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=-7.0 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no 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 AD234C4361A for ; Thu, 3 Dec 2020 18:24:19 +0000 (UTC) Received: from merlin.infradead.org (merlin.infradead.org [205.233.59.134]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 43E5B207D0 for ; Thu, 3 Dec 2020 18:24:19 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 43E5B207D0 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=merlin.20170209; h=Sender:Content-Transfer-Encoding: Content-Type:Cc:List-Subscribe:List-Help:List-Post:List-Archive: List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References:Message-ID: Subject:To:From:Date:Reply-To:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=HOmo4KmRxEYzPeTpFbAVL7mjXSV59AEs9vIPWV63j04=; b=MEedjq0aLS4mD0AjcafIikoiq XHCWR6Gmqkzom4Il0qb/tajSuGPCVDAWDQ3rO0MMNZr9X6L0P/oYkdmhCQBnhOonhFdyqbhmDnZTI IdQhTcbdK/Uhm1HfBEYPe4znM6MG0DXz3cY8zZwNWj1J7GvvOJtC5RLHIEVjhspfjOt2M/FMshPE9 f7K8jtgjvX7xnWru+iIw57xVyUQiTzRz8HzaKKZLLnCV+g/i1tBEfyfqOdapbd69qKC2zAzai+x8l R9m39aus9nEYWbX9p+QGCJLQPffCDKaA0M8WnQp3c02sQw5bujZw3FanqPCyJ8XiBd3up7CBoiS+/ KUhwtv3dw==; Received: from localhost ([::1] helo=merlin.infradead.org) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1kktFw-0001mo-Vl; Thu, 03 Dec 2020 18:23:05 +0000 Received: from mail.kernel.org ([198.145.29.99]) by merlin.infradead.org with esmtps (Exim 4.92.3 #3 (Red Hat Linux)) id 1kktFt-0001lH-Tx for linux-arm-kernel@lists.infradead.org; Thu, 03 Dec 2020 18:23:03 +0000 Date: Thu, 3 Dec 2020 18:22:53 +0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1607019780; bh=5a8ME2oAXsKqPoeqJfEFvJGi2qwglFd3HcH42x+ZJqE=; h=From:To:Cc:Subject:References:In-Reply-To:From; b=j4tuv3canVKeGWPc00whrPf+yMo+rCeJBSSu9hQ8JKopGOeh22g/l4jLBxki+Q3S4 GSy6xiQfH2GEr5ORIucdbbEVNr0+FmT1NytvINuFt62ibIHSwQQe+XYxmKQbFUBoQm Pe3O1nyWoKMJPEEb4rf10UT8GM0TMsNCoKs0CA1Y6VDqMD3Fa15DWO2+8KzDpouaH/ jk1NY9n6DPz7lGFPTSZZ3NhJoqHMrLxT9UBGdtIKfzs7C+/Z/dfeNnE5pmTpJgadKr WuTbKHZQrQHeug6qVN5E7eo5w7WLqpt9nlELvChtCWZI3JyjnMxiQXEC4zYRHhnct0 /Nj+EdgBLhJYA== From: Will Deacon To: Sami Tolvanen Subject: Re: [PATCH v8 00/16] Add support for Clang LTO Message-ID: <20201203182252.GA32011@willie-the-truck> References: <20201201213707.541432-1-samitolvanen@google.com> <20201203112622.GA31188@willie-the-truck> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20201203_132302_138682_0F8D89F9 X-CRM114-Status: GOOD ( 30.74 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-arch , Kees Cook , "Paul E. McKenney" , Kernel Hardening , Peter Zijlstra , Greg Kroah-Hartman , Masahiro Yamada , linux-kbuild , Nick Desaulniers , LKML , Steven Rostedt , clang-built-linux , PCI , Josh Poimboeuf , Nathan Chancellor , linux-arm-kernel Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Thu, Dec 03, 2020 at 09:07:30AM -0800, Sami Tolvanen wrote: > On Thu, Dec 3, 2020 at 3:26 AM Will Deacon wrote: > > On Tue, Dec 01, 2020 at 01:36:51PM -0800, Sami Tolvanen wrote: > > > This patch series adds support for building the kernel with Clang's > > > Link Time