linux-kernel - Re: [PATCH 1/2] x86/nmi: Optimize the check for being in the repeat

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Date:   Thu, 09 Mar 2017 22:49:49 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Andy Lutomirski <luto@...capital.net>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/2] x86/nmi: Optimize the check for being in the repeat_nmi code



On March 9, 2017 9:42:57 PM EST, Andy Lutomirski <luto@...capital.net> wrote:
>On Thu, Mar 9, 2017 at 2:42 PM, Steven Rostedt <rostedt@...dmis.org>
>wrote:
>> From: "Steven Rostedt (Red Hat)" <rostedt@...dmis.org>
>>
>> Linus mentioned that doing two compares can be replaced by a single
>> compare. That is, instead of:
>>
>>    movq $repeat_nmi, %rdx
>>    cmpq 8(%rsp), %rdx
>>    ja not_in_region
>>    movq $end_repeat_nmi, %rdx
>>    cmpq 8(%rsp), %rdx
>>    ja in_region
>>
>> we can replace that with:
>>
>>    movq 8(%rsp), %rdx
>>    subq $repeat_nmi, %rdx
>>    cmpq $end_repeat_nmi-repeat_nmi, %rdx
>>    jb in_region
>
>Seems reasonable to me.  Good luck ever noticing the speedup :)
>

It had nothing to do with speedup. Linus said that the current code makes the assembly programmer in him die a little. I want to cure that.

-- Steve 

>--Andy

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