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

[Igor Mammedov <imammedo@redhat.com>]

On Wed, 20 Jan 2021 11:18:01 -0500
Eduardo Habkost <ehabkost@redhat.com> wrote:

> 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/
that's ancient :)

He recently started this revolution himself :)
https://www.mail-archive.com/qemu-devel@nongnu.org/msg757280.html

That's why I have -cpu +/-foo deprecation on my not too far away TODO list.