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

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Date:   Thu, 08 Jul 2021 16:31:28 +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:

> 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*.

Yes, but that is largely a documentation matter.  We should have said
something about “userspace” there, and that the bit needs to be known to
glibc.  There is another exception: FSGSBASE, and that's a real bug we
need to fix (it has to go through AT_HWCAP2).

If we want to avoid that, we need to go down the road of a curated set
of CPUID bits, where a bit only exists if we have taught glibc its
semantics.  You still might get a false negative by running against an
older glibc than the application was built for.  (We are not going to
force applications that e.g. look for FSGSBASE only run with a glibc
that is at least of that version which implemented semantics for the
FSGSBASE bit.)

Thanks,
Florian