linux-kernel - Re: Why does glibc use AVX-512?

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Date:   Fri, 26 Mar 2021 11:06:20 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     libc-alpha <libc-alpha@...rceware.org>,
        "H. J. Lu" <hjl.tools@...il.com>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Bae, Chang Seok" <chang.seok.bae@...el.com>,
        Florian Weimer <fweimer@...hat.com>,
        Carlos O'Donell <carlos@...hat.com>,
        Rich Felker <dalias@...c.org>
Subject: Re: Why does glibc use AVX-512?

On Thu, Mar 25, 2021 at 09:38:24PM -0700, Andy Lutomirski wrote:
> I think we should seriously consider solutions in which, for new
> tasks, XCR0 has new giant features (e.g. AMX) and possibly even
> AVX-512 cleared, and programs need to explicitly request enablement.

I totally agree with making this depend on an explicit user request,
but...

> This would allow programs to opt into not saving/restoring across
> signals or to save/restore in buffers supplied when the feature is
> enabled.  This has all kinds of pros and cons, and I'm not sure it's a
> great idea.  But, in the absence of some change to the ABI, the
> default outcome is that, on AMX-enabled kernels on AMX-enabled
> hardware, the signal frame will be more than 8kB, and this will affect
> *every* signal regardless of whether AMX is in use.

... what's stopping the library from issuing that new ABI call before it
starts the app and get <insert fat feature here> automatically enabled
for everything by default?

And then we'll get the lazy FPU thing all over again.

So the ABI should be explicit user interaction or a kernel cmdline param
or so.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette