Re: [PATCH v1 0/2] s390x/cpumodel: Introduce "best" model variants

From: Peter Maydell <peter.maydell@linaro.org>
To: Eduardo Habkost <ehabkost@redhat.com>
Cc: "Thomas Huth" <thuth@redhat.com>,
	"Daniel P . Berrangé" <berrange@redhat.com>,
	"Janosch Frank" <frankja@linux.ibm.com>,
	"David Hildenbrand" <david@redhat.com>,
	"Cornelia Huck" <cohuck@redhat.com>,
	"Richard Henderson" <richard.henderson@linaro.org>,
	"QEMU Developers" <qemu-devel@nongnu.org>,
	"Markus Armbruster" <armbru@redhat.com>,
	"Halil Pasic" <pasic@linux.ibm.com>,
	"Christian Borntraeger" <borntraeger@de.ibm.com>,
	qemu-s390x <qemu-s390x@nongnu.org>,
	"Michael Mueller" <mimu@linux.ibm.com>,
	"Jiri Denemark" <jdenemar@redhat.com>
Subject: Re: [PATCH v1 0/2] s390x/cpumodel: Introduce "best" model variants
Date: Tue, 19 Nov 2019 09:22:42 +0000	[thread overview]
Message-ID: <CAFEAcA8XoyeAfHuKe0AEvecCzo748Yk1VD1+VD=C3ACZdfzwsw@mail.gmail.com> (raw)
In-Reply-To: <20191118220417.GF3812@habkost.net>

On Mon, 18 Nov 2019 at 22:04, Eduardo Habkost <ehabkost@redhat.com> wrote:
>
> On Mon, Nov 18, 2019 at 09:19:55PM +0000, Peter Maydell wrote:
> > Why should it matter whether a feature is enabled
> > or disabled by default in the CPU type? It ought to be probeable
> > by QMP either way (ie there is a difference between
> > "this CPU has this feature switch and it is on by default",
> > "this CPU has this feature switch and it is off by default"
> > and "this CPU does not have this feature switch at all",
> > and presumably libvirt needs to distinguish them).
>
> Its use case is neither "this CPU has this feature switch" or for
> "it is on|off by default".  We use it to probe for "this feature
> can be enabled in this host hardware+kernel+QEMU combination".

OK. Well, the answer to that depends on the name of the CPU,
in general. So you can't use a fake CPU name to try to answer
the question.

> In other words, in x86 and s390x "max" is just a reserved name
> for the query-cpu-model-expansion command arguments in s390x and
> x86.  The fact that it is visible to users can be considered a
> bug, and we can fix that.

I think 'max' is useful to users, and we've provided it to users,
so removing it again would be a compatibility break. I'm not
entirely sure where we go from here...

> If you still don't like how query-cpu-model-expansion works, we
> can also discuss that.  But I'm not sure it would be a good use
> of our (and libvirt developers') time.

I don't hugely care about query-cpu-model-expansion. I
just don't want it to have bad effects on the semantics
of user-facing stuff like x- properties.

thanks
-- PMM