[v8,20/54] Return path: Send responses from destination to source

diff --git a/include/migration/migration.h b/include/migration/migration.h
index 98a6d07..3ce3fda 100644
--- a/include/migration/migration.h
+++ b/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);
diff --git a/migration/migration.c b/migration/migration.c
index ba23a65..4fad6a5 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -89,6 +89,7 @@  MigrationIncomingState *migration_incoming_state_new(QEMUFile* f)
     mis_current = g_malloc0(sizeof(MigrationIncomingState));
     mis_current->from_src_file = f;
     QLIST_INIT(&mis_current->loadvm_handlers);
+    qemu_mutex_init(&mis_current->rp_mutex);
 
     return mis_current;
 }
@@ -325,6 +326,50 @@  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
diff --git a/migration/savevm.c b/migration/savevm.c
index 819ab1e..f51cbcd 100644
--- a/migration/savevm.c
+++ b/migration/savevm.c
@@ -1100,7 +1100,7 @@  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;
     }
 
diff --git a/trace-events b/trace-events
index 4d4e9dc..be58b47 100644
--- a/trace-events
+++ b/trace-events
@@ -1419,6 +1419,7 @@  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"