From: Dave Hansen <dave.hansen@intel.com>
To: Florian Weimer <fweimer@redhat.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)
Date: Thu, 8 Jul 2021 07:36:44 -0700 [thread overview]
Message-ID: <ba004c50-a630-27f4-5fb0-c6ad2b1f1e06@intel.com> (raw)
In-Reply-To: <87sg0oswqn.fsf@oldenburg.str.redhat.com>
On 7/8/21 7:31 AM, Florian Weimer wrote:
>> 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.)
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.
next prev parent reply other threads:[~2021-07-08 14:36 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-23 15:04 Florian Weimer
2021-06-23 15:32 ` Dave Hansen
2021-07-08 6:05 ` Florian Weimer
2021-07-08 14:19 ` Dave Hansen
2021-07-08 14:31 ` Florian Weimer
2021-07-08 14:36 ` Dave Hansen [this message]
2021-07-08 14:41 ` Florian Weimer
2021-06-25 23:31 ` Thiago Macieira
[not found] ` <3c5c29e2-1b52-3576-eda2-018fb1e58ff9@metux.net>
2021-06-28 13:20 ` Peter Zijlstra
[not found] ` <534d0171-2cc5-cd0a-904f-cd3c499b55af@metux.net>
2021-06-30 15:36 ` Thiago Macieira
2021-06-28 15:08 ` Thiago Macieira
2021-06-28 15:27 ` Peter Zijlstra
2021-06-28 16:13 ` Thiago Macieira
2021-06-28 17:11 ` Peter Zijlstra
2021-06-28 17:23 ` Thiago Macieira
2021-06-28 19:08 ` Peter Zijlstra
2021-06-28 19:26 ` Thiago Macieira
2021-06-28 17:43 ` Peter Zijlstra
2021-06-28 19:05 ` Thiago Macieira
[not found] ` <e07294c9-b02a-e1c5-3620-7fae7269fdf1@metux.net>
2021-06-30 14:34 ` Florian Weimer
[not found] ` <030f1462-2bf9-39bc-d620-6d9fbe454a27@metux.net>
2021-06-30 15:38 ` Florian Weimer
[not found] ` <4ba30cb7-6854-0691-fad6-4ca9ce674ac2@metux.net>
2021-07-01 8:21 ` Florian Weimer
[not found] ` <034dcf9b-1f8c-23ee-86a6-791122bc0f8c@metux.net>
2021-07-06 12:57 ` Florian Weimer
2021-06-30 15:29 ` Thiago Macieira
2021-07-08 7:08 ` Florian Weimer
2021-07-08 15:13 ` Thiago Macieira
2021-07-08 17:56 ` Mark Brown
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