linux-kernel - Re: [PATCH] x86-64: use 32-bit XOR to zero registers

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: <5B31DDFF02000078001CDC03@prv1-mh.provo.novell.com>
Date:   Tue, 26 Jun 2018 00:32:31 -0600
From:   "Jan Beulich" <JBeulich@...e.com>
To:     <mingo@...e.hu>, <rdunlap@...radead.org>, <tglx@...utronix.de>,
        <hpa@...or.com>
Cc:     <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86-64: use 32-bit XOR to zero registers

>>> On 25.06.18 at 18:33, <rdunlap@...radead.org> wrote:
> On 06/25/2018 03:25 AM, Jan Beulich wrote:
>> Some Intel CPUs don't recognize 64-bit XORs as zeroing idioms - use
>> 32-bit ones instead.
> 
> Hmph.  Is that considered a bug (errata)?

No.

> URL/references?

Intel's Optimization Reference Manual says so (in rev 040 this is in section
16.2.2.5 "Zeroing Idioms" as a subsection of the Goldmont/Silvermont
descriptions).

> Are these changes really only zeroing the lower 32 bits of the register?
> and that's all that the code cares about?

No - like all operations targeting a 32-bit register, the result is zero
extended to the entire 64-bit destination register.

Jan