linux-kernel - Re: [PATCH] perf/x86/intel: Mark expected switch fall-throughs

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Message-ID: <20190628133105.GD3463@hirez.programming.kicks-ass.net>
Date:   Fri, 28 Jun 2019 15:31:05 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Joe Perches <joe@...ches.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Kan Liang <kan.liang@...ux.intel.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Shawn Landden <shawn@....icu>,
        Nathan Chancellor <natechancellor@...il.com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Chandler Carruth <chandlerc@...gle.com>
Subject: Re: [PATCH] perf/x86/intel: Mark expected switch fall-throughs

On Thu, Jun 27, 2019 at 09:12:50AM +0200, Peter Zijlstra wrote:

> Josh came up with the following:
> 
> +		/* If the jump target is close, do a 2-byte nop: */
> +		".skip -(%l[l_yes] - 1b <= 126), 0x66\n"
> +		".skip -(%l[l_yes] - 1b <= 126), 0x90\n"
> +		/* Otherwise do a 5-byte nop: */
> +		".skip -(%l[l_yes] - 1b > 126), 0x0f\n"
> +		".skip -(%l[l_yes] - 1b > 126), 0x1f\n"
> +		".skip -(%l[l_yes] - 1b > 126), 0x44\n"
> +		".skip -(%l[l_yes] - 1b > 126), 0x00\n"
> +		".skip -(%l[l_yes] - 1b > 126), 0x00\n"
> 
> Which is a wonderfully gruesome hack :-) So I'll be playing with that
> for a bit.

For those with interest; full patches at:

  https://lkml.kernel.org/r/20190628102113.360432762@infradead.org