From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id D412FC48BD7 for ; Thu, 27 Jun 2019 07:16:25 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A633C20828 for ; Thu, 27 Jun 2019 07:16:25 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="dyamLpzz" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726506AbfF0HQY (ORCPT ); Thu, 27 Jun 2019 03:16:24 -0400 Received: from merlin.infradead.org ([205.233.59.134]:54898 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725787AbfF0HQX (ORCPT ); Thu, 27 Jun 2019 03:16:23 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=merlin.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=Adl0vr62TYYsl73Le7yrJGhjQV3e374BPJaykChMxu0=; b=dyamLpzz1keSN5aTYUNqAD0Ow e4n10Dl2RmO0cDmpL1WgzWoDtGx9fZhVhp/VE4pB4+KyLm6GSLi6nKfDvMq+KX/o9SDT+xTKkUOR0 FC9Ck/Kek6z8lCc2ynD3HiGrvJkRj/D0tFQcFkx5Mw03Tq4cUiAEZouTLb81eWltIK5Z49HxIsO+y cwm38yVJiS+vV21yzwhH7BxXe39IjDTFICXvLCJrQqJ6ttUwYhNAwxoGdfHwRcYrJm7xCA9OWwiYP GUErFHTQpq1Ajn8UWwwgiGJFQsulyi1lQs6l7jB7tRV/2Noj7ZIozHA1o//vwbEMC5i29hKMDuPz+ MTRj9QCZQ==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=hirez.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.92 #3 (Red Hat Linux)) id 1hgOdd-0007RE-UE; Thu, 27 Jun 2019 07:16:10 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id A2CFF20A714BA; Thu, 27 Jun 2019 09:16:08 +0200 (CEST) Date: Thu, 27 Jun 2019 09:16:08 +0200 From: Peter Zijlstra To: Nick Desaulniers Cc: Thomas Gleixner , Nathan Chancellor , Kees Cook , Miguel Ojeda , "Gustavo A. R. Silva" , Joe Perches , Ingo Molnar , Arnaldo Carvalho de Melo , Alexander Shishkin , Jiri Olsa , Namhyung Kim , Borislav Petkov , "H. Peter Anvin" , "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" , Kan Liang , linux-kernel , Shawn Landden , clang-built-linux , Josh Poimboeuf Subject: Re: [PATCH] perf/x86/intel: Mark expected switch fall-throughs Message-ID: <20190627071608.GA3402@hirez.programming.kicks-ass.net> References: <3dc75cd4-9a8d-f454-b5fb-64c3e6d1f416@embeddedor.com> <20190625071846.GN3436@hirez.programming.kicks-ass.net> <201906251009.BCB7438@keescook> <20190625180525.GA119831@archlinux-epyc> <20190625202746.GA83499@archlinux-epyc> <20190626092432.GJ3419@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jun 26, 2019 at 03:15:38PM -0700, Nick Desaulniers wrote: > On Wed, Jun 26, 2019 at 2:24 AM Peter Zijlstra 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 > > > 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 > > > 43a: 44 89 35 00 00 00 00 mov %r14d,0x0(%rip) # 441 > > > > > > 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? As I wrote later; the above suggestion is actually wrong :/ > 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? As I wrote in that same later email; the way we influence the actual code-layout is with the __builtin_expect() thing. Let me expand on that in another email. Sadly, I've no clue what so ever about compiler internals, be it GCC or LLVM.