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=-23.3 required=3.0 tests=BAYES_00,DKIMWL_WL_MED, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_IN_DEF_DKIM_WL autolearn=unavailable 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 0CCA6C433ED for ; Fri, 2 Apr 2021 18:07:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D112B61103 for ; Fri, 2 Apr 2021 18:07:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235847AbhDBSH1 (ORCPT ); Fri, 2 Apr 2021 14:07:27 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:36980 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235657AbhDBSHZ (ORCPT ); Fri, 2 Apr 2021 14:07:25 -0400 Received: from mail-lj1-x22d.google.com (mail-lj1-x22d.google.com [IPv6:2a00:1450:4864:20::22d]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E229FC061788 for ; Fri, 2 Apr 2021 11:07:23 -0700 (PDT) Received: by mail-lj1-x22d.google.com with SMTP id y1so6325279ljm.10 for ; Fri, 02 Apr 2021 11:07:23 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=PM3yJgN8KwxpiHp9xCT4k+uyrPB+t4rWGu++QPJE560=; b=B58X00VnP5DFpJ+hL8Sz5d26e4DTVke1e4z5O+a75+KasfQG3lYwT9qRWCl0q/gnrJ Y38HI6ApVd5WAXD/KHTtgTfdlLvR4UJ1F2kYEMFjiijCFZZ3g8w2SgzdJHWkE+fefxJ5 Q1T97IDU5TpcquAzm+FdjkDl9YBJGmELHnkOxIELa+kKpnuSoAakWEgHIPyaDtHHJ8sC 3yoftMjuwHr7lLOp4HL/l4Q6lbBMAjXWYkuV7NvrBuF1S7pK8/RFjBeUszrRMzT1vRVZ t2Awm28ebPD5p/Yb26LbKyVo/QOfMdwOSkuazsdoH7LGSeL7KT9UbMNaQO9B9dCg4H2f yPog== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=PM3yJgN8KwxpiHp9xCT4k+uyrPB+t4rWGu++QPJE560=; b=lPPcLi5P3cCjeHiBE8VF9GzSs4WRi6w5BDo8U2wbY7eh8V9DX1iVjqrdE6z72kfORn zFpRJWVAfvFhZw1zXYqrB77uT0wNZIdSlyCyEWMG9INYa2QabWv6dhqeAqsNs6zejoY3 fHZ/LRC6HwcJYDl2ICmjCj9rdqj9KG99N50UUIAfxHZmk+sY8Lop6Yp7rM5cF1Kxt3fW /WE8SauvJ679FGH8iGKXGnV+15Hndb46p5lnBCLeQLBmyem1Hjqco9RXYX4KB64ARCQG LbOlIiKhYXnCKMKM21i6D+3+hj9oDOkfZ3JLIPyovA9w5WQ4WIpknxPIsdnutW+pFZ8k Mt/A== X-Gm-Message-State: AOAM531RmruIff7WZt3rpymx97k4RSIWJxE+Ws5RbJF7Gyv0Bev6Jpxw PMIjY1wqFIy5hl0uYXstXyivagP+jSAjsDsdsB+MQA== X-Google-Smtp-Source: ABdhPJyqiPydlwE2rpVAqQ8yNauCEz83cMMFmNvzIzt55MPhWfwMmB22LgPejZ7fmWLvnphIAttlAgmJ939SrbYBJFM= X-Received: by 2002:a2e:5716:: with SMTP id l22mr8865244ljb.244.1617386841985; Fri, 02 Apr 2021 11:07:21 -0700 (PDT) MIME-Version: 1.0 References: <20210401232723.3571287-1-yhs@fb.com> In-Reply-To: <20210401232723.3571287-1-yhs@fb.com> From: Nick Desaulniers Date: Fri, 2 Apr 2021 11:07:10 -0700 Message-ID: Subject: Re: [PATCH kbuild v4] kbuild: add an elfnote for whether vmlinux is built with lto To: Yonghong Song Cc: Linux Kbuild mailing list , Alexei Starovoitov , Arnaldo Carvalho de Melo , bpf , kernel-team@fb.com, Masahiro Yamada , Michal Marek , Bill Wendling , clang-built-linux Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org On Thu, Apr 1, 2021 at 4:27 PM Yonghong Song wrote: > > 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 a new type LINUX_ELFNOTE_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/ > > Suggested-by: Arnaldo Carvalho de Melo > Signed-off-by: Yonghong Song LGTM thanks Yonghong! Reviewed-by: Nick Desaulniers > --- > include/linux/elfnote-lto.h | 14 ++++++++++++++ > init/version.c | 2 ++ > scripts/mod/modpost.c | 2 ++ > 3 files changed, 18 insertions(+) > create mode 100644 include/linux/elfnote-lto.h > > Changelogs: > v3 -> v4: > . put new lto note in its own header file similar to > build-salt.h. (Nick) > v2 -> v3: > . abandoned the approach of adding -grecord-gcc-switches, > instead create a note to indicate whether it is a lto build > or not. The note definition is in compiler.h. (Arnaldo) > v1 -> v2: > . limited to add -grecord-gcc-switches for LTO_CLANG > instead of all clang build > > diff --git a/include/linux/elfnote-lto.h b/include/linux/elfnote-lto.h > new file mode 100644 > index 000000000000..d4635a3ecc4f > --- /dev/null > +++ b/include/linux/elfnote-lto.h > @@ -0,0 +1,14 @@ > +#ifndef __ELFNOTE_LTO_H > +#define __ELFNOTE_LTO_H > + > +#include > + > +#define LINUX_ELFNOTE_LTO_INFO 0x101 > + > +#ifdef CONFIG_LTO > +#define BUILD_LTO_INFO ELFNOTE32("Linux", LINUX_ELFNOTE_LTO_INFO, 1) > +#else > +#define BUILD_LTO_INFO ELFNOTE32("Linux", LINUX_ELFNOTE_LTO_INFO, 0) > +#endif > + > +#endif /* __ELFNOTE_LTO_H */ > diff --git a/init/version.c b/init/version.c > index 92afc782b043..1a356f5493e8 100644 > --- a/init/version.c > +++ b/init/version.c > @@ -9,6 +9,7 @@ > > #include > #include > +#include > #include > #include > #include > @@ -45,3 +46,4 @@ const char linux_proc_banner[] = > " (" LINUX_COMPILER ") %s\n"; > > BUILD_SALT; > +BUILD_LTO_INFO; > diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c > index 24725e50c7b4..98fb2bb024db 100644 > --- a/scripts/mod/modpost.c > +++ b/scripts/mod/modpost.c > @@ -2191,10 +2191,12 @@ static void add_header(struct buffer *b, struct module *mod) > */ > buf_printf(b, "#define INCLUDE_VERMAGIC\n"); > buf_printf(b, "#include \n"); > + buf_printf(b, "#include \n"); > buf_printf(b, "#include \n"); > buf_printf(b, "#include \n"); > buf_printf(b, "\n"); > buf_printf(b, "BUILD_SALT;\n"); > + buf_printf(b, "BUILD_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"); > -- > 2.30.2 > -- Thanks, ~Nick Desaulniers