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Date:   Mon, 22 Aug 2016 18:47:48 -0400
From:   Brian Gerst <brgerst@...il.com>
To:     "H. Peter Anvin" <hpa@...or.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 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

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