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Date: Tue, 24 Nov 2009 18:41:11 +0100
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: [Qemu-devel] Re: Live migration protocol, device features,
	ABIs and other beasts
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To: Juan Quintela <quintela@trasno.org>
Cc: qemu-devel@nongnu.org, Paul Brook <paul@codesourcery.com>, "Michael S. Tsirkin" <mst@redhat.com>

On 11/24/2009 03:30 PM, Juan Quintela wrote:
> No, new ->  old is way, way more difficult.

New->old is way more difficult with the current migration file format. 
The current migration file format is not at all designed to be read by 
an older version.

Or for that matter a tool that only cares about the state of a couple 
devices (this is IMNSHO much more important if it wasn't for the RHEL 
case that started the discussion).

Thinking about the latter and using it to solve the former would be the 
right way to do it.  Some time ago when I had just started at Red Hat I 
wrote a mini-library to parse qemu migrate data, and I couldn't believe 
that there was no length field anywhere.  Luckily I needed only RAM and 
CPU data, so I only needed to know about a couple of extraneous chunks.

Paolo