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

[David Edmondson]

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.

dme.
-- 
I just bite it, it's for the look I don't light it.