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From:   Florian Weimer <fweimer@redhat.com>
To:     Borislav Petkov <bp@alien8.de>
Cc:     Andy Lutomirski <luto@kernel.org>,
        "Bae, Chang Seok" <chang.seok.bae@intel.com>,
        Dave Hansen <dave.hansen@intel.com>, X86 ML <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, linux-abi@vger.kernel.org,
        "libc-alpha@sourceware.org" <libc-alpha@sourceware.org>,
        Rich Felker <dalias@libc.org>, Kyle Huey <me@kylehuey.com>,
        Keno Fischer <keno@juliacomputing.com>
Subject: Re: Candidate Linux ABI for Intel AMX and hypothetical new related
 features
References: <CALCETrW2QHa2TLvnUuVxAAheqcbSZ-5_WRXtDSAGcbG8N+gtdQ@mail.gmail.com>
        <87lf9nk2ku.fsf@oldenburg.str.redhat.com>
        <20210412143139.GE24283@zn.tnic>
Date:   Mon, 12 Apr 2021 16:38:15 +0200
In-Reply-To: <20210412143139.GE24283@zn.tnic> (Borislav Petkov's message of
        "Mon, 12 Apr 2021 16:31:39 +0200")
Message-ID: <878s5nk1pk.fsf@oldenburg.str.redhat.com>
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* Borislav Petkov:

> On Mon, Apr 12, 2021 at 04:19:29PM +0200, Florian Weimer wrote:
>> Maybe we could have done this in 2016 when I reported this for the first
>> time.  Now it is too late, as more and more software is using
>> CPUID-based detection for AVX-512.
>
> So as I said on another mail today, I don't think a library should rely
> solely on CPUID-based detection of features especially if those features
> need kernel support too. IOW, it should ask whether the kernel can
> handle those too, first.

Yes, that's why we have the XGETBV handshake.  I was imprecise.  It's
CPUID + XGETBV of course.  Or even AT_HWCAP2 (for FSGSBASE).

> And the CPUID-faulting thing would solve stuff like that because then
> the kernel can *actually* get involved into answering something where it
> has a say in, too.

But why wouldn't we use a syscall or an entry in the auxiliary vector
for that?  Why fault a potentially performance-critical instruction?

Thanks,
Florian