Re: [Qemu-devel] Re: Live migration protocol, device features, ABIs and other beasts

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Nov 25, 2009 at 02:59:58PM +0100, Gerd Hoffmann wrote:
> 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

There might be many reasons to use -M pc-0.11.  Migrating to old qemu is
only one of them.  We should not force old savevm bugs on all users that
use -M pc-0.11.  In partucular, I think with time (years) we might drop
support for old savevm bugs, but I see no reason not to support old
machines indefinitely.


-- 
MST