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

From: "Daniel P. Berrangé" <berrange@redhat.com>
To: Eduardo Habkost <ehabkost@redhat.com>
Cc: Laurent Vivier <lvivier@redhat.com>,
	Thomas Huth <thuth@redhat.com>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Richard Henderson <richard.henderson@linaro.org>,
	qemu-devel@nongnu.org, Markus Armbruster <armbru@redhat.com>,
	David Edmondson <david.edmondson@oracle.com>,
	Igor Mammedov <imammedo@redhat.com>,
	Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [RFC PATCH 0/2] x86 CPU feature +/- fiddling and +kvm-no-defaults
Date: Tue, 19 Jan 2021 16:41:42 +0000	[thread overview]
Message-ID: <20210119164142.GO2335568@redhat.com> (raw)
In-Reply-To: <20210119163518.GH1227584@habkost.net>

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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