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

[Eduardo Habkost]

On Fri, Aug 28, 2015 at 12:00:58PM +0200, Markus Armbruster wrote:
> Eduardo Habkost <address@hidden> writes:
> 
> > 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.
> 
> While I don't think maintaining the old, not-really-functional q35 types
> is worth the bother, I wouldn't mind these aliases...
> 
> > 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.
> 
> ... *if* the compat code *demonstrably* has no guest-visible effects.

Most of the compat code has guest-visible effects, it seems. Enumerating
some of them:

* Lots of CPUID compat code
* virtio and PCI compat properties at PC_COMPAT_2_0: I assume
  at least some of them have guest-visible effects
* USB msos-desc property on PC_COMPAT_1_7
* no_floppy on pc_q35_2_3_machine_options()

-- 
Eduardo