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

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Date:   Wed, 26 Jun 2019 10:49:27 +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 Tue, Jun 25, 2019 at 11:15:57AM -0700, Nick Desaulniers wrote:

> Unreleased versions of Clang built from source can;

I've bad experiences with using unreleased compilers; life is too short.

> We're currently planning multiple output constraint support w/ asm
> goto, and have recently implemented things like
> __GCC_ASM_FLAG_OUTPUTS__.

That's good to hear.

> If there's other features that we should
> start implementing, please let us know.

If you've got any ideas on how to make this:

  https://lkml.kernel.org/r/20190621120923.GT3463@hirez.programming.kicks-ass.net

work, that'd be nice. Basically I wanted the asm goto to emit a 2 or 5
byte JMP/NOP depending on the displacement size. We can trivially get
JMP right by using:

	jmp \l_yes

and letting the assembler sort it, but getting the NOP right has so far
eluded me:

.if \l_yes - (. + 2) < 127
	.byte 0x66, 0x90
.else
	.byte STATIC_KEY_INIT_NOP
.endif

doesn't work. We can ofcourse unconditionally emit the JMP and then
rewrite the binary afterward, and replace the emitted jumps with the
right size NOP, but that's a bit yuck.

Once it emits the variable size instruction consistently, we can update
the patching side to use the same condition to select the new
instruction (and fix objtool).