On Mon, Aug 04, 2014 at 05:46:58PM +0100, Alex Bligh wrote: > Michael, > > On 4 Aug 2014, at 17:22, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote: > > >> > >> These are capable of modifying fields within the > >> VMStateDescription of the relevant object. > >> > >> However, the PIIX4 change modifies the minimum_version_id > >> (outside fields); I don't quite see how that would work. > >> Can you help here? > > > > If you want to support lower version IDs, > > you can just decrease minimum_version_id. > > > > field_exists gets the version ID so you can > > parse different fields depending on the > > version. > > The issue is that qemu-kvm 1.0 used version ID 2 > but is actually sending a version 3 structure. > > I don't think I can just reduce the minimum version > ID. > As per the comment in the original patch from > Cole Robinson: > > http://pkgs.fedoraproject.org/cgit/qemu.git/tree/0001-Fix-migration-from-qemu-kvm.patch?h=f20 > > --- a/hw/acpi/piix4.c > +++ b/hw/acpi/piix4.c > @@ -289,7 +289,13 @@ static int acpi_load_old(QEMUFile *f, void *opaque, int version_id) > static const VMStateDescription vmstate_acpi = { > .name = "piix4_pm", > .version_id = 3, > - .minimum_version_id = 3, > + /* > + * qemu-kvm 1.2 uses qemu.git version 3 format, but advertised as 2. > + * This allows incoming migration from qemu-kvm, but breaks incoming > + * migration from qemu < 1.3. > + */ > + //minimum_version_id = 3, > + .minimum_version_id = 2, > .minimum_version_id_old = 1, > .load_state_old = acpi_load_old, > .post_load = vmstate_acpi_post_load, > > An inbound migration from qemu-1.0-git, qemu-1.1 or > qemu-1.2 will have version ID 2 and actually mean version > 2; currently (i.e. with minimum_version_id = 3), these > use the minimum_version_id_old field (1) and > acpi_load_old routine. > > If I decrease minimum_version_id to 2, as far as I can > tell this will break inbound migration from the 'good' > earlier versions, whilst fixing qemu-1.0-kvm (which uses > version 2 to mean version 3). > > So as far as I can tell, the minimum version ID needs > to be dependent upon inbound machine type (or machine > parameters). By the time the command line is parsed, > these structures have already been built. That's why > I took the approach I did, but again I'm new to this > so may be missing something. > > Or were you suggesting I introduced a > test_force_use_new_load_vm_state > type field into VMStateDescription so it can dynamically > choose not to use the load_state_old but rather to > use the new method, irrespective of the minimum version > mismatch? I was merely suggesting changing acpi_load_old to detect the new flag and parse the qemu-kvm format. > -- > Alex Bligh > > > -- libvir-list mailing list libvir-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvir-list