On 03/23/17 10:54, Peter Krempa wrote:
> On Thu, Mar 23, 2017 at 10:48:01 +0100, Laszlo Ersek wrote:
>> On 03/23/17 10:29, Peter Krempa wrote:
>>> If the variable store (<nvram>) file is raw qemu can't do a snapshot of
>>> it and thus the snapshot would be incomplete. QEMU does no reject such
>>> snapshot.
>>>
>>> Additionally allowing to use a qcow2 variable store backing file would
>>> solve this issue but then it would become eligible to become target of
>>> the memory dump.
>>>
>>> Offline internal snapshot would be incomplete too with either storage
>>> format since libvirt does not handle the pflash file in this case.
>>>
>>> Forbid such snapshot so that we can avoid problems.
>>> ---
>>>
>>> Notes:
>>> v2:
>>> - changed error code to OPERATION_UNSUPPORTED (from CONFIG_UNSUPPORTED)
>>> - dropped mention of QEMU from the error message
>>> - dropped mentions of OVMF or the firmware itself altoghether, the culprit is
>>> the pflash device regardless of the software it contains
>>> - mentioned all the stuff in the commit message and comment
>>>
>>> We also will need to introduce a way to snapshot the pflash for external
>>> snapshots which is currently impossible as well, but fortunately does not
>>> have inherent drawbacks as internal snapshots.
>>>
>>> src/qemu/qemu_driver.c | 10 ++++++++++
>>> 1 file changed, 10 insertions(+)
>>>
>>> diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c
>>> index 676295208..202d190b3 100644
>>> --- a/src/qemu/qemu_driver.c
>>> +++ b/src/qemu/qemu_driver.c
>>> @@ -13873,6 +13873,16 @@ qemuDomainSnapshotPrepare(virConnectPtr conn,
>>> goto cleanup;
>>> }
>>>
>>> + /* Internal snapshots don't work with VMs with OVMF loader since qemu does
>>> + * not snapshot the variable store */
>>> + if (found_internal &&
>>> + vm->def->os.loader->type == VIR_DOMAIN_LOADER_TYPE_PFLASH) {
>>> + virReportError(VIR_ERR_OPERATION_UNSUPPORTED, "%s",
>>> + _("internal snapshots of a VM with pflash based "
>>> + "firmware are not supported"));
>>> + goto cleanup;
>>> + }
>>> +
>>> /* Alter flags to let later users know what we learned. */
>>> if (external && !active)
>>> *flags |= VIR_DOMAIN_SNAPSHOT_CREATE_DISK_ONLY;
>>>
>
> I apparently forgot to commit this despite mentioning it in the commit
> message:
>
> @@ -13873,8 +13873,14 @@ qemuDomainSnapshotPrepare(virConnectPtr conn,
> goto cleanup;
> }
>
> - /* Internal snapshots don't work with VMs with OVMF loader since qemu does
> - * not snapshot the variable store */
> + /* internal snapshots + pflash based loader have the following problems:
> + * - if the variable store is raw, the snapshot is incomplete
> + * - alowing a qcow2 image as the varstore would make it eligible to receive
> + * the vmstate dump, which would make it huge
> + * - offline snapshot would not snapshot the varstore at all
> + *
> + * Avoid the issues by forbidding this completely.
> + */
>
Yeah I saw the OVMF mention in the v2 comment, despite "dropped mentions
of OVMF", but I figured I wouldn't obsess about it. :)
Anyway, if you post a v3 with the comment updated, you can keep my R-b
with it -- the new comment looks great.
Thanks!
Laszlo
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