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

From: David Hildenbrand
Subject: Re: [PATCH v1 0/2] s390x/cpumodel: Introduce "best" model variants
Date: Tue, 19 Nov 2019 12:00:14 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.1

On 19.11.19 11:36, Peter Maydell wrote:
On Tue, 19 Nov 2019 at 09:59, David Hildenbrand <address@hidden> wrote:

On 19.11.19 10:22, Peter Maydell wrote:
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.

IMHO, max should really include all features (yes, also the bad
x-features on arm :) ) and we should have a way to give users the
opportunity to specify "just give me the best model independent of the
accelerator" - something like a "best" model, but I don't care about the

How would "max includes all features" work if we have two
x- features (or even two normal features!) which are incompatible
with each other? How does it work for features which are

I assume the "max" model should at least be consistent, see below.

valid for some other CPU type but not for 'max'? The design
seems to assume a rather simplified system where every
feature is independent and can always be applied to every
CPU, which I don't think is guaranteed to be the case.

I do agree that the use case of "max" for detecting which features can be enabled for any CPU model is quite simplified and would also not work like this on s390x. It really means "give me the maximum/latest/greatest you can". Some examples on s390x:

On s390x, we don't allow to enable new features for older CPU generation. "vx" was introduced with a z13. If "max" is a z13, it can include "vx", if available. However, if you select a z12 (zEC12), you cannot enable "vx":

"Feature '%s' is not available for CPU model '%s', it was introduced with later models."

Also, we have dependency checks (target/s390x/cpu_models.c:check_consistency()), that at least warn on inconsistencies between features (feature X requires feature Y).

We would need more fine-grained "max" models. E.g., z13-max vs. z13-best vs. z13-base.

- z13-max: Maximum features that can be enabled on the current
           accel/host for a z13.
- z13-best: Best features that can be enabled on the current
           accel/host for a z13.
- z13-base: base feature set, independent of current
           accel/host for a z13. (we do have this already on s390x)

-- PMM



David / dhildenb

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