Re: [Qemu-devel] [PATCH] q35: Remove old machine versions

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

On Tue, Aug 25, 2015 at 05:21:16PM +0100, Daniel P. Berrange wrote:
> On Mon, Aug 24, 2015 at 11:54:48AM +0200, Markus Armbruster wrote:
> > John Snow <jsnow@redhat.com> writes:
> > 
> > > On 08/19/2015 02:55 AM, Dr. David Alan Gilbert wrote:
> > >> * Eduardo Habkost (ehabkost@redhat.com) wrote:
> > >>> Migration with q35 was not possible before commit
> > >>> 04329029a8c539eb5f75dcb6d8b016f0c53a031a, because q35 unconditionally creates
> > >>> an ich9-ahci device, that was marked as unmigratable. So all q35 machines
> > >>> before pc-q35-2.4 were unmigratable, and there's no point in keeping
> > >>> compatibility code for them.
> > >>>
> > >>> Remove all old pc-q35 machine classes and keep only pc-q35-2.4.
> > >> 
> > >> But doesn't that mean that anyone who has a machine configured with one
> > >> of those machine types will suddenly find it wont start?
> > >> 
> > >> Dave
> > >> 
> > >
> > > To some extent, all versions of this board prior to 2.4 should be
> > > considered unsupported and we should discourage their use anyway.
> > >
> > > If you really want, I suppose we could just alias them to 2.4 ...
> > 
> > I'd very much prefer an honest "won't start" over a silent change of the
> > machine type.
> > 
> > If we really want to bend over backwards for existing uses of these
> > machine types, we could make them error out with "use pc-q35-2.5
> > instead".  Since I don't think they exist outside testing, I wouldn't
> > bother.
> 
> Agreed, we should be reporting a hard error for any machine types we
> have deleted. Or if we care about smooth upgrade path then we shouldn't
> be deleting them in the first place. Silently changing the user's
> requested machine type into a different machine type is violating
> the semantics of stable machine types.
> 
> Regards,
> Daniel

The reason we are deleting them is because changes in behaviour are not
user visible implementation details, and live migration is unsupported.

In other words 2.4 is identical to <2.3 in all respect except live
migration, which didn't work in <2.3 and works in 2.4, that's why
aliasing them is fine.

But if people really want "2.3 = 2.4 that does not migrate" we could
do that with very little code, too.



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