Date: Thu, 09 Mar 2017 22:49:49 -0500
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Subject: Re: [PATCH 1/2] x86/nmi: Optimize the check for being in the repeat_nmi code
To: Andy Lutomirski <luto@amacapital.net>
CC: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
        "H. Peter Anvin" <hpa@zytor.com>, Andy Lutomirski <luto@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>
From: Steven Rostedt <rostedt@goodmis.org>
Message-ID: <B0A59A10-BA5C-42AA-A23B-4CF832E6B764@goodmis.org>
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On March 9, 2017 9:42:57 PM EST, Andy Lutomirski <luto@amacapital.net> wrote:
>On Thu, Mar 9, 2017 at 2:42 PM, Steven Rostedt <rostedt@goodmis.org>
>wrote:
>> From: "Steven Rostedt (Red Hat)" <rostedt@goodmis.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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