Postcopy: Check for support when setting the capability

Knowing whether the destination host supports migration with
postcopy can be tricky.
The destination doesn't need the capability set, however
if we set it then use the opportunity to do the test and
tell the user/management layer early.
Signed-off-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Message-id: 1465816605-29488-7-git-send-email-dgilbert@redhat.com
Message-Id: <1465816605-29488-7-git-send-email-dgilbert@redhat.com>
Signed-off-by: NAmit Shah <amit.shah@redhat.com>

migration/migration.c

@@ -720,6 +720,7 @@ void qmp_migrate_set_capabilities(MigrationCapabilityStatusList *params,
 {
     MigrationState *s = migrate_get_current();
     MigrationCapabilityStatusList *cap;
+    bool old_postcopy_cap = migrate_postcopy_ram();
 
     if (migration_is_setup_or_active(s->state)) {
         error_setg(errp, QERR_MIGRATION_ACTIVE);
@@ -742,6 +743,19 @@ void qmp_migrate_set_capabilities(MigrationCapabilityStatusList *params,
             s->enabled_capabilities[MIGRATION_CAPABILITY_POSTCOPY_RAM] =
                 false;
         }
+        /* This check is reasonably expensive, so only when it's being
+         * set the first time, also it's only the destination that needs
+         * special support.
+         */
+        if (!old_postcopy_cap && runstate_check(RUN_STATE_INMIGRATE) &&
+            !postcopy_ram_supported_by_host()) {
+            /* postcopy_ram_supported_by_host will have emitted a more
+             * detailed message
+             */
+            error_report("Postcopy is not supported");
+            s->enabled_capabilities[MIGRATION_CAPABILITY_POSTCOPY_RAM] =
+                false;
+        }
     }
 }