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

[Daniel P . Berrangé]

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


Regards,
Daniel
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