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From:   Florian Weimer <fweimer@redhat.com>
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     libc-alpha@sourceware.org, linux-api@vger.kernel.org,
        x86@kernel.org, linux-arch@vger.kernel.org,
        "H.J. Lu" <hjl.tools@gmail.com>, linux-kernel@vger.kernel.org
Subject: Re: x86 CPU features detection for applications (and AMX)
References: <87tulo39ms.fsf@oldenburg.str.redhat.com>
        <e376bcb9-cd79-7665-5859-ae808dd286f1@intel.com>
        <878s2hz6g3.fsf@oldenburg.str.redhat.com>
        <b3b104cd-72d9-7f5c-116b-414c6ebf448d@intel.com>
        <87sg0oswqn.fsf@oldenburg.str.redhat.com>
        <ba004c50-a630-27f4-5fb0-c6ad2b1f1e06@intel.com>
Date:   Thu, 08 Jul 2021 16:41:09 +0200
In-Reply-To: <ba004c50-a630-27f4-5fb0-c6ad2b1f1e06@intel.com> (Dave Hansen's
        message of "Thu, 8 Jul 2021 07:36:44 -0700")
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* 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