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Re: [RFC PATCH 0/2] x86 CPU feature +/- fiddling and +kvm-no-defaults
From: |
Daniel P . Berrangé |
Subject: |
Re: [RFC PATCH 0/2] x86 CPU feature +/- fiddling and +kvm-no-defaults |
Date: |
Tue, 19 Jan 2021 16:41:42 +0000 |
User-agent: |
Mutt/1.14.6 (2020-07-11) |
On Tue, Jan 19, 2021 at 11:35:18AM -0500, Eduardo Habkost wrote:
> 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.
Historically libvirt was using +/- syntax, but we finally removed the last
usage of it in June 2019 / libvirt v5.4.0. So for modern QEMU libvirt will
always use =on|off.
> In either case, I thought we had already deprecated the weird
> ordering rules of "-feature,+feature".
Regards,
Daniel
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- Re: [External] : Re: [RFC PATCH 1/2] hw/i386: -cpu model,-feature,+feature should enable feature, (continued)
- Re: [External] : Re: [RFC PATCH 1/2] hw/i386: -cpu model,-feature,+feature should enable feature, Daniel P . Berrangé, 2021/01/20
- Re: [RFC PATCH 1/2] hw/i386: -cpu model,-feature,+feature should enable feature, David Edmondson, 2021/01/20
- Re: [RFC PATCH 1/2] hw/i386: -cpu model,-feature,+feature should enable feature, Eduardo Habkost, 2021/01/20
- Re: [RFC PATCH 1/2] hw/i386: -cpu model,-feature,+feature should enable feature, Igor Mammedov, 2021/01/20
- [PATCH] docs/system: Deprecate `-cpu ...,+feature,-feature` syntax, Eduardo Habkost, 2021/01/20
- Re: [PATCH] docs/system: Deprecate `-cpu ..., +feature, -feature` syntax, David Edmondson, 2021/01/20
- Re: [PATCH] docs/system: Deprecate `-cpu ...,+feature,-feature` syntax, Daniel P . Berrangé, 2021/01/21
- Re: [PATCH] docs/system: Deprecate `-cpu ...,+feature,-feature` syntax, Igor Mammedov, 2021/01/21
Re: [RFC PATCH 0/2] x86 CPU feature +/- fiddling and +kvm-no-defaults, Daniel P . Berrangé, 2021/01/19