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.
+ */
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