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Revision feeee5ac


Added by Miguel Di Ciurcio Filho over 10 years ago

savevm: Really verify if a drive supports snapshots

Both bdrv_can_snapshot() and bdrv_has_snapshot() does not work as advertized.

First issue: Their names implies different porpouses, but they do the same thing
and have exactly the same code. Maybe copied and pasted and forgotten?
bdrv_has_snapshot() is called in various places for actually checking if there
is snapshots or not.

Second issue: the way bdrv_can_snapshot() verifies if a block driver supports or
not snapshots does not catch all cases. E.g.: a raw image.

So when do_savevm() is called, first thing it does is to set a global
BlockDriverState to save the VM memory state calling get_bs_snapshots().

static BlockDriverState *get_bs_snapshots(void) {
BlockDriverState *bs;
DriveInfo *dinfo;

if (bs_snapshots)
return bs_snapshots;
QTAILQ_FOREACH(dinfo, &drives, next) {
bs = dinfo->bdrv;
if (bdrv_can_snapshot(bs))
goto ok;
return NULL;
bs_snapshots = bs;
return bs;

bdrv_can_snapshot() may return a BlockDriverState that does not support
snapshots and do_savevm() goes on.

Later on in do_savevm(), we find:

QTAILQ_FOREACH(dinfo, &drives, next) {
bs1 = dinfo->bdrv;
if (bdrv_has_snapshot(bs1)) {
/* Write VM state size only to the image that contains the state */
sn->vm_state_size = (bs == bs1 ? vm_state_size : 0);
ret = bdrv_snapshot_create(bs1, sn);
if (ret < 0) {
monitor_printf(mon, "Error while creating snapshot on '%s'\n",

bdrv_has_snapshot(bs1) is not checking if the device does support or has
snapshots as explained above. Only in bdrv_snapshot_create() the device is
actually checked for snapshot support.

So, in cases where the first device supports snapshots, and the second does not,
the snapshot on the first will happen anyways. I believe this is not a good
behavior. It should be an all or nothing process.

This patch addresses these issues by making bdrv_can_snapshot() actually do
what it must do and enforces better tests to avoid errors in the middle of
do_savevm(). bdrv_has_snapshot() is removed and replaced by bdrv_can_snapshot()
where appropriate.

bdrv_can_snapshot() was moved from savevm.c to block.c. It makes more sense to me.

The loadvm_state() function was updated too to enforce that when loading a VM at
least all writable devices must support snapshots too.

Signed-off-by: Miguel Di Ciurcio Filho <>
Signed-off-by: Kevin Wolf <>


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