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=-18.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_GIT 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 A94BCC433B4 for ; Thu, 1 Apr 2021 01:25:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6D25B61076 for ; Thu, 1 Apr 2021 01:25:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229497AbhDABZG (ORCPT ); Wed, 31 Mar 2021 21:25:06 -0400 Received: from mx0b-00082601.pphosted.com ([67.231.153.30]:19800 "EHLO mx0b-00082601.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233173AbhDABYf (ORCPT ); Wed, 31 Mar 2021 21:24:35 -0400 Received: from pps.filterd (m0148460.ppops.net [127.0.0.1]) by mx0a-00082601.pphosted.com (8.16.0.43/8.16.0.43) with SMTP id 1311FSrX030963 for ; Wed, 31 Mar 2021 18:24:34 -0700 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=fb.com; h=from : to : cc : subject : date : message-id : in-reply-to : references : content-type : content-transfer-encoding : mime-version; s=facebook; bh=J71BE9RoPiHh1SugZt8jJTFl8tcrLgSSpv9QcXnD2cw=; b=AthmvRUZIipNzMMRBBH9Yei7uAgWcDPj/6nW3l+8U0JgjAV4nMvEaCL9dhJDmBGHk6xX TlUeXq2+B0w/o/o5MRT8UOQ8rb3KnlYAtb0C7WFR9gKKmEAka+TcBD5CkHg5isTIK8rC ylEnFKXjrwfrGiOkM4hyMghvHVtHdYRkxxk= Received: from mail.thefacebook.com ([163.114.132.120]) by mx0a-00082601.pphosted.com with ESMTP id 37n2810kkq-3 (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128 verify=NOT) for ; Wed, 31 Mar 2021 18:24:34 -0700 Received: from intmgw001.38.frc1.facebook.com (2620:10d:c085:208::f) by mail.thefacebook.com (2620:10d:c085:21d::7) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2176.2; Wed, 31 Mar 2021 18:24:33 -0700 Received: by devbig003.ftw2.facebook.com (Postfix, from userid 128203) id AB83AEB0CE5; Wed, 31 Mar 2021 18:24:17 -0700 (PDT) From: Yonghong Song To: CC: Alexei Starovoitov , Arnaldo Carvalho de Melo , , , Masahiro Yamada , Michal Marek , Nick Desaulniers Subject: [PATCH kbuild v3 2/2] kbuild: add an elfnote with type BUILD_COMPILER_LTO_INFO Date: Wed, 31 Mar 2021 18:24:17 -0700 Message-ID: <20210401012417.1802681-1-yhs@fb.com> X-Mailer: git-send-email 2.30.2 In-Reply-To: <20210401012406.1800957-1-yhs@fb.com> References: <20210401012406.1800957-1-yhs@fb.com> X-FB-Internal: Safe Content-Type: text/plain X-Proofpoint-GUID: mBhDopQCJZt8aq_XImBVpMmM4XY532fo X-Proofpoint-ORIG-GUID: mBhDopQCJZt8aq_XImBVpMmM4XY532fo Content-Transfer-Encoding: quoted-printable X-Proofpoint-UnRewURL: 0 URL was un-rewritten MIME-Version: 1.0 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.369,18.0.761 definitions=2021-03-31_11:2021-03-31,2021-03-31 signatures=0 X-Proofpoint-Spam-Details: rule=fb_default_notspam policy=fb_default score=0 clxscore=1015 bulkscore=0 impostorscore=0 spamscore=0 mlxlogscore=999 priorityscore=1501 mlxscore=0 suspectscore=0 phishscore=0 lowpriorityscore=0 adultscore=0 malwarescore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2103310000 definitions=main-2104010006 X-FB-Internal: deliver Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org Currently, clang LTO built vmlinux won't work with pahole. LTO introduced cross-cu dwarf tag references and broke current pahole model which handles one cu as a time. The solution is to merge all