[libvirt] [PATCH v2] qemu: snapshot: Forbid internal snapshots with pflash firmware

[Peter Krempa]

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;
-- 
2.12.1