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=-10.0 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED 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 8DDA6C433E2 for ; Mon, 7 Sep 2020 15:31:18 +0000 (UTC) Received: from mother.openwall.net (mother.openwall.net [195.42.179.200]) by mail.kernel.org (Postfix) with SMTP id 993F62065E for ; Mon, 7 Sep 2020 15:31:17 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=nifty.com header.i=@nifty.com header.b="Yna4t3lc" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 993F62065E Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=kernel-hardening-return-19797-kernel-hardening=archiver.kernel.org@lists.openwall.com Received: (qmail 21622 invoked by uid 550); 7 Sep 2020 15:31:11 -0000 Mailing-List: contact kernel-hardening-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Received: (qmail 21581 invoked from network); 7 Sep 2020 15:31:09 -0000 DKIM-Filter: OpenDKIM Filter v2.10.3 conssluserg-05.nifty.com 087FUq8K012338 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=nifty.com; s=dec2015msa; t=1599492652; bh=LaAhPv7xf0fzyzAeNRC0oSM953AkWxA0K40O3j1oCJU=; h=References:In-Reply-To:From:Date:Subject:To:Cc:From; b=Yna4t3lcBn3ihm+SsMQJlSW4XLVip7NZS99xKV6wjegTr21Y5C+6/U7bVUwTlcnB3 dAmf1vUY7jje4VMpEF5B9l3HdyZldxrn5AXwW+dJOhdygQuD1gpo1B7WlJLi4042rB wpL8A6q4haTi7v3IdY0Yz7k421Qa5DJ7HjSymgZP9UOkdS5L+VUN1HdPhWxO0fbpqq sqd7DXuvbF/TUg/5jmbJC5BH0SP6sb2Rjrmz2EPA8uMz+CX6itEvKBHdMkyZwmZ2lm UGnHdDg5OmpNRGnyEq7vt5ZnO4AVkuczrZKm7pug4SY+XcKT7YxjN66TWcAmcWHrm9 GYuGjQJXsXUnw== X-Nifty-SrcIP: [209.85.216.46] X-Gm-Message-State: AOAM532nqTPadnxInhEwvB9Wy5L2fo043VNKtO+epVrQ5vKAvwL8/ogF OHqFC+rLk5tAdfiqILV+3kMfZsieKvUtQFwYUT0= X-Google-Smtp-Source: ABdhPJyEfw+RTulZSFTnkTeBt2WLDcgnnJBCvvqMaD56gohQ35yMLXLHW+vK/ERS8QsJRkz5skzAxWakdk+WfgH2rog= X-Received: by 2002:a17:90b:208:: with SMTP id fy8mr4808973pjb.153.1599492651374; Mon, 07 Sep 2020 08:30:51 -0700 (PDT) MIME-Version: 1.0 References: <20200624203200.78870-1-samitolvanen@google.com> <20200903203053.3411268-1-samitolvanen@google.com> <20200903203053.3411268-10-samitolvanen@google.com> In-Reply-To: <20200903203053.3411268-10-samitolvanen@google.com> From: Masahiro Yamada Date: Tue, 8 Sep 2020 00:30:14 +0900 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v2 09/28] kbuild: add support for Clang LTO To: Sami Tolvanen Cc: Will Deacon , Peter Zijlstra , Steven Rostedt , Greg Kroah-Hartman , "Paul E. McKenney" , Kees Cook , Nick Desaulniers , clang-built-linux , Kernel Hardening , linux-arch , linux-arm-kernel , Linux Kbuild mailing list , Linux Kernel Mailing List , linux-pci@vger.kernel.org, X86 ML Content-Type: text/plain; charset="UTF-8" On Fri, Sep 4, 2020 at 5:31 AM Sami Tolvanen wrote: > > This change adds build system support for Clang's Link Time > Optimization (LTO). With -flto, instead of ELF object files, Clang > produces LLVM bitcode, which is compiled into native code at link > time, allowing the final binary to be optimized globally. For more > details, see: > > https://llvm.org/docs/LinkTimeOptimization.html > > The Kconfig option CONFIG_LTO_CLANG is implemented as a choice, > which defaults to LTO being disabled. To use LTO, the architecture > must select ARCH_SUPPORTS_LTO_CLANG and support: > > - compiling with Clang, > - compiling inline assembly with Clang's integrated assembler, > - and linking with LLD. > > While using full LTO results in the best runtime performance, the > compilation is not scalable in time or memory. CONFIG_THINLTO > enables ThinLTO, which allows parallel optimization and faster > incremental builds. ThinLTO is used by default if the architecture > also selects ARCH_SUPPORTS_THINLTO: > > https://clang.llvm.org/docs/ThinLTO.html > > To enable LTO, LLVM tools must be used to handle bitcode files. The > easiest way is to pass the LLVM=1 option to make: > > $ make LLVM=1 defconfig > $ scripts/config -e LTO_CLANG > $ make LLVM=1 > > Alternatively, at least the following LLVM tools must be used: > > CC=clang LD=ld.lld AR=llvm-ar NM=llvm-nm > > To prepare for LTO support with other compilers, common parts are > gated behind the CONFIG_LTO option, and LTO can be disabled for > specific files by filtering out CC_FLAGS_LTO. > > Note that support for DYNAMIC_FTRACE and MODVERSIONS are added in > follow-up patches. > > Signed-off-by: Sami Tolvanen > --- > Makefile | 18 +++++++- > arch/Kconfig | 68 +++++++++++++++++++++++++++++++ > include/asm-generic/vmlinux.lds.h | 11 +++-- > scripts/Makefile.build | 9 +++- > scripts/Makefile.modfinal | 9 +++- > scripts/Makefile.modpost | 24 ++++++++++- > scripts/link-vmlinux.sh | 32 +++++++++++---- > 7 files changed, 154 insertions(+), 17 deletions(-) > #define TEXT_MAIN .text > diff --git a/scripts/Makefile.build b/scripts/Makefile.build > index 6ecf30c70ced..a5f4b5d407e6 100644 > --- a/scripts/Makefile.build > +++ b/scripts/Makefile.build > @@ -111,7 +111,7 @@ endif > # --------------------------------------------------------------------------- > > quiet_cmd_cc_s_c = CC $(quiet_modtag) $@ > - cmd_cc_s_c = $(CC) $(filter-out $(DEBUG_CFLAGS), $(c_flags)) $(DISABLE_LTO) -fverbose-asm -S -o $@ $< > + cmd_cc_s_c = $(CC) $(filter-out $(DEBUG_CFLAGS) $(CC_FLAGS_LTO), $(c_flags)) -fverbose-asm -S -o $@ $< > > $(obj)/%.s: $(src)/%.c FORCE > $(call if_changed_dep,cc_s_c) > @@ -428,8 +428,15 @@ $(obj)/lib.a: $(lib-y) FORCE > # Do not replace $(filter %.o,^) with $(real-prereqs). When a single object > # module is turned into a multi object module, $^ will contain header file > # dependencies recorded in the .*.cmd file. > +ifdef CONFIG_LTO_CLANG > +quiet_cmd_link_multi-m = AR [M] $@ > +cmd_link_multi-m = \ > + rm -f $@; \ > + $(AR) rcsTP$(KBUILD_ARFLAGS) $@ $(filter %.o,$^) KBUILD_ARFLAGS no longer exists in the mainline. (commit 13dc8c029cabf52ba95f60c56eb104d4d95d5889) > +else > quiet_cmd_link_multi-m = LD [M] $@ > cmd_link_multi-m = $(LD) $(ld_flags) -r -o $@ $(filter %.o,$^) > +endif > > $(multi-used-m): FORCE > $(call if_changed,link_multi-m) > diff --git a/scripts/Makefile.modfinal b/scripts/Makefile.modfinal > index 411c1e600e7d..1005b147abd0 100644 > --- a/scripts/Makefile.modfinal > +++ b/scripts/Makefile.modfinal > @@ -6,6 +6,7 @@ > PHONY := __modfinal > __modfinal: > > +include $(objtree)/include/config/auto.conf > include $(srctree)/scripts/Kbuild.include > > # for c_flags > @@ -29,6 +30,12 @@ quiet_cmd_cc_o_c = CC [M] $@ > > ARCH_POSTLINK := $(wildcard $(srctree)/arch/$(SRCARCH)/Makefile.postlink) > > +ifdef CONFIG_LTO_CLANG > +# With CONFIG_LTO_CLANG, reuse the object file we compiled for modpost to > +# avoid a second slow LTO link > +prelink-ext := .lto > +endif > + > quiet_cmd_ld_ko_o = LD [M] $@ > cmd_ld_ko_o = \ > $(LD) -r $(KBUILD_LDFLAGS) \ > @@ -37,7 +44,7 @@ quiet_cmd_ld_ko_o = LD [M] $@ > -o $@ $(filter %.o, $^); \ > $(if $(ARCH_POSTLINK), $(MAKE) -f $(ARCH_POSTLINK) $@, true) > > -$(modules): %.ko: %.o %.mod.o $(KBUILD_LDS_MODULE) FORCE > +$(modules): %.ko: %$(prelink-ext).o %.mod.o $(KBUILD_LDS_MODULE) FORCE > +$(call if_changed,ld_ko_o) > > targets += $(modules) $(modules:.ko=.mod.o) > diff --git a/scripts/Makefile.modpost b/scripts/Makefile.modpost > index f54b6ac37ac2..a70f1f7da6aa 100644 > --- a/scripts/Makefile.modpost > +++ b/scripts/Makefile.modpost > @@ -102,12 +102,32 @@ $(input-symdump): > @echo >&2 'WARNING: Symbol version dump "$@" is missing.' > @echo >&2 ' Modules may not have dependencies or modversions.' > > +ifdef CONFIG_LTO_CLANG > +# With CONFIG_LTO_CLANG, .o files might be LLVM bitcode, or, .o files might be even thin archives. For example, $ file net/ipv6/netfilter/nf_defrag_ipv6.o net/ipv6/netfilter/nf_defrag_ipv6.o: thin archive with 6 symbol entries Now we have 3 possibilities for .o files: - ELF (real .o) - LLVM bitcode (.bc) - Thin archive (.a) Let me discuss how to proceed with this... -- Best Regards Masahiro Yamada