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

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Message-ID: <87blb5d7zx.fsf@mid.deneb.enyo.de>
Date:   Fri, 26 Mar 2021 20:34:26 +0100
From:   Florian Weimer <fw@...eb.enyo.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     "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?

* 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>

The dynamic RTM check might trap due to this bug.  (We have a bit more
information about the nature of the bug, currently missing from
Bugzilla.)

I'm also worried that the new dynamic RTM check in the string
functions has a performance impact.  Due to its nature, it will be
enabled for every program once running on RTM-capable hardware, not
just those that actually use RTM.