Re: [External] : Re: [RFC PATCH 1/2] hw/i386: -cpu model,-feature,+feature should enable feature

[Igor Mammedov]

On Tue, 19 Jan 2021 11:30:52 -0500
Eduardo Habkost <ehabkost@redhat.com> wrote:

> On Tue, Jan 19, 2021 at 04:27:56PM +0000, David Edmondson wrote:
> > On Tuesday, 2021-01-19 at 10:20:56 -05, Eduardo Habkost wrote:
> >   
> > > Hi,
> > >
> > > Thanks for the patch.  Getting rid of special -feature/+feature
> > > behavior was in our TODO list for a long time.
> > >
> > > On Tue, Jan 19, 2021 at 02:22:06PM +0000, David Edmondson wrote:  
> > >> "Minus" features are applied after "plus" features, so ensure that a
> > >> later "plus" feature causes an earlier "minus" feature to be removed.
> > >> 
> > >> This has no effect on the existing "-feature,feature=on" backward
> > >> compatibility code (which warns and turns the feature off).  
> > >
> > > If we are changing behavior, why not change behavior of
> > > "-feature,feature=on" at the same time?  This would allow us to
> > > get rid of plus_features/minus_features completely and just make
> > > +feature/-feature synonyms to feature=on/feature=off.  
> > 
> > Okay, I'll do that.
> > 
> > Given that there have been warnings associated with
> > "-feature,feature=on" for a while, changing that behaviour seems
> > acceptable.
> > 
> > Would the same be true for changing "-feature,+feature"? (i.e. what this
> > patch does) Really: can this just be changed, or does there have to be
> > some period where the behaviour stays the same with a warning?  
> 
> I actually expected warnings to be triggered when using
> "-feature,+feature" as well.  If we were not generating warnings
> for that case, it will need more careful evaluation, just to be
> sure it's safe.  Igor, do you remember the details here?
As part of preparation to define/create machines via QMP,
I plan to post patch(s) to deprecate +-features in 6.0
(including special casing for -feat behavior (affects x86/sparc only))
and drop support for +-feat in 2 releases.
So we should end up with canonical property behavior only like all other
CPUs and devices.