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=-12.7 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 65F66C433B4 for ; Fri, 2 Apr 2021 18:31:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2802B61167 for ; Fri, 2 Apr 2021 18:31:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236255AbhDBSby (ORCPT ); Fri, 2 Apr 2021 14:31:54 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42318 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230092AbhDBSbx (ORCPT ); Fri, 2 Apr 2021 14:31:53 -0400 Received: from mail-il1-x12f.google.com (mail-il1-x12f.google.com [IPv6:2607:f8b0:4864:20::12f]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 87401C0613E6; Fri, 2 Apr 2021 11:31:52 -0700 (PDT) Received: by mail-il1-x12f.google.com with SMTP id t14so5294635ilu.3; Fri, 02 Apr 2021 11:31:52 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:references:in-reply-to:reply-to:from:date:message-id :subject:to:cc; bh=BFBhTWbA18iJBRtXWFRSWL3NbbovxojieVF5twnWirs=; b=Tc5Kq3mPbIujxRM/yG4dLepyGe3i0xnWqqzhk0AX9o3ilPmgjdIqJgnqpM3YTpWGw3 5QNeJsKtJ9G6JGR33u+dP5fkuVE/OpuixUnS6K/1Fa9tF0g/imuFkfKFqa9uuU/6hEFZ D5M9kJFAt9qEDOh9S3Bb6RWsgJgd7UydPZ3YDEUKC/gPAsPfK75tfukc1WqL7tAEqOAQ /TaPTZnPToIQU0ve6z18nvcDSQUx9ClWQHo6S8boDEnkn9HkLeJll3U0m7kwIKUfIH/j 07xTY1Ic2AdUYrFgHehO935S2I9Dnzfkyg965q7eWrCFyHkorM2OmJycEY8nTIPixPql /mzA== 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:reply-to :from:date:message-id:subject:to:cc; bh=BFBhTWbA18iJBRtXWFRSWL3NbbovxojieVF5twnWirs=; b=S+WCwtL6dbek+jLzQjJBwZe5ukgv28BPIn+ecqKeNxYH5Kj6M0uL0MfDubCxId6b7h RvrDx795l3wxuKuuxM+Ua+N0VmQLMdrmIfBKXHf6FmjavWatCV81GPqvcBZq361xiK1M SPv9dnmPPoc+z/yWRe29LFy8LkVbliRdledgmCqnNrohxRgmJsxw4YLRvb40iBlgWKcf 4KP9sGxgwRhChOVp7dtMvaun1A3LqCihlEHnVmUYXLTg9tlh54Fdx9wsTMNVeRJmUoX/ 46rJu07u1AyqjXAwftR/MxRRrQC3AurJXolq+YkA2Rg8DeuHMAlmzdmuKuydjc3J05Tr Is8Q== X-Gm-Message-State: AOAM5313alhR40BZ6TsGaFfDpHBVe9iHgRRGYEw0wBslC3lWvoW2lSbx E5H9SG/dtHdrM1N5TW3sf/IxIrm2I+Plumbs1Y0= X-Google-Smtp-Source: ABdhPJyUdEuM58vXDHGVrWHdUM1vJXdEDdizX87W+x8pFH0Z7996KSbwX45KJxBPjWNVMtOpctxKXIYExrqnCJkXe38= X-Received: by 2002:a92:444e:: with SMTP id a14mr11798341ilm.215.1617388311933; Fri, 02 Apr 2021 11:31:51 -0700 (PDT) MIME-Version: 1.0 References: <20210401232723.3571287-1-yhs@fb.com> In-Reply-To: Reply-To: sedat.dilek@gmail.com From: Sedat Dilek Date: Fri, 2 Apr 2021 20:31:15 +0200 Message-ID: Subject: Re: [PATCH kbuild v4] kbuild: add an elfnote for whether vmlinux is built with lto To: Nick Desaulniers Cc: Yonghong Song , 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 Fri, Apr 2, 2021 at 8:07 PM 'Nick Desaulniers' via Clang Built Linux wrote: > > 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) That is a bit smarter (and smaller) than v3. Queued up and building a new clang-lto kernel... Will report later. - Sedat - > > 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 > > -- > You received this message because you are subscribed to the Google Groups "Clang Built Linux" group. > To unsubscribe from this group and stop receiving emails from it, send an email to clang-built-linux+unsubscribe@googlegroups.com. > To view this discussion on the web visit https://groups.google.com/d/msgid/clang-built-linux/CAKwvOdmX8d3XTzJFk5rN_PnOQYJ8bXMrh8DrhzqN%3DUBNdQiO3g%40mail.gmail.com.