linux-kernel - Re: [PATCH 5/8] x86/head: remove useless zeroed word

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Message-ID: <EE54BE00-1B92-4D8C-B54B-71A7D8206C67@zytor.com>
Date:   Mon, 22 Aug 2016 16:10:19 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Brian Gerst <brgerst@...il.com>
CC:     Andi Kleen <andi@...stfloor.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Byungchul Park <byungchul.park@....com>,
        Nilay Vaish <nilayvaish@...il.com>
Subject: Re: [PATCH 5/8] x86/head: remove useless zeroed word

On August 22, 2016 3:47:48 PM PDT, Brian Gerst <brgerst@...il.com> wrote:
>On Mon, Aug 22, 2016 at 6:23 PM, H. Peter Anvin <hpa@...or.com> wrote:
>> On August 22, 2016 2:48:46 PM PDT, Andi Kleen <andi@...stfloor.org>
>wrote:
>>>Josh Poimboeuf <jpoimboe@...hat.com> writes:
>>>
>>>> This zeroed word has no apparent purpose, so remove it.
>>>>
>>>> Brian Gerst says:
>>>>
>>>>   "FYI the word used to be the SS segment selector for the LSS
>>>>   instruction, which isn't needed in 64-bit mode."
>>>>
>>>
>>>Seems dangerous. It wouldn't surprise me if some CPUs or x86
>emulations
>>>load it anyways and trigger page faults if there is really nothing
>>>there.
>>>
>>>-Andi
>>
>> Yes, seems pointlessly risky.
>
>It was added in commit 9cf4f298e29abba25c16679fe7be70898223167e,
>copied from 32-bit but never used.  It doesn't look like we ever used
>LSS on 64-bit.
>
>--
>Brian Gerst

Ah, ok.  Wasn't in front of a computer.
-- 
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