linux-kernel - Re: [PATCH v1 0/2] x86: Avoid using INC and DEC instructions on hot paths

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Message-ID: <11dec988-0b4f-00f2-6fa6-8d08be95d106@gnuweeb.org>
Date:   Mon, 7 Mar 2022 20:37:59 +0700
From:   Ammar Faizi <ammarfaizi2@...weeb.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tony Luck <tony.luck@...el.com>,
        Youquan Song <youquan.song@...el.com>,
        linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org,
        gwml@...r.gnuweeb.org, x86@...nel.org
Subject: Re: [PATCH v1 0/2] x86: Avoid using INC and DEC instructions on hot
 paths

On 3/7/22 7:38 PM, Borislav Petkov wrote:
> On Mon, Mar 07, 2022 at 06:45:56PM +0700, Ammar Faizi wrote:
>> In order to take maximum advantage of out-of-order execution,
>> avoid using INC/DEC instructions when appropriate. INC/DEC only
>> writes to part of the flags register, which can cause a partial
>> flag register stall. This series replaces INC/DEC with ADD/SUB.
> 
> "Improvements" like that need to show in benchmark runs - not
> microbenchmark - that they bring anything. Just by looking at them, I'd
> say they won't show any difference. But I'm always open to surprises.

OK, thanks for taking a look. I will play a bit more with this. Not sure
how much the visible improvement. If I can win some numbers (probably can't),
I will be back to this thread.

> Btw, you don't have to send all your patches directly to me - there are
> other x86 maintainers. IOW, you can use scripts/get_maintainer.pl to
> figure out who to send them to.

I did anyway, all CC list here I took from that script. I will try to give
other maintainers a turn next time.

> Also, I'd advise going over Documentation/process/ if you're new to this.
> Especially Documentation/process/submitting-patches.rst.
I might've missed the benchmark backup part. Will review those documents again.

-- 
Ammar Faizi