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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 79219C43219 for ; Mon, 9 May 2022 23:13:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232311AbiEIXRu (ORCPT ); Mon, 9 May 2022 19:17:50 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54670 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232281AbiEIXRn (ORCPT ); Mon, 9 May 2022 19:17:43 -0400 Received: from mail-yw1-x112d.google.com (mail-yw1-x112d.google.com [IPv6:2607:f8b0:4864:20::112d]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 937DA2A3BEE for ; Mon, 9 May 2022 16:13:48 -0700 (PDT) Received: by mail-yw1-x112d.google.com with SMTP id 00721157ae682-2f7d19cac0bso161379217b3.13 for ; Mon, 09 May 2022 16:13:48 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=oazinpgomaqUG0jSYMm8ubizFPTzpD9YluLXTH1Uf+8=; b=f6fWCk6p41OZaY/aYptJIOS/H7gQoR2LQU/vFrk5Gn4NSNwuGdiZDiDStk7AbG3yZD IKBXb4w884/50UAgGFMYKZ1TW7UWPSrJsR6aU/0xVkQloWI1gv9dqJSDbnU+7ZRlaq+H 5q6FzLE9Jch5W51SjYRwMzmvNSaAVh59pmK1la7G6Ce7QS8+phGNXM1poUTxBJEghkkL rkNzGIHiR6cg7h2j0E68I08LETn6PDCH3Bk2j2CtnK3mFxMH+qkZYlQ5Mbk0pgP96vbx k86u84TYrd/iZlrt9edUpHqGeZQt6sH9DbxxljCpeCMl8DqbX8Q9LTMC9vVENf9lnOH2 8xnA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=oazinpgomaqUG0jSYMm8ubizFPTzpD9YluLXTH1Uf+8=; b=ggCRmKixQk5YABhd+fkdT4XtH5h8w2d/9l5/S3LcvDMFsLNljr/SpiX+0trNk+f3qy FGGgVHZwMj9/Sp1D7SxMQ3SeT2vKBCZj+GomzE5ymcCa5GBi0W1X8+Q0XVRPG3wvUqRW uCOWJOuZpeUbrTDhrM5vPGPV5QeDSGA6Oem15ZYG+YwoLoWGtlFAOmKdzXqAbFE8khAH PDji3kK/fEgJRSfsRPloweFoep6mvQ6HHwDZrsd1mTnTZBLYVN3qSzLwIV6fCg+1pr/R 1pMxesM5Si5ZYJWv7F2REVMJ2+tR6CypCGqeiWaBPAvqRB7g6PYpE7zpIg2vaAwELqu4 Dt1A== X-Gm-Message-State: AOAM532vK4/0xJLBShHp4Iuxf6xBeKSJlyV60JigxH+JhYBkP3ME0k+k PrRuIHmC18u9wIj4kR+jPl/AsLruQAsdetEgVB2sYg== X-Google-Smtp-Source: ABdhPJxssHqsbnbsTj81I2dSgjtFlZace94GadSqyvpd5jqz1pk64Y4lHyhr+1TXl7QDxSK2F5OuNrrpLo+K4Kc2AN8= X-Received: by 2002:a81:1108:0:b0:2f7:bf11:3fb9 with SMTP id 8-20020a811108000000b002f7bf113fb9mr17011668ywr.305.1652138027673; Mon, 09 May 2022 16:13:47 -0700 (PDT) MIME-Version: 1.0 References: <20220508190631.2386038-1-masahiroy@kernel.org> <20220508190631.2386038-10-masahiroy@kernel.org> In-Reply-To: <20220508190631.2386038-10-masahiroy@kernel.org> From: Sami Tolvanen Date: Mon, 9 May 2022 16:13:11 -0700 Message-ID: Subject: Re: [PATCH v4 09/14] kbuild: do not create *.prelink.o for Clang LTO or IBT To: Masahiro Yamada Cc: linux-kbuild , LKML , Nathan Chancellor , Nick Desaulniers , Nicolas Schier , Peter Zijlstra , linux-modules@vger.kernel.org, linux-s390@vger.kernel.org, linuxppc-dev , clang-built-linux , Ard Biesheuvel Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, May 8, 2022 at 12:10 PM Masahiro Yamada wrote: > > When CONFIG_LTO_CLANG=y, additional intermediate *.prelink.o is created > for each module. Also, objtool is postponed until LLVM bitcode is > converted to ELF. > > CONFIG_X86_KERNEL_IBT works in a similar way to postpone objtool until > objects are merged together. > > This commit stops generating *.prelink.o, so the build flow will look > the same with/without LTO. > > The following figures show how the LTO build currently works, and > how this commit is changing it. > > Current build flow > ================== > > [1] single-object module > > $(LD) > $(CC) +objtool $(LD) > foo.c --------------------> foo.o -----> foo.prelink.o -----> foo.ko > (LLVM bitcode) (ELF) | > | > foo.mod.o --/ > > [2] multi-object module > $(LD) > $(CC) $(AR) +objtool $(LD) > foo1.c -----> foo1.o -----> foo.o -----> foo.prelink.o -----> foo.ko > | (archive) (ELF) | > foo2.c -----> foo2.o --/ | > (LLVM bitcode) foo.mod.o --/ > > One confusion is foo.o in multi-object module is an archive despite of > its suffix. > > New build flow > ============== > > [1] single-object module > > Since there is only one object, we do not need to have the LLVM > bitcode stage. Use $(CC)+$(LD) to generate an ELF object in one > build rule. When LTO is disabled, $(LD) is unneeded because $(CC) > produces an ELF object. > > $(CC)+$(LD)+objtool $(LD) > foo.c ------------------------> foo.o -------> foo.ko > (ELF) | > | > foo.mod.o --/ > > [2] multi-object module > > Previously, $(AR) was used to combine LLVM bitcode into an archive, > but there was no technical reason to do so. > This commit just uses $(LD) to combine and convert them into a single > ELF object. > > $(LD) > $(CC) +objtool $(LD) > foo1.c -------> foo1.o -------> foo.o -------> foo.ko > | (ELF) | > foo2.c -------> foo2.o ---/ | > (LLVM bitcode) foo.mod.o --/ > > Signed-off-by: Masahiro Yamada > Reviewed-by: Nicolas Schier > Tested-by: Nathan Chancellor Looks good, thanks for cleaning this up! Reviewed-by: Sami Tolvanen Sami