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

[Anthony Liguori <anthony@codemonkey.ws>]

Paolo Bonzini wrote:
> On 11/23/2009 02:51 PM, Eduardo Habkost wrote:
>> Right, but I wouldn't be surprised if a user complains that "I know that
>> my guest don't use that VM feature, so I want to be able to migrate to
>> an older version anyway".
>
> That's a bit more tricky.  What if the older version doesn't support 
> sound (just making up an example) and you know that you're not using a 
> client that plays sound, but still the sound card is part of the 
> machine?  I think there is no doubt that the migration (or 
> save/restore) should be aborted in that case.
>
> I would not generalize very much, the problem that Dor posed is very 
> specific and probably quite rare.  However, it's real.

I think the problem is that you shouldn't be changing the guest visible 
state in a stable update of qemu.  If you change the guest visible state 
in a stable update, then you won't be able to support live migration 
between arbitrary stable versions.  You can't introduce features without 
introducing forward compatibility issues.  If you're adding new guest 
visible state, you've added a feature.

This is not a live migration problem, this is a problem with your stable 
branch policy.

Regards,

Anthony Liguori

> Paolo