Re: [Qemu-devel] [PATCH v3] hw/i386: Deprecate the machines pc-0.10 to pc-1.2

[Daniel P. Berrange]

On Wed, Jul 12, 2017 at 06:00:46PM +0200, Thomas Huth wrote:
> On 12.07.2017 17:04, Daniel P. Berrange wrote:
> > On Wed, Jul 12, 2017 at 10:22:33AM +0200, Thomas Huth wrote:
> >> We don't want to carry along old machine types forever. If we are able to
> >> remove the pc machines up to 0.13 one day for example, this would allow
> >> us to eventually kill the code for rombar=0 (i.e. where QEMU copies ROM
> >> BARs directly to low memory). Everything up to pc-1.2 is also known to
> >> have issues with migration.  So let's start with a deprecation message
> >> for the old machine types so that the (hopefully) few users of these old
> >> systems start switching over to newer machine types instead.
> > 
> > I think we must document & agree on our support policy for machine
> > types, before we start marking them as deprecated. eg please consider
> > the following document before accepting this deprecation patch:
> > 
> >  https://lists.gnu.org/archive/html/qemu-devel/2017-07/msg00652.html
> > 
> > Note in that proposal there, I say we do *not* go through trouble of
> > explicitly marking machines as deprecated. We just document upfront
> > the intended lifecycle and then delete them when it is done.
> > 
> > Just use deprecation warnings for things where there is no predictable
> > lifecycle upfront.
> 
> I'm still not 100% sure whether that auto-deprecation of machine types
> is such a good idea ... since we might need to maintain machines in
> downstream a little bit longer than specified there, it might be better
> to rather deprecate them manually from time to time.

Downstreams usually maintain custom machine types, so fact that the
upstream machine types get deleted is not a problem in itself. The problem
comes if followup internal code removal then prevents downstream from
creating their custom machine type.  I don't think we need tie these
issues together. We can remove old machine types, without immediately
removing features that our harm creation of downstream machine types.

> Anyway, concerning my patch - I'll stop here and won't send another
> version. There is too much bikeshed painting going on in this area for
> my taste, and since I'm rather a powerpc / s390x guy, I'm also fine if
> the pc-0.x machines stay around forever. If somebody else wants to push
> this topic instead, feel free to do so.

FWIW, I think your proposals have been very useful in general. It has
been way overdue to have this kind of discussion. I just want us to
focus on defining a policy, rather than making adhoc decisions each
time around, as the later is rather unpredictable for users of qemu.

Regards,
Daniel
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