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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: Sat, 9 Nov 2019 16:07:14 +0000

On Fri, 8 Nov 2019 at 19:11, Eduardo Habkost <address@hidden> wrote:
> On Fri, Nov 08, 2019 at 01:02:28PM +0000, Peter Maydell wrote:
> > On Fri, 8 Nov 2019 at 12:46, David Hildenbrand <address@hidden> wrote:
> > > There is a small but important difference between "max"/"host" and
> > > "best". Max really means "all features", including deprecated ones.
> > > "best", however, can disable experimental or deprecated features. Or any
> > > other features we don't want to be enabled when somebody selects a model
> > > manually.
> On x86, this is implemented by "host".  "max" gives you the full
> set of features that can be enabled by the user.  "host" gives
> you a reasonable set of features you will want to see enabled by
> default when the user says "use the host CPU".

How does "host" work for TCG on x86?

> > Hmm. I see the distinction, but is it one that's sufficiently
> > worth making that we want to expose it to our users, possibly
> > try to add it to the other architectures, etc ? How bad is it
> > if the CPU provides some legacy deprecated feature that the
> > guest just doesn't use ?
> >
> "max" isn't something we want to expose to end users.  It is
> something we need to expose to other software components.

We seem to have a disagreement here about what 'max' is intended
for and what its semantics for. That seems unfortunate...

For Arm, "max" is absolutely something we want to expose to
end users. It's the easiest way for a user to say "give me
something that's going to work". "host" doesn't work on TCG,
only with KVM.

> > 'max' already shouldn't include experimental features, at least
> > for Arm -- those should be off by default, because they're
> > experimental and you only want users to get them if they
> > explicitly opt in via '-cpu something,+x-my-feature'.
> The whole point of "max" is to tell management software which
> features are valid to be enabled in a host.  If "+x-my-feature"
> works, "x-my-feature" must be included in "max".

No, definitely not. Experimental features should never be
enabled by default -- the user must explicitly opt into them
so they are aware that they're using something that might
change behaviour or go away in a future QEMU version.

Also, from my point of view "max" is not for the benefit
of management software in particular. It's a convenience
for users primarily (and also for management software if
it doesn't want to care whether it's running KVM or TCG).

If management software wants a way to ask "what features
might be valid" we should provide them with one which
doesn't require those features to be enabled-by-default
in the 'max' CPU.

-- PMM

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