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

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Message-ID: <CALCETrUXMnutTV=SEs6ot58j32_5=K5Z=G7_57gVZt_GFcuDiw@mail.gmail.com>
Date:   Fri, 26 Mar 2021 12:47:42 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Florian Weimer <fw@...eb.enyo.de>
Cc:     Andy Lutomirski <luto@...nel.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>,
        "Carlos O'Donell" <carlos@...hat.com>,
        Rich Felker <dalias@...c.org>,
        libc-alpha <libc-alpha@...rceware.org>
Subject: Re: Why does glibc use AVX-512?

On Fri, Mar 26, 2021 at 12:34 PM Florian Weimer <fw@...eb.enyo.de> wrote:
>
> * Andy Lutomirski:
>
> >> > AVX-512 cleared, and programs need to explicitly request enablement.
> >> > This would allow programs to opt into not saving/restoring across
> >> > signals or to save/restore in buffers supplied when the feature is
> >> > enabled.
> >>
> >> Isn't XSAVEOPT already able to handle that?
> >>
> >
> > Yes, but we need a place to put the data, and we need to acknowledge
> > that, with the current save-everything-on-signal model, the amount of
> > time and memory used is essentially unbounded.  This isn't great.
>
> The size has to have a known upper bound, but the save amount can be
> dynamic, right?
>
> How was the old lazy FPU initialization support for i386 implemented?
>
> >> Assuming you can make XSAVEOPT work for you on the kernel side, my
> >> instincts tell me that we should have markup for RTM, not for AVX-512.
> >> This way, we could avoid use of the AVX-512 registers and keep using
> >> VZEROUPPER, without run-time transaction checks, and deal with other
> >> idiosyncrasies needed for transaction support that users might
> >> encounter once this feature sees more use.  But the VZEROUPPER vs RTM
> >> issues is currently stuck in some internal process issue on my end (or
> >> two, come to think of it), which I hope to untangle next month.
> >
> > Can you elaborate on the issue?
>
> This is the bug:
>
>   vzeroupper use in AVX2 multiarch string functions cause HTM aborts
>   <https://sourceware.org/bugzilla/show_bug.cgi?id=27457>
>
> Unfortunately we have a bug (outside of glibc) that makes me wonder if
> we can actually roll out RTM transaction checks (or any RTM
> instruction) on a large scale:
>
>   x86: Sporadic failures in tst-cpu-features-cpuinfo
>   <https://sourceware.org/bugzilla/show_bug.cgi?id=27398#c3>

It's worth noting that recent microcode updates have make RTM
considerably less likely to actually work on many parts.  It's
possible you should just disable it. :(