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

From: Anthony Liguori
Subject: Re: [Qemu-devel] Re: Live migration protocol, device features, ABIs and other beasts
Date: Mon, 23 Nov 2009 09:00:05 -0600
User-agent: Thunderbird (X11/20090825)

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.


Anthony Liguori


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