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

[David Edmondson]

On Wednesday, 2021-01-20 at 10:59:24 +01, Igor Mammedov wrote:

> 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.

In that case I will abandon this change and focus on getting my upstack
consumer to switch away from using +-.

Thanks.

dme.
-- 
Sometimes these eyes, forget the face they're peering from.