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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: Fri, 8 Nov 2019 13:02:28 +0000

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 s390x, the feature "csske" is deprecated. New HW still has it, but we
> want new guests to run without this facility. Dropping it from "max"
> would affect existing setups. We already changed the default model
> (e.g., -cpu z13) to disable it with never QEMU machines.
> E.g., nested virtualization features on some architectures could be a
> feature set you want to disable, although contained in the "max" model.
> (e.g., no migration support yet).
> I am not completely against calling these "max" models instead of "best"
> models, but I think this makes it clearer that there is indeed a difference.

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' 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'.

-- PMM

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