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Date: Mon, 23 Nov 2009 07:04:08 -0600
From: Anthony Liguori <anthony@codemonkey.ws>
MIME-Version: 1.0
Subject: Re: [Qemu-devel] Re: Live migration protocol, device features, ABIs
	and   other beasts
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: qemu-devel@nongnu.org

Paolo Bonzini wrote:
> On 11/23/2009 03:17 AM, Anthony Liguori wrote:
>> You mean, each device would have multiple sections?  We already use
>> chunks for each device state.
>
> If they want to, yes.
>
>> We only migrate things that are guest visible.  Everything else is left
>> to the user to configure.  We wouldn't migrate the state of a RNG
>> emulation provided that it doesn't have an impact on the guest.
>
> The project doing lockstep virtualization would need to migrate it, 
> for example.

Lock step is an entirely different beast.  The live migration protocol 
is not suitable for it.

>> By definition, anything that is guest visible is important because it
>> affects the guest's behavior.
>
> Yes, but vendors want backwards-compatibility whenever possible. 
> Anything that is guest visible is important, but some things are less 
> important than others (or they wouldn't have been overlooked in the 
> first place).

I disagree.  Everything is equally important if we want migration to be 
correct.

I don't see how backwards compatibility fits into this picture though.  
The only argument I've heard for a change here is forwards compatibility 
which is not something I would ever expect any vendor to want to support.

Regards,

Anthony Liguori