both the start or recover of a postcopy migration.  We'll
+     * post to this sem every time a new preempt channel is created in the
+     * main thread, and we keep post() and wait() in pair.
+     */
+    QemuSemaphore postcopy_qemufile_src_sem;
     QIOChannelBuffer *bioc;
     /*
      * Protects to_dst_file/from_dst_file pointers.  We need to make sure we
diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
index 84f7b1526e..70b21e9d51 100644
--- a/migration/postcopy-ram.c
+++ b/migration/postcopy-ram.c
@@ -1552,10 +1552,50 @@ bool postcopy_preempt_new_channel(MigrationIncomingState *mis, QEMUFile *file)
     return true;
 }
 
-int postcopy_preempt_setup(MigrationState *s, Error **errp)
+static void
+postcopy_preempt_send_channel_new(QIOTask *task, gpointer opaque)
 {
-    QIOChannel *ioc;
+    MigrationState *s = opaque;
+    QIOChannel *ioc = QIO_CHANNEL(qio_task_get_source(task));
+    Error *local_err = NULL;
+
+    if (qio_task_propagate_error(task, &local_err)) {
+        /* Something wrong happened.. */
+        migrate_set_error(s, local_err);
+        error_free(local_err);
+    } else {
+        migration_ioc_register_yank(ioc);
+        s->postcopy_qemufile_src = qemu_file_new_output(ioc);
+        trace_postcopy_preempt_new_channel();
+    }
+
+    /*
+     * Kick the waiter in all cases.  The waiter should check upon
+     * postcopy_qemufile_src to know whether it failed or not.
+     */
+    qemu_sem_post(&s->postcopy_qemufile_src_sem);
+    object_unref(OBJECT(ioc));
+}
 
+/* Returns 0 if channel established, -1 for error. */
+int postcopy_preempt_wait_channel(MigrationState *s)
+{
+    /* If preempt not enabled, no need to wait */
+    if (!migrate_postcopy_preempt()) {
+        return 0;
+    }
+
+    /*
+     * We need the postcopy preempt channel to be established before
+     * starting doing anything.
+     */
+    qemu_sem_wait(&s->postcopy_qemufile_src_sem);
+
+    return s->postcopy_qemufile_src ? 0 : -1;
+}
+
+int postcopy_preempt_setup(MigrationState *s, Error **errp)
+{
     if (!migrate_postcopy_preempt()) {
         return 0;
     }
@@ -1566,16 +1606,8 @@ int postcopy_preempt_setup(MigrationState *s, Error **errp)
         return -1;
     }
 
-    ioc = socket_send_channel_create_sync(errp);
-
-    if (ioc == NULL) {
-        return -1;
-    }
-
-    migration_ioc_register_yank(ioc);
-    s->postcopy_qemufile_src = qemu_file_new_output(ioc);
-
-    trace_postcopy_preempt_new_channel();
+    /* Kick an async task to connect */
+    socket_send_channel_create(postcopy_preempt_send_channel_new, s);
 
     return 0;
 }
diff --git a/migration/postcopy-ram.h b/migration/postcopy-ram.h
index 34b1080cde..6147bf7d1d 100644
--- a/migration/postcopy-ram.h
+++ b/migration/postcopy-ram.h
@@ -192,5 +192,6 @@ enum PostcopyChannels {
 
 bool postcopy_preempt_new_channel(MigrationIncomingState *mis, QEMUFile *file);
 int postcopy_preempt_setup(MigrationState *s, Error **errp);
+int postcopy_preempt_wait_channel(MigrationState *s);
 
 #endif