savevm: Ignore minimum_version_id_old if there is no load_state_old

At the moment we require vmstate definitions to set minimum_version_id_old
to the same value as minimum_version_id if they do not provide a
load_state_old handler. Since the load_state_old functionality is
required only for a handful of devices that need to retain migration
compatibility with a pre-vmstate implementation, this means the bulk
of devices have pointless boilerplate. Relax the definition so that
minimum_version_id_old is ignored if there is no load_state_old handler.

Note that under the old scheme we would segfault if the vmstate
specified a minimum_version_id_old that was less than minimum_version_id
but did not provide a load_state_old function, and the incoming state
specified a version number between minimum_version_id_old and
minimum_version_id. Under the new scheme this will just result in
our failing the migration.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

docs/migration.txt

@@ -139,7 +139,6 @@ static const VMStateDescription vmstate_kbd = {
     .name = "pckbd",
     .version_id = 3,
     .minimum_version_id = 3,
-    .minimum_version_id_old = 3,
     .fields      = (VMStateField []) {
         VMSTATE_UINT8(write_cmd, KBDState),
         VMSTATE_UINT8(status, KBDState),
@@ -168,12 +167,13 @@ You can see that there are several version fields:
 - minimum_version_id: the minimum version_id that VMState is able to understand
   for that device.
 - minimum_version_id_old: For devices that were not able to port to vmstate, we can
-  assign a function that knows how to read this old state.
+  assign a function that knows how to read this old state. This field is
+  ignored if there is no load_state_old handler.
 
 So, VMState is able to read versions from minimum_version_id to
-version_id.  And the function load_state_old() is able to load state
+version_id.  And the function load_state_old() (if present) is able to
-from minimum_version_id_old to minimum_version_id.  This function is
+load state from minimum_version_id_old to minimum_version_id.  This
-deprecated and will be removed when no more users are left.
+function is deprecated and will be removed when no more users are left.
 
 ===  Massaging functions ===
 
@@ -255,7 +255,6 @@ const VMStateDescription vmstate_ide_drive_pio_state = {
     .name = "ide_drive/pio_state",
     .version_id = 1,
     .minimum_version_id = 1,
-    .minimum_version_id_old = 1,
     .pre_save = ide_drive_pio_pre_save,
     .post_load = ide_drive_pio_post_load,
     .fields      = (VMStateField []) {
@@ -275,7 +274,6 @@ const VMStateDescription vmstate_ide_drive = {
     .name = "ide_drive",
     .version_id = 3,
     .minimum_version_id = 0,
-    .minimum_version_id_old = 0,
     .post_load = ide_drive_post_load,
     .fields      = (VMStateField []) {
         .... several fields ....

vmstate.c

@@ -63,11 +63,12 @@ int vmstate_load_state(QEMUFile *f, const VMStateDescription *vmsd,
     if (version_id > vmsd->version_id) {
         return -EINVAL;
     }
-    if (version_id < vmsd->minimum_version_id_old) {
-        return -EINVAL;
-    }
     if  (version_id < vmsd->minimum_version_id) {
-        return vmsd->load_state_old(f, opaque, version_id);
+        if (vmsd->load_state_old &&
+            version_id >= vmsd->minimum_version_id_old) {
+            return vmsd->load_state_old(f, opaque, version_id);
+        }
+        return -EINVAL;
     }
     if (vmsd->pre_load) {
         int ret = vmsd->pre_load(opaque);