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

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Nov 24, 2009 at 03:20:47PM +0100, Juan Quintela wrote:
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> >> But to really make it work, we need to take a list of each savevm format
> >> change and put it here.  Notice that several changes are needed:
> >> - savevm infrastructure save functions don't know about version id
> >> - devices don't know to "behave" as other version
> >> - other things that I have probably missed
> >> 
> >> Later, Juan.
> >
> > Why do you think this the right place for it, I wonder?
> > This describes the machine, it does not seem to have
> > anything to do with how we migrate it.
> 
> Becase here is where we know that _this_ machine had _this_ versions of
> each device.

So? migration format is not related to what devices
you have at all. It's how you save them.

>  And this is also the trivial part to describe: I want a
> machine like the one in qemu-0.11.

Yes, but there might be a ton of reasons to want a
machine like the one in qemu 0.11.
The need to migrate to old qemu is very rare,
it is a completely separate decision
one might take long after starting qemu,

> What is more, it makes trivial for downstream to do things like:
> - I define my machine, called foo-7.0, and the changes that it has over
>   any other machine.
> 
> Later, Juan.

Yes, machine desacription is a good thing.
No, using it for things that have nothing to do with
describing the machine is not a good thing.

-- 
MST