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

Brian Gerst
2016-08-22T22:55:08Z

On Mon, Aug 22, 2016 at 6:23 PM, H. Peter Anvin <hpa@zytor.com> wrote:
> On August 22, 2016 2:48:46 PM PDT, Andi Kleen <andi@firstfloor.org> wrote:
>>Josh Poimboeuf <jpoimboe@redhat.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