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Date: Wed, 25 Nov 2009 16:09:55 +0200
From: "Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [Qemu-devel] Re: Live migration protocol, device features,
	ABIs and other beasts
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To: Gerd Hoffmann <kraxel@redhat.com>
Cc: dlaor@redhat.com, qemu-devel <qemu-devel@nongnu.org>, Juan Quintela <quintela@redhat.com>

On Wed, Nov 25, 2009 at 03:10:16PM +0100, Gerd Hoffmann wrote:
>   Hi,
>
>>> Doesn't work.  If you have a qemu 0.11 machine, a virtio nic and your
>>> guest uses MSI-X you simply can't migrate to qemu 0.10.  End of story.
>>> If you want to be able to migrate to 0.10 you have to start in 0.10
>>> compat mode with MSI-X disabled.  So IMHO it does makes sense to tie the
>>> savevm format to -M pc-<version>.
>>>
>>> cheers,
>>>    Gerd
>>
>> MSI-X is an orthogonal issue, let's not mix it in.
>
> It isn't.  I was making that point with feature=MSI-X.  The same  
> argument is true for any other feature:  If $feature is new in qemu  
> $newversion and you are using it you can't migrate to qemu $oldversion  
> which hasn't $feature.  The versioned machine types turn $feature in  
> $newversion, so migration to $oldversion could work.
>
> cheers,
>   Gerd

We were discussing features that are (mostly) not user-visible.
It is clear that if you have a user-visible change you have
a different machine, so you can not migrate.

Now if you fix a bug by changing savevm format, without user visible
changes you *also* can not migrate, but this does not make it into
feature or make it a good fit for machine description.

-- 
MST