linux-kernel - Re: [PATCH -tip 1/2] x86/hweight: Fix false output register dependency of POPCNT insn

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAFULd4ZbYmwTOR0vc8+Xiq8uPgqbte=ruKVGiKKngjHmkNunFw@mail.gmail.com>
Date: Tue, 25 Mar 2025 18:17:01 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: Borislav Petkov <bp@...en8.de>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...nel.org>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH -tip 1/2] x86/hweight: Fix false output register
 dependency of POPCNT insn

On Tue, Mar 25, 2025 at 6:10 PM Borislav Petkov <bp@...en8.de> wrote:
>
> On Tue, Mar 25, 2025 at 05:48:37PM +0100, Uros Bizjak wrote:
> > +/*
> > + * On Sandy/Ivy Bridge and later Intel processors, the POPCNT instruction
> > + * appears to have a false dependency on the destination register. Even
> > + * though the instruction only writes to it, the instruction will wait
> > + * until destination is ready before executing. This false dependency
> > + * was fixed for Cannon Lake (and later) processors.
>
> Any official documentation about that?

Please see [1], errata 026.

[1] https://www.intel.com/content/dam/www/public/us/en/documents/specification-updates/8th-gen-core-spec-update.pdf

>
> Any performance numbers to justify that change?

There is a lot of performance analysis at [2] and [3].

[2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62011
[3] https://stackoverflow.com/questions/25078285/replacing-a-32-bit-loop-counter-with-64-bit-introduces-crazy-performance-deviati

> Because if it doesn't matter, why do it in the first place? Especially if
> you're doing this XORing now for *everyone* - not just the affected parties.

Thanks,
Uros.