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

From: Eduardo Habkost
Subject: Re: [Qemu-devel] Re: Live migration protocol, device features, ABIs and other beasts
Date: Mon, 23 Nov 2009 13:21:42 -0200
User-agent: Sup/git

Excerpts from Anthony Liguori's message of Mon Nov 23 12:49:23 -0200 2009:
> Juan Quintela wrote:
> > But if you know substitute qemu-0.11 and qemu-0.12 for RHEL5.4 and
> > RHEL5.4.1, you will see that the code bases are going to be really,
> > really similar.  And if any savevm format is changed, it is because
> > there are no other solution.
> >   
> In our own stable branch, we do not introduce any savevm changes.  I 
> would recommend the same policy for RHEL :-)

But what if you need to add a savevm change to make migration work
properly on the stable branch? You can't just tell users "migration is
known to be broken on the stable branch, please don't run migrations
when using the stable branch". That's the case for the pvclock MSR
migration fix.

In a perfect world, the set of state data that is migrated by the
current implementation would always match exactly the expected behavior
of the virtual machine. Unfortunately sometimes the implementation
doesn't follow the "contract" (be it some written specification,
documentation, or just user expectations).  When that happens, it is a
bug on qemu and it needs to be fixed on the stable branch.

Note that (right now) I am not arguing for backward migration, but just
arguing that we can't have a strict "no savevm changes" policy on the
stable branch.

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