cu's as one pahole cu as in [1]. We would like to do this merging only if cross-cu dwarf references happens. The LTO build mode is a pretty good indication for that. In earlier version of this patch ([2]), clang flag -grecord-gcc-switches is proposed to add to compilation flags so pahole could detect "-flto" and then merging cu's. This will increate the binary size of 1% without LTO though. Arnaldo suggested to use a note to indicate the vmlinux is built with LTO. Such a cheap way to get whether the vmlinux is built with LTO or not helps pahole but is also useful for tracing as LTO may inline/delete/demote global functions, promote static functions, etc. So this patch added an elfnote with type BUILD_COMPILER_LTO_INFO. The owner of the note is "Linux". With gcc 8.4.1 and clang trunk, without LTO, I got $ readelf -n vmlinux Displaying notes found in: .notes Owner Data size Description ... Linux 0x00000004 func description data: 00 00 00 00 ... With "readelf -x ".notes" vmlinux", I can verify the above "func" with type code 0x101. With clang thin-LTO, I got the same as above except the following: description data: 01 00 00 00 which indicates the vmlinux is built with LTO. [1] https://lore.kernel.org/bpf/20210325065316.3121287-1-yhs@fb.com/ [2] https://lore.kernel.org/bpf/20210331001623.2778934-1-yhs@fb.com/ Signed-off-by: Yonghong Song --- include/linux/compiler.h | 8 ++++++++ include/linux/elfnote.h | 1 + init/version.c | 2 ++ scripts/mod/modpost.c | 1 + 4 files changed, 12 insertions(+) diff --git a/include/linux/compiler.h b/include/linux/compiler.h index df5b405e6305..b92930877277 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -245,6 +245,14 @@ static inline void *offset_to_ptr(const int *off) */ #define prevent_tail_call_optimization() mb() =20 +#include + +#ifdef CONFIG_LTO +#define BUILD_COMPILER_LTO_INFO ELFNOTE32("Linux", LINUX_ELFNOTE_BUILD_LTO= , 1) +#else +#define BUILD_COMPILER_LTO_INFO ELFNOTE32("Linux", LINUX_ELFNOTE_BUILD_LTO= , 0) +#endif + #include =20 #endif /* __LINUX_COMPILER_H */ diff --git a/include/linux/elfnote.h b/include/linux/elfnote.h index 04af7ac40b1a..f5ec2b50ab7d 100644 --- a/include/linux/elfnote.h +++ b/include/linux/elfnote.h @@ -100,5 +100,6 @@ * The types for "Linux" owned notes. */ #define LINUX_ELFNOTE_BUILD_SALT 0x100 +#define LINUX_ELFNOTE_BUILD_LTO 0x101 =20 #endif /* _LINUX_ELFNOTE_H */ diff --git a/init/version.c b/init/version.c index 92afc782b043..a4f74b06fe78 100644 --- a/init/version.c +++ b/init/version.c @@ -9,6 +9,7 @@ =20 #include #include +#include #include #include #include @@ -45,3 +46,4 @@ const char linux_proc_banner[] =3D " (" LINUX_COMPILER ") %s\n"; =20 BUILD_SALT; +BUILD_COMPILER_LTO_INFO; diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c index 24725e50c7b4..713c0d5d5525 100644 --- a/scripts/mod/modpost.c +++ b/scripts/mod/modpost.c @@ -2195,6 +2195,7 @@ static void add_header(struct buffer *b, struct modul= e *mod) buf_printf(b, "#include \n"); buf_printf(b, "\n"); buf_printf(b, "BUILD_SALT;\n"); + buf_printf(b, "BUILD_COMPILER_LTO_INFO;\n"); buf_printf(b, "\n"); buf_printf(b, "MODULE_INFO(vermagic, VERMAGIC_STRING);\n"); buf_printf(b, "MODULE_INFO(name, KBUILD_MODNAME);\n"); --=20 2.30.2