linux-kernel - RE: [PATCH] x86: bitops: fix build regression

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: <A6FD91D2-8D3A-4767-B6AD-D35B056C58C4@zytor.com>
Date:   Thu, 07 May 2020 12:31:19 -0700
From:   hpa@...or.com
To:     David Laight <David.Laight@...LAB.COM>,
        "'Brian Gerst'" <brgerst@...il.com>
CC:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Sedat Dilek <sedat.dilek@...il.com>,
        stable <stable@...r.kernel.org>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        "kernelci . org bot" <bot@...nelci.org>,
        Andy Shevchenko <andriy.shevchenko@...el.com>,
        Ilie Halip <ilie.halip@...il.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Marco Elver <elver@...gle.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Daniel Axtens <dja@...ens.net>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "clang-built-linux@...glegroups.com" 
        <clang-built-linux@...glegroups.com>
Subject: RE: [PATCH] x86: bitops: fix build regression

On May 7, 2020 8:09:35 AM PDT, David Laight <David.Laight@...LAB.COM> wrote:
>From: Brian Gerst
>> Sent: 07 May 2020 14:32
>...
>> I think the bug this worked around was that the compiler didn't
>detect
>> that CONST_MASK(nr) was also constant and doesn't need to be put into
>> a register.  The question is does that bug still exist on compiler
>> versions we care about?
>
>Hmmm...
>That ought to have been fixed instead of worrying about the fact
>that an invalid register was used.
>
>Alternatively is there any reason not to use the bts/btc instructions?
>Yes, I know they'll do wider accesses, but variable bit numbers do.
>It is also possible that the assembler will support constant bit
>numbers >= 32 by adding to the address offset.
>
>	David
>
>-
>Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes,
>MK1 1PT, UK
>Registration No: 1397386 (Wales)

They're slower, and for unaligned locked fields can be severely so.
-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.