From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753571AbeDTSGv (ORCPT ); Fri, 20 Apr 2018 14:06:51 -0400 Received: from mx0a-00082601.pphosted.com ([67.231.145.42]:48200 "EHLO mx0a-00082601.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752749AbeDTSGs (ORCPT ); Fri, 20 Apr 2018 14:06:48 -0400 Subject: Re: [PATCH v2] x86/cpufeature: guard asm_volatile_goto usage with NO_BPF_WORKAROUND To: Peter Zijlstra CC: , , , , , References: <20180415042738.1788215-1-yhs@fb.com> <20180420081936.GD4064@hirez.programming.kicks-ass.net> From: Yonghong Song Message-ID: Date: Fri, 20 Apr 2018 11:06:03 -0700 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.13; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 MIME-Version: 1.0 In-Reply-To: <20180420081936.GD4064@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset="utf-8"; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [2620:10d:c090:200::5:bd22] X-ClientProxiedBy: CO1PR15CA0112.namprd15.prod.outlook.com (2603:10b6:101:21::32) To CO2PR15MB0073.namprd15.prod.outlook.com (2a01:111:e400:5068::23) X-MS-PublicTrafficType: Email X-Microsoft-Antispam: UriScan:;BCL:0;PCL:0;RULEID:(7020095)(4652020)(5600026)(2017052603328)(7153060)(7193020);SRVR:CO2PR15MB0073; 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NO_BPF_WORKAROUND is set >> at toplevel Makefile when compiler supports asm-goto. That is, >> if the compiler supports asm-goto, the kernel build will use >> asm-goto version of static_cpu_has(). > > Hurm, so adding __BPF__ for BPF compiles isn't an option? It seems to me > having a CPP flag to identify BPF compile context might be useful in > general. With "clang -target bpf", we already have __BPF__ defined. For tracing, esp. ptrace.h is included, "clang -target " where "-target " can be omitted, is typically used. The reason is the native architecture header files typically include a lot of various asm related stuff where "-target bpf" cannot really handle. We relay on native clang to flush out all these asm constructs and only bpf program needed stuff survives reach to backend compiler. The backend compiler, llc, will have option "-march=bpf" to do right thing to generate bpf byte codes. So the patch is really a workaround for "clang -target x86_64" with intention of using "llc -march=bpf" later on.