[Qemu-devel] [PATCH v2] wdt_i6300esb: fix vmstate versioning

[Michael Roth <mdroth@linux.vnet.ibm.com>]

When this VMSD was introduced it's version fields were set to
sizeof(I6300State), making them essentially random from build to build,
version to version.

To fix this, we lock in a high version id and low minimum version id to
support old->new migration from all prior versions of this device's
state. This should work since the device state has not changed since
its introduction.

The potentially breaks migration from 1.5+ to 1.5, but since the
versioning was essentially random prior to this patch, new->old
migration was not consistently functional to begin with.

Reported-by: Nicholas Thomas <nick@bytemark.co.uk>
Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Cc: qemu-stable@nongnu.org
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
---
v2:
 * Fixed s/except/accept/ typo (Laszlo)

 hw/watchdog/wdt_i6300esb.c |   19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/hw/watchdog/wdt_i6300esb.c b/hw/watchdog/wdt_i6300esb.c
index 1407fba..05af0b1 100644
--- a/hw/watchdog/wdt_i6300esb.c
+++ b/hw/watchdog/wdt_i6300esb.c
@@ -374,9 +374,22 @@ static const MemoryRegionOps i6300esb_ops = {
 
 static const VMStateDescription vmstate_i6300esb = {
     .name = "i6300esb_wdt",
-    .version_id = sizeof(I6300State),
-    .minimum_version_id = sizeof(I6300State),
-    .minimum_version_id_old = sizeof(I6300State),
+    /* With this VMSD's introduction, version_id/minimum_version_id were
+     * erroneously set to sizeof(I6300State), causing a somewhat random
+     * version_id to be set for every build. This eventually broke
+     * migration.
+     *
+     * To correct this without breaking old->new migration for older versions
+     * of QEMU, we've set version_id to a value high enough to exceed all past
+     * values of sizeof(I6300State) across various build environments, and have
+     * reset minimum_version_id_old/minimum_version_id to 1, since this VMSD
+     * has never changed and thus can accept all past versions.
+     *
+     * For future changes we can treat these values as we normally would.
+     */
+    .version_id = 10000,
+    .minimum_version_id = 1,
+    .minimum_version_id_old = 1,
     .fields      = (VMStateField []) {
         VMSTATE_PCI_DEVICE(dev, I6300State),
         VMSTATE_INT32(reboot_enabled, I6300State),
-- 
1.7.9.5