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

From: Peter Maydell
Subject: Re: [PATCH v1 0/2] s390x/cpumodel: Introduce "best" model variants
Date: Tue, 19 Nov 2019 09:22:42 +0000

On Mon, 18 Nov 2019 at 22:04, Eduardo Habkost <address@hidden> 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.

-- PMM

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