Re: [RFC v2 01/14] machine,virtio-net: add early-mig property

[Eugenio Perez Martin <eperezma@redhat.com>]

On Tue, Mar 24, 2026 at 3:07 PM Jonah Palmer <jonah.palmer@oracle.com> wrote:
>
>
>
> On 3/23/26 6:25 AM, Eugenio Perez Martin wrote:
> > On Fri, Mar 20, 2026 at 3:20 PM Jonah Palmer <jonah.palmer@oracle.com> wrote:
> >>
> >> Introduce a new 'early-mig' property that enables the early migration
> >> path for virtio-net devices on machine types >= 10.2:
> >>
> >>   - virtio-net-device,early-mig=on
> >>   - virtio-net-pci,early-mig=on
> >>   - virtio-net-pci-transitional,early-mig=on
> >>   - virtio-net-pci-non-transitional,early-mig=on
> >>   - virtio-net-ccw,early-mig=on
> >>
> >> To preserve compatibility for older machine types (<= 10.1), add compat
> >> overrides in hw_compat_10_1 to keep the legacy default (off).
> >>
> >> With this, machine types 10.2 and newer enable early migration by
> >> default while older machine types retain the previous behavior. Users
> >> may still override explicitly via:
> >>
> >>   -device virtio-net-pci,early-mig=off
> >>   or
> >>   -global virtio-net-device.early-mig=off
> >>
> >> Follow-up patches will implement the actual early migration feature for
> >> virtio-net devices.
> >>
> >> Signed-off-by: Jonah Palmer <jonah.palmer@oracle.com>
> >> ---
> >>   hw/core/machine.c              | 5 +++++
> >>   hw/net/virtio-net.c            | 1 +
> >>   include/hw/virtio/virtio-net.h | 1 +
> >>   3 files changed, 7 insertions(+)
> >>
> >> diff --git a/hw/core/machine.c b/hw/core/machine.c
> >> index 6cf0e2f404..1b6c7db119 100644
> >> --- a/hw/core/machine.c
> >> +++ b/hw/core/machine.c
> >> @@ -45,6 +45,11 @@ const size_t hw_compat_10_2_len = G_N_ELEMENTS(hw_compat_10_2);
> >>
> >>   GlobalProperty hw_compat_10_1[] = {
> >>       { TYPE_ACPI_GED, "x-has-hest-addr", "false" },
> >> +    { "virtio-net-device", "early-mig", "off" },
> >> +    { "virtio-net-pci", "", "off" },
> >> +    { "virtio-net-pci-transitional", "early-mig", "off" },
> >> +    { "virtio-net-pci-non-transitional", "early-mig", "off" },
> >> +    { "virtio-net-ccw", "early-mig", "off" },
> >
> > Is this needed even if we mark the VMState as optional?
> >
> > The source might not be able to send the device configuration in some
> > cases. For example, if it cannot shadow CVQ. If the destination knows
> > this field is optional, the QEMU source version is irrelevant.
> >
>
> I think it is still needed, yes. This series adds new VMSDs that are
> registered as top-level savevm sections, and, IIUC, a destination guest
> using an older version of Qemu would not be able to ignore these
> "unknown" sections, even if they're optional for the source to send.
>
> The compat property also preserves the pre-10.2 machine-type default, so
> a 10.1 machine type doesn't silently change the migration behavior when
> it's being run on a newer Qemu version.
>

Ok good point. I'm tempted to propose removing the cmdline argument
then, but I see virtio-mem "x-early-migration" also adds it. I don't
think increasing the migration's complexity by adding command-line
cases is worth it, but I guess there is a reason for that. What about
setting it to true by default?

Also, maybe it is worth exploring using the same name for both
properties. At the same time, I remember that x- prefix was
discouraged so... I'll be happy with any option
{x-,}early-mig{,ration}.

Again, I wouldn't object to adding the cmdline parameter, I'm just
trying to make this as easy to maintain in the future as possible :).

> I do agree though that being unable to shadow some state should be
> handled as a runtime check for early migration (like we do for
> vhost-user), but that's separate from a machine-type compat default.
>

Yes, I missed that vhost-user and vhost-vdpa were out of the series at
this patch. It happened with a few comments. I should have gone back
and solved them myself; apologies! If possible, and if you see it
makes sense, you can expand the patch descriptions with the info.