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

[Eduardo Habkost]

On Thu, Aug 27, 2015 at 01:50:10PM +0300, Michael S. Tsirkin wrote:
> 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 <address@hidden> writes:
> > > 
> > > > On 08/19/2015 02:55 AM, Dr. David Alan Gilbert wrote:
> > > >> * Eduardo Habkost (address@hidden) 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.

I don't know what you mean by "not user visible implementation details"
and "identical in all respect", because I see lots of compat code that
implement user-visible differences inside pc_compat_*(), PC_COMPAT_*,
pc_q35_*_machine_options() for 2.3 and older.

-- 
Eduardo