Re: [libvirt PATCH 02/11] qemu_snapshot: revert: always restart QEMU process for running VM

On Tue, Nov 16, 2021 at 03:17:56PM +0100, Peter Krempa wrote: > On Mon, Nov 15, 2021 at 17:22:45 +0100, Pavel Hrdina wrote: > > Our compatibility check code isn't complete and there are cases where it > > fails to detect incompatible configuration and the revert fails. In > > addition future support for external snapshot will always require > > restarting the QEMU process. > > > > To unify the behavior drop the compatibility check code and always > > restart the QEMU process. > > > > Signed-off-by: Pavel Hrdina <phrdina(a)redhat.com&gt; > > --- > > src/qemu/qemu_snapshot.c | 66 ++++++---------------------------------- > > 1 file changed, 10 insertions(+), 56 deletions(-) > > > > diff --git a/src/qemu/qemu_snapshot.c b/src/qemu/qemu_snapshot.c > > index 3d6ec490ab..661f74146c 100644 > > --- a/src/qemu/qemu_snapshot.c > > +++ b/src/qemu/qemu_snapshot.c > > @@ -1989,62 +1989,16 @@ qemuSnapshotRevert(virDomainObj *vm, > > [...] > > > /* Transitions 5, 6, 8, 9 */ > > - /* If using VM GenID, there is no way currently to change > > - * the genid for the running guest, so set an error, > > - * mark as incompatible, and don't allow change of genid > > - * if the revert force flag would start the guest again. */ > > - if (compatible && config->genidRequested) { > > - virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s", > > - _("domain genid update requires restart")); > > - compatible = false; > > - start_flags &= ~VIR_QEMU_PROCESS_START_GEN_VMID; > > We still need this bit. If genid is requested we must ensure that the > new start of the VM will regenerate it to ensure that the guest can > detect the reversion. At the beginning of the function qemuSnapshotRevert() there is this line: unsigned int start_flags = VIR_QEMU_PROCESS_START_GEN_VMID; so generating of VMID is enabled by default. This part of code disabled the VMID regeneration if FORCE flag had to be used to revert the snapshot. That change was introduced by commit <e5d7064be02a0105b7c> but it doesn't make sense to me. The commit messages states: If the the snapshot revert involves a forced revert option, then let's not cause startup to change the genid flag in order to signify that we're still running the same/previous guest and not some snapshot reversion. but how we would run the same guest if this code is reverting to different snapshot?