linux-kernel - Re: x86 CPU features detection for applications (and AMX)

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Message-ID: <87lf6gswai.fsf@oldenburg.str.redhat.com>
Date:   Thu, 08 Jul 2021 16:41:09 +0200
From:   Florian Weimer <fweimer@...hat.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     libc-alpha@...rceware.org, linux-api@...r.kernel.org,
        x86@...nel.org, linux-arch@...r.kernel.org,
        "H.J. Lu" <hjl.tools@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: x86 CPU features detection for applications (and AMX)

* Dave Hansen:

> That's kinda my whole point.
>
> These *MUST* be curated to be meaningful.  Right now, someone just
> dumped a set of CPUID bits into the documentation.
>
> The interface really needs *three* modes:
>
> 1. Yes, the CPU/OS supports this feature
> 2. No, the CPU/OS doesn't support this feature
> 3. Hell if I know, never heard of this feature
> 	
> The interface really conflates 2 and 3.  To me, that makes it
> fundamentally flawed.

That's an interesing point.

3 looks potentially more useful than the feature/usable distinction to
me.

The recent RTM change suggests that there are more states, but we
probably can't do much about such soft-disable changes.

Thanks,
Florian