From: Nick Desaulniers <ndesaulniers@google.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Nathan Chancellor <natechancellor@gmail.com>,
Kees Cook <keescook@chromium.org>,
Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
"Gustavo A. R. Silva" <gustavo@embeddedor.com>,
Joe Perches <joe@perches.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>,
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>,
Shawn Landden <shawn@git.icu>,
clang-built-linux <clang-built-linux@googlegroups.com>,
Josh Poimboeuf <jpoimboe@redhat.com>
Subject: Re: [PATCH] perf/x86/intel: Mark expected switch fall-throughs
Date: Wed, 26 Jun 2019 15:15:38 -0700 [thread overview]
Message-ID: <CAKwvOdm3qvaFH47MxJ9PWMbhYumt9V7gTASO5-ZAzwTVBUQqyA@mail.gmail.com> (raw)
In-Reply-To: <20190626092432.GJ3419@hirez.programming.kicks-ass.net>
On Wed, Jun 26, 2019 at 2:24 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Tue, Jun 25, 2019 at 11:47:06PM +0200, Thomas Gleixner wrote:
> > > On Tue, Jun 25, 2019 at 09:53:09PM +0200, Thomas Gleixner wrote:
>
> > > > but it also makes objtool unhappy:
> > > >
> > > > arch/x86/events/intel/core.o: warning: objtool: intel_pmu_nhm_workaround()+0xb3: unreachable instruction
> > > > kernel/fork.o: warning: objtool: free_thread_stack()+0x126: unreachable instruction
> > > > mm/workingset.o: warning: objtool: count_shadow_nodes()+0x11f: unreachable instruction
> > > > arch/x86/kernel/cpu/mtrr/generic.o: warning: objtool: get_fixed_ranges()+0x9b: unreachable instruction
> > > > arch/x86/kernel/platform-quirks.o: warning: objtool: x86_early_init_platform_quirks()+0x84: unreachable instruction
> > > > drivers/iommu/irq_remapping.o: warning: objtool: irq_remap_enable_fault_handling()+0x1d: unreachable instruction
>
> > I just checked two of them in the disassembly. In both cases it's jump
> > label related. Here is one:
> >
> > asm volatile("1: rdmsr\n"
> > 410: b9 59 02 00 00 mov $0x259,%ecx
> > 415: 0f 32 rdmsr
> > 417: 49 89 c6 mov %rax,%r14
> > 41a: 48 89 d3 mov %rdx,%rbx
> > return EAX_EDX_VAL(val, low, high);
> > 41d: 48 c1 e3 20 shl $0x20,%rbx
> > 421: 48 09 c3 or %rax,%rbx
> > 424: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
> > 429: eb 0f jmp 43a <get_fixed_ranges+0xaa>
> > do_trace_read_msr(msr, val, 0);
> > 42b: bf 59 02 00 00 mov $0x259,%edi <------- "unreachable"
> > 430: 48 89 de mov %rbx,%rsi
> > 433: 31 d2 xor %edx,%edx
> > 435: e8 00 00 00 00 callq 43a <get_fixed_ranges+0xaa>
> > 43a: 44 89 35 00 00 00 00 mov %r14d,0x0(%rip) # 441 <get_fixed_ranges+0xb1>
> >
> > Interestingly enough there are some more hunks of the same pattern in that
> > function which look all the same. Those are not upsetting objtool. Josh
> > might give an hint where to stare at.
>
> That's pretty atrocious code-gen :/ Does LLVM support things like label
> attributes? Back when we did jump labels GCC didn't, or rather, it
> ignored it completely when combined with asm goto (and it might still).
