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

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Date:   Thu, 8 Jul 2021 07:19:21 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Florian Weimer <fweimer@...hat.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)

On 7/7/21 11:05 PM, Florian Weimer wrote:
>> This looks basically like someone dumped a bunch of CPUID bit values and
>> exposed them to applications without considering whether applications
>> would ever need them.  For instance, why would an app ever care about:
>>
>> 	PKS – Protection keys for supervisor-mode pages.
>>
>> And how could glibc ever give applications accurate information about
>> whether PKS "is supported by the operating system"?  It just plain
>> doesn't know, or at least only knows from a really weak ABI like
>> /proc/cpuinfo.
> glibc is expected to mask these bits for CPU_FEATURE_USABLE because they
> have unknown semantics (to glibc).

OK, so if I call CPU_FEATURE_USABLE(PKS) on a system *WITH* PKS
supported in the operating system, I'll get false from an interface that
claims to be:

> This macro returns a nonzero value (true) if the processor has the
> feature name and the feature is supported by the operating system.

The interface just seems buggy by *design*.