Re: [Qemu-arm] [PATCH 06/10] target-i386: print obsolete warnings if +-features are used

[Igor Mammedov]

On Tue, 7 Jun 2016 15:00:04 +0200
Paolo Bonzini <address@hidden> wrote:

> On 07/06/2016 14:54, Igor Mammedov wrote:
> > On Tue, 7 Jun 2016 14:36:51 +0200
> > Paolo Bonzini <address@hidden> wrote:
> >   
> >> On 07/06/2016 14:32, Igor Mammedov wrote:  
> >>>>> Could you detect using +foo together with foo=off, and -foo together
> >>>>> with foo=on?  Those are the really problematic cases, without them +foo
> >>>>> and -foo can become synonyms for =on and =off.    
> >>> That's (legacy)current semantic of -cpu +-foo where it overrides any 
> >>> foo=x,
> >>> potentially it's possible to track foo=x locally in parser
> >>> and then compare with +-foo both ways.
> >>> But all we can do currently is to print warning about such use case.
> >>>
> >>> I think Eduardo's suggestion to just warn that +-foo is obsolete for now
> >>> and drop support for it in several releases is sufficient(good) enough.   
> >>>  
> >>
> >> kvm-unit-tests and libvirt both use it, especially because =on and =off
> >> are relatively new I think?  It seems like it's really widespread.  
> > 
> > Yep, that's why it's not removed now.
> > Looks like libvirt would be able to switch to foo=x syntax,
> > I can take a look at kvm-unit-tests and make it use foo=x too.  
> 
> And all tutorials, and all scripts.  It's really too hard.
> 
> I'd really prefer to make an incompatible change straight in 2.7 for the
> case of mixed foo=x and [+-]foo.
I've tried to make a bit more extreme incompatible change starting from 2.7
machine type in RFC (i.e. allow only foo=x syntax).
But Eduardo prefers to keep current +-foo, just marking it obsolete
so that users would have time to adapt before support for legacy +-foo
is dropped.

Anyways, we can introduce "mixed foo=x and [+-]foo" check on top of
this series with warning and probably switch to fail later so not
break existing users without giving them time to adapt.

> Paolo