>
> That is, would something like this:
>
> diff --git a/arch/x86/include/asm/jump_label.h b/arch/x86/include/asm/jump_label.h
> index 06c3cc22a058..1761b1e76ddc 100644
> --- a/arch/x86/include/asm/jump_label.h
> +++ b/arch/x86/include/asm/jump_label.h
> @@ -32,7 +32,7 @@ static __always_inline bool arch_static_branch(struct static_key *key, bool bran
> : : "i" (key), "i" (branch) : : l_yes);
>
> return false;
> -l_yes:
> +l_yes: __attribute__((cold));
> return true;
> }
>
> @@ -49,7 +49,7 @@ static __always_inline bool arch_static_branch_jump(struct static_key *key, bool
> : : "i" (key), "i" (branch) : : l_yes);
>
> return false;
> -l_yes:
> +l_yes: __attribute__((hot));
> return true;
> }
>
> Help LLVM?
So Clang definitely complains about putting attribute hot/cold on
labels: https://godbolt.org/z/N-Z33Q
In my test case I wasn't able to influence code gen with them though
in GCC at -O2 or -O0. Maybe GCC has a test case that shows how they
should work?
--
Thanks,
~Nick Desaulniers
next prev parent reply other threads:[~2019-06-26 22:15 UTC|newest]
Thread overview: 52+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-06-24 16:19 [PATCH] perf/x86/intel: Mark expected switch fall-throughs Gustavo A. R. Silva
2019-06-24 19:31 ` Peter Zijlstra
2019-06-24 19:45 ` Joe Perches
2019-06-24 20:37 ` Peter Zijlstra
2019-06-24 20:53 ` Gustavo A. R. Silva
2019-06-24 20:57 ` Joe Perches
2019-06-25 7:20 ` Peter Zijlstra
2019-06-24 22:28 ` Miguel Ojeda
2019-06-25 7:18 ` Peter Zijlstra
2019-06-25 12:47 ` Miguel Ojeda
2019-06-25 18:15 ` Nick Desaulniers
2019-06-25 22:29 ` Joe Perches
2019-06-25 22:57 ` Nick Desaulniers
2019-06-25 23:25 ` Joe Perches
2019-06-26 8:49 ` Peter Zijlstra
2019-06-26 22:14 ` Nick Desaulniers
2019-06-27 7:12 ` Peter Zijlstra
2019-06-28 13:31 ` Peter Zijlstra
2019-06-28 18:44 ` Nick Desaulniers
2019-06-29 7:10 ` Peter Zijlstra
2019-06-25 17:12 ` Kees Cook
2019-06-25 18:05 ` Nathan Chancellor
2019-06-25 19:53 ` Thomas Gleixner
2019-06-25 20:27 ` Nathan Chancellor
2019-06-25 20:37 ` Nick Desaulniers
2019-06-25 21:47 ` Thomas Gleixner
2019-06-26 5:10 ` Nathan Chancellor
2019-06-26 15:18 ` Thomas Gleixner
2019-06-26 19:00 ` Nathan Chancellor
2019-06-26 19:46 ` Thomas Gleixner
2019-06-26 20:03 ` Nathan Chancellor
2019-06-26 9:24 ` Peter Zijlstra
2019-06-26 9:55 ` Peter Zijlstra
2019-06-26 22:23 ` Nick Desaulniers
2019-06-27 7:35 ` Peter Zijlstra
2019-06-26 10:43 ` Peter Zijlstra
2019-06-26 22:15 ` Nick Desaulniers [this message]
2019-06-27 7:16 ` Peter Zijlstra
2019-06-26 16:30 ` Peter Zijlstra
2019-06-26 22:33 ` Nick Desaulniers
2019-06-26 23:11 ` Thomas Gleixner
2019-06-27 7:11 ` Peter Zijlstra
2019-06-25 23:46 ` Arnaldo Carvalho de Melo
2019-06-26 5:14 ` Nathan Chancellor
2019-06-25 20:09 ` Kees Cook
2019-06-26 8:06 ` Peter Zijlstra
2019-06-25 7:15 ` Peter Zijlstra
2019-07-25 16:27 ` [tip:perf/urgent] " tip-bot for Gustavo A. R. Silva
2019-07-25 17:06 ` Borislav Petkov
2019-07-25 17:35 ` Peter Zijlstra
2019-07-25 23:18 ` Joe Perches
2019-07-25 23:28 ` Kees Cook
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