savevm: fail if migration blockers are present

QEMU has currently two ways to prevent migration to occur:
- migration blocker when it depends on runtime state
- VMStateDescription.unmigratable when migration is not supported at all

This patch gathers all the logic into a single function to be called from
both the savevm and the migrate paths.

This fixes a bug with 9p, at least, where savevm would succeed and the
following would happen in the guest after loadvm:

$ ls /host
ls: cannot access /host: Protocol error

With this patch:

(qemu) savevm foo
Migration is disabled when VirtFS export path '/' is mounted in the guest
using mount_tag 'host'
Signed-off-by: NGreg Kurz <gkurz@linux.vnet.ibm.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Message-Id: <146239057139.11271.9011797645454781543.stgit@bahia.huguette.org>

[Update subject according to Paolo's suggestion - Amit]
Signed-off-by: NAmit Shah <amit.shah@redhat.com>
(cherry picked from commit 24f3902b)
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

include/migration/migration.h

@@ -210,6 +210,7 @@ int migrate_fd_close(MigrationState *s);
 void add_migration_state_change_notifier(Notifier *notify);
 void remove_migration_state_change_notifier(Notifier *notify);
 MigrationState *migrate_init(const MigrationParams *params);
+bool migration_is_blocked(Error **errp);
 bool migration_in_setup(MigrationState *);
 bool migration_has_finished(MigrationState *);
 bool migration_has_failed(MigrationState *);

migration/migration.c

@@ -992,6 +992,20 @@ void qmp_migrate_incoming(const char *uri, Error **errp)
     once = false;
 }
 
+bool migration_is_blocked(Error **errp)
+{
+    if (qemu_savevm_state_blocked(errp)) {
+        return true;
+    }
+
+    if (migration_blockers) {
+        *errp = error_copy(migration_blockers->data);
+        return true;
+    }
+
+    return false;
+}
+
 void qmp_migrate(const char *uri, bool has_blk, bool blk,
                  bool has_inc, bool inc, bool has_detach, bool detach,
                  Error **errp)
@@ -1014,12 +1028,7 @@ void qmp_migrate(const char *uri, bool has_blk, bool blk,
         return;
     }
 
-    if (qemu_savevm_state_blocked(errp)) {
-        return;
-    }
-
-    if (migration_blockers) {
-        *errp = error_copy(migration_blockers->data);
+    if (migration_is_blocked(errp)) {
         return;
     }
 

migration/savevm.c

@@ -1169,7 +1169,7 @@ static int qemu_savevm_state(QEMUFile *f, Error **errp)
     MigrationState *ms = migrate_init(&params);
     ms->to_dst_file = f;
 
-    if (qemu_savevm_state_blocked(errp)) {
+    if (migration_is_blocked(errp)) {
         return -EINVAL;
     }