Re: [Qemu-devel] balloon config change seems to break live migration from 3.0.1 to 4.0

[Stefan Hajnoczi]

On Tue, Jul 9, 2019 at 4:23 PM Dr. David Alan Gilbert
<address@hidden> wrote:
> * Wolfgang Bumiller (address@hidden) wrote:
> > On Thu, Jun 27, 2019 at 03:12:52PM +0200, Wolfgang Bumiller wrote:
> > With this changing compatibility options I'm not sure if this is a
> > desired upstream change since 4.0 is already released?
>
> There's no great answer here - one of them is going to stay broken;
> we can either fix migration earlier 4.0 and break migration to 4.0
> or leave migration from earlier broken.
> Let's add the  property at least - downstream I know I'll need
> it.
>
> As for your actual patch; it's way bigger than I expected - can't you
> just use DEFINE_PROP - like for exmaple the 'disable-modern' in
> virtio/virtio-pci.c ?
>
> Stefan/mst: What do you reckon - should we:
>    a) Fix it so migration with older qemu's works but break 4.0
>    b) Or leave 4.0 working and keep older broken.

I suggest we keep the 4.0 machine type as it is (with the larger
config size) but fix older machine types so they use the smaller
config size.

QEMU 4.1 and newer machine types should use a config size that depends
on the virtio-balloon features enabled.

With this approach 3.1 -> 4.1 and 4.0 -> 4.1 migration works.
However, migrating from QEMU 4.0 to 4.1 with a machine type earlier
than 4.0 will fail.  I think this is okay because 3.1 -> 4.0 already
failed in this case.

Please see the patch I have sent separately and let me know if you like it.

Stefan