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Message-ID: <CAKwvOdkp7qnwLGY2=TOx=FQa1k2hEkdi1PO+9GfZkTQEUh49Rg@mail.gmail.com>
Date: Wed, 26 Jun 2019 15:14:05 -0700
From: Nick Desaulniers <ndesaulniers@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
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 Wed, Jun 26, 2019 at 1:49 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> 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.
Yes; but before release is when they need the help the most in order
for testing to find regressions.
>
> > 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).
Not sure; the assembler directives and their requirements aren't
something I'm too familiar with.
--
Thanks,
~Nick Desaulniers
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