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

From: Peter Zijlstra <peterz@infradead.org>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: "Gustavo A. R. Silva" <gustavo@embeddedor.com>,
	Joe Perches <joe@perches.com>,
	Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
	Ingo Molnar <mingo@redhat.com>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
	<x86@kernel.org>, Kan Liang <kan.liang@linux.intel.com>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Kees Cook <keescook@chromium.org>, Shawn Landden <shawn@git.icu>,
	Nathan Chancellor <natechancellor@gmail.com>,
	Luc Van Oostenryck <luc.vanoostenryck@gmail.com>,
	Chandler Carruth <chandlerc@google.com>
Subject: Re: [PATCH] perf/x86/intel: Mark expected switch fall-throughs
Date: Wed, 26 Jun 2019 10:49:27 +0200	[thread overview]
Message-ID: <20190626084927.GI3419@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <CAKwvOdn5j8Hkc_jrLMbhg-4jbNya+agtMJi=c9o01RPCno1Q+w@mail.gmail.com>

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).