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

[Eduardo Habkost]

On Wed, Jan 20, 2021 at 10:17:36AM +0000, David Edmondson wrote:
> On Wednesday, 2021-01-20 at 10:08:03 GMT, Daniel P. Berrangé wrote:
> 
> > On Tue, Jan 19, 2021 at 11:30:52AM -0500, Eduardo Habkost 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?
> >
> > Where are you expecting warnings ? I don't see any when launching QEMU
> 
> qemu-system-x86_64 -display none -cpu Westmere,-vmx,+vmx
> 
> Warnings because the result of this is "-vmx".
> 
> > IMHO just leave the parsing unchanged, deprecate it, and then delete
> > the code.  We don't need to "improve" usability semantics of something
> > that we want to delete anyway.
> 
> /me nods.

I agree, but I guess we need to convince Paolo:
https://lore.kernel.org/qemu-devel/1990888058.22417362.1465939000140.JavaMail.zimbra@redhat.com/

-- 
Eduardo