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

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Message-ID: <YiX81kD/668UdFBr@nazgul.tnic>
Date:   Mon, 7 Mar 2022 13:38:59 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Ammar Faizi <ammarfaizi2@...weeb.org>
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 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.

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.

Also, I'd advise going over Documentation/process/ if you're new to this.
Especially Documentation/process/submitting-patches.rst.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette