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Date: Wed, 25 Nov 2009 14:59:58 +0100
From: Gerd Hoffmann <kraxel@redhat.com>
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Subject: Re: [Qemu-devel] Re: Live migration protocol, device features,	ABIs
	and other beasts
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To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: dlaor@redhat.com, qemu-devel <qemu-devel@nongnu.org>, Juan Quintela <quintela@redhat.com>

On 11/25/09 14:40, Michael S. Tsirkin wrote:

>> We could add a DeviceState->savevm field and make that available as
>> property for devices which need to support multiple versions.  Then you
>> we can use the compat properties to switch back to the older format with
>> -M pc-0.10.

> I'm confused sorry. Of course when you want to migrate to qemu 0.10
> you must have a compatible machine. And savevm format has nothing
> to do with it IMO, so MSI is orthogonal to this discussion.
> It just shows that it was smart not to save MSI state when
> MSI is not present (/me pats self on the back).
>
> In this thread we were discussing changes like pvclock bug,
> where we change savevm format without changing the machine,
> or almost without changing the machine.

If 0.12 has this fixed (and thus a new version) and 0.11 hasn't, then 
you'll want -M pc-0.11 use the old (buggy) savevm version.  You have to 
stay bug compatible otherwise you can't migrate to the old buggy version 
because the old qemu can't handle the new format.

Could be implemented via DeviceState->savevm as outlined above.

cheers,
   Gerd