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Date: Wed, 25 Nov 2009 15:52:40 +0100
From: Gerd Hoffmann <kraxel@redhat.com>
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Subject: Re: [Qemu-devel] Re: Live migration protocol, device features,	ABIs
	and other beasts
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To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: dlaor@redhat.com, qemu-devel <qemu-devel@nongnu.org>, Juan Quintela <quintela@redhat.com>

On 11/25/09 15:09, Michael S. Tsirkin wrote:
> 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.

Well, it does fit into the machine description IMHO.  When you want 
migrate to a old qemu version you need to know what the old qemu version 
understands, and the machine description for old qemu is a natural fit.

We still could make the use of this information optional though, so 
savevm/migration could use either the most recent savevm format 
(ignoring DeviceState->savevm_version) or the savevm format matching the 
machine type (using DeviceState->savevm_version) depending on what the 
user chooses.

cheers,
   Gerd