Return path: Send responses from destination to source

Add migrate_send_rp_message to send a message from destination to source along the return path.
  (It uses a mutex to let it be called from multiple threads)
Add migrate_send_rp_shut to send a 'shut' message to indicate
  the destination is finished with the RP.
Add migrate_send_rp_ack to send a 'PONG' message in response to a PING
  Use it in the MSG_RP_PING handler

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Amit Shah <amit.shah@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

include/migration/migration.h

@@ -43,12 +43,22 @@ struct MigrationParams {
     bool shared;
 };
 
+/* Messages sent on the return path from destination to source */
+enum mig_rp_message_type {
+    MIG_RP_MSG_INVALID = 0,  /* Must be 0 */
+    MIG_RP_MSG_SHUT,         /* sibling will not send any more RP messages */
+    MIG_RP_MSG_PONG,         /* Response to a PING; data (seq: be32 ) */
+
+    MIG_RP_MSG_MAX
+};
+
 typedef QLIST_HEAD(, LoadStateEntry) LoadStateEntry_Head;
 /* State for the incoming migration */
 struct MigrationIncomingState {
     QEMUFile *from_src_file;
 
     QEMUFile *to_src_file;
+    QemuMutex rp_mutex;    /* We send replies from multiple threads */
 
     /* See savevm.c */
     LoadStateEntry_Head loadvm_handlers;
@@ -181,6 +191,15 @@ int migrate_compress_threads(void);
 int migrate_decompress_threads(void);
 bool migrate_use_events(void);
 
+/* Sending on the return path - generic and then for each message type */
+void migrate_send_rp_message(MigrationIncomingState *mis,
+                             enum mig_rp_message_type message_type,
+                             uint16_t len, void *data);
+void migrate_send_rp_shut(MigrationIncomingState *mis,
+                          uint32_t value);
+void migrate_send_rp_pong(MigrationIncomingState *mis,
+                          uint32_t value);
+
 void ram_control_before_iterate(QEMUFile *f, uint64_t flags);
 void ram_control_after_iterate(QEMUFile *f, uint64_t flags);
 void ram_control_load_hook(QEMUFile *f, uint64_t flags, void *data);

migration/migration.c

@@ -97,6 +97,7 @@ MigrationIncomingState *migration_incoming_state_new(QEMUFile* f)
     mis_current = g_new0(MigrationIncomingState, 1);
     mis_current->from_src_file = f;
     QLIST_INIT(&mis_current->loadvm_handlers);
+    qemu_mutex_init(&mis_current->rp_mutex);
 
     return mis_current;
 }
@@ -344,6 +345,50 @@ void process_incoming_migration(QEMUFile *f)
     qemu_coroutine_enter(co, f);
 }
 
+/*
+ * Send a message on the return channel back to the source
+ * of the migration.
+ */
+void migrate_send_rp_message(MigrationIncomingState *mis,
+                             enum mig_rp_message_type message_type,
+                             uint16_t len, void *data)
+{
+    trace_migrate_send_rp_message((int)message_type, len);
+    qemu_mutex_lock(&mis->rp_mutex);
+    qemu_put_be16(mis->to_src_file, (unsigned int)message_type);
+    qemu_put_be16(mis->to_src_file, len);
+    qemu_put_buffer(mis->to_src_file, data, len);
+    qemu_fflush(mis->to_src_file);
+    qemu_mutex_unlock(&mis->rp_mutex);
+}
+
+/*
+ * Send a 'SHUT' message on the return channel with the given value
+ * to indicate that we've finished with the RP.  Non-0 value indicates
+ * error.
+ */
+void migrate_send_rp_shut(MigrationIncomingState *mis,
+                          uint32_t value)
+{
+    uint32_t buf;
+
+    buf = cpu_to_be32(value);
+    migrate_send_rp_message(mis, MIG_RP_MSG_SHUT, sizeof(buf), &buf);
+}
+
+/*
+ * Send a 'PONG' message on the return channel with the given value
+ * (normally in response to a 'PING')
+ */
+void migrate_send_rp_pong(MigrationIncomingState *mis,
+                          uint32_t value)
+{
+    uint32_t buf;
+
+    buf = cpu_to_be32(value);
+    migrate_send_rp_message(mis, MIG_RP_MSG_PONG, sizeof(buf), &buf);
+}
+
 /* amount of nanoseconds we are willing to wait for migration to be down.
  * the choice of nanoseconds is because it is the maximum resolution that
  * get_clock() can achieve. It is an internal measure. All user-visible

migration/savevm.c

@@ -1103,7 +1103,7 @@ static int loadvm_process_command(QEMUFile *f)
                          tmp32);
             return -1;
         }
-        /* migrate_send_rp_pong(mis, tmp32); TODO: gets added later */
+        migrate_send_rp_pong(mis, tmp32);
         break;
     }
 

trace-events

@@ -1431,6 +1431,7 @@ migrate_fd_cleanup(void) ""
 migrate_fd_error(void) ""
 migrate_fd_cancel(void) ""
 migrate_pending(uint64_t size, uint64_t max) "pending size %" PRIu64 " max %" PRIu64
+migrate_send_rp_message(int msg_type, uint16_t len) "%d: len %d"
 migrate_transferred(uint64_t tranferred, uint64_t time_spent, double bandwidth, uint64_t size) "transferred %" PRIu64 " time_spent %" PRIu64 " bandwidth %g max_size %" PRId64
 migrate_state_too_big(void) ""
 migrate_global_state_post_load(const char *state) "loaded state: %s"