Re: [RFC PATCH 0/2] x86 CPU feature +/- fiddling and +kvm-no-defaults

[Eduardo Habkost]

On Tue, Jan 19, 2021 at 04:28:26PM +0000, Daniel P. Berrangé wrote:
> On Tue, Jan 19, 2021 at 02:22:05PM +0000, David Edmondson wrote:
> > Currently "-cpu -feature,+feature" will disable -feature, which seems
> > contrary to the intention of the user. Fix this such that the later
> > flag wins. There are no changes to the interaction of +/- and =on/=off.
> 
> The -feature/+feature syntax is the legacy  way of configuring
> features, with feature=on|off being the preferred, since that matches
> the general QEMU standard for boolean properties.
> 
> Your proposed change in ordering of + vs - makes conceptual sense, but
> it is none the less a semantic change in behaviour that may well cause
> breakage for existing deployed VMs. This impacts guest ABI so could
> particularly cause live migration problems.
> 
> IOW, we should have implemented it the way you propose in the first
> place, but I don't think it is safe to change it now, unless you can
> tie that new semantic to a machine type version.
> 
> Before we consider that though, Paolo has just deprecated many of the
> legacy approaches for boolean properties in this:
> 
>   https://lists.nongnu.org/archive/html/qemu-devel/2021-01/msg04341.html
> 
> I'm inclined to say that we just follow on from that and finally
> deprecate the +feature/-feature CPU syntax which we're already considering
> legacy. This would remove the need to care about changing its behaviour

I believe we had multiple proposal in the past do deprecate
+feature/-feature, but there were objections.  I couldn't find
the original threads, though.

In either case, I thought we had already deprecated the weird
ordering rules of "-feature,+feature".

-- 
Eduardo