migration: add "return-path" capability

When this capability is enabled, QEMU will use the return path even for
precopy migration. This is helpful at least in one case when destination
failed to load the image while source quited without confirmation. With
return path, source will wait for the last response from destination,
and if destination fails, it'll fail the migration on source, then the
guest can be run again on the source (rather than assuming to be good,
then the guest will be lost after source quits).

It needs to be enabled explicitly on source, otherwise disabled.
Signed-off-by: NPeter Xu <peterx@redhat.com>
Message-Id: <1498472935-14461-1-git-send-email-peterx@redhat.com>
Reviewed-by: NJuan Quintela <quintela@redhat.com>
Reviewed-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: NJuan Quintela <quintela@redhat.com>

migration/migration.c

@@ -1311,6 +1311,15 @@ bool migrate_use_block(void)
     return s->enabled_capabilities[MIGRATION_CAPABILITY_BLOCK];
 }
 
+bool migrate_use_return_path(void)
+{
+    MigrationState *s;
+
+    s = migrate_get_current();
+
+    return s->enabled_capabilities[MIGRATION_CAPABILITY_RETURN_PATH];
+}
+
 bool migrate_use_block_incremental(void)
 {
     MigrationState *s;
@@ -1975,10 +1984,11 @@ void migrate_fd_connect(MigrationState *s)
     notifier_list_notify(&migration_state_notifiers, s);
 
     /*
-     * Open the return path; currently for postcopy but other things might
-     * also want it.
+     * Open the return path. For postcopy, it is used exclusively. For
+     * precopy, only if user specified "return-path" capability would
+     * QEMU uses the return path.
      */
-    if (migrate_postcopy_ram()) {
+    if (migrate_postcopy_ram() || migrate_use_return_path()) {
         if (open_return_path_on_source(s)) {
             error_report("Unable to open return-path for postcopy");
             migrate_set_state(&s->state, MIGRATION_STATUS_SETUP,

migration/migration.h

@@ -177,6 +177,7 @@ bool migrate_colo_enabled(void);
 
 bool migrate_use_block(void);
 bool migrate_use_block_incremental(void);
+bool migrate_use_return_path(void);
 
 bool migrate_use_compression(void);
 int migrate_compress_level(void);

qapi-schema.json

@@ -900,12 +900,15 @@
 #          offers more flexibility.
 #          (Since 2.10)
 #
+# @return-path: If enabled, migration will use the return path even
+#               for precopy. (since 2.10)
+#
 # Since: 1.2
 ##
 { 'enum': 'MigrationCapability',
   'data': ['xbzrle', 'rdma-pin-all', 'auto-converge', 'zero-blocks',
            'compress', 'events', 'postcopy-ram', 'x-colo', 'release-ram',
-           'block' ] }
+           'block', 'return-path' ] }
 
 ##
 # @MigrationCapabilityStatus: