qmp/migration: new command migrate-recover

The first allow-oob=true command.  It's used on destination side when
the postcopy migration is paused and ready for a recovery.  After
execution, a new migration channel will be established for postcopy to
continue.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180502104740.12123-21-peterx@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
---
s/2.12/2.13/

migration/migration.c

@@ -1475,6 +1475,30 @@ void qmp_migrate_incoming(const char *uri, Error **errp)
     once = false;
 }
 
+void qmp_migrate_recover(const char *uri, Error **errp)
+{
+    MigrationIncomingState *mis = migration_incoming_get_current();
+
+    if (mis->state != MIGRATION_STATUS_POSTCOPY_PAUSED) {
+        error_setg(errp, "Migrate recover can only be run "
+                   "when postcopy is paused.");
+        return;
+    }
+
+    if (atomic_cmpxchg(&mis->postcopy_recover_triggered,
+                       false, true) == true) {
+        error_setg(errp, "Migrate recovery is triggered already");
+        return;
+    }
+
+    /*
+     * Note that this call will never start a real migration; it will
+     * only re-setup the migration stream and poke existing migration
+     * to continue using that newly established channel.
+     */
+    qemu_start_incoming_migration(uri, errp);
+}
+
 bool migration_is_blocked(Error **errp)
 {
     if (qemu_savevm_state_blocked(errp)) {

migration/migration.h

@@ -77,6 +77,7 @@ struct MigrationIncomingState {
     struct PostcopyBlocktimeContext *blocktime_ctx;
 
     /* notify PAUSED postcopy incoming migrations to try to continue */
+    bool postcopy_recover_triggered;
     QemuSemaphore postcopy_pause_sem_dst;
     QemuSemaphore postcopy_pause_sem_fault;
 };

migration/savevm.c

@@ -2190,6 +2190,9 @@ static bool postcopy_pause_incoming(MigrationIncomingState *mis)
 {
     trace_postcopy_pause_incoming();
 
+    /* Clear the triggered bit to allow one recovery */
+    mis->postcopy_recover_triggered = false;
+
     migrate_set_state(&mis->state, MIGRATION_STATUS_POSTCOPY_ACTIVE,
                       MIGRATION_STATUS_POSTCOPY_PAUSED);
 

qapi/migration.json

@@ -1191,3 +1191,23 @@
 # Since: 2.9
 ##
 { 'command': 'xen-colo-do-checkpoint' }
+
+##
+# @migrate-recover:
+#
+# Provide a recovery migration stream URI.
+#
+# @uri: the URI to be used for the recovery of migration stream.
+#
+# Returns: nothing.
+#
+# Example:
+#
+# -> { "execute": "migrate-recover",
+#      "arguments": { "uri": "tcp:192.168.1.200:12345" } }
+# <- { "return": {} }
+#
+# Since: 2.13
+##
+{ 'command': 'migrate-recover', 'data': { 'uri': 'str' },
+  'allow-oob': true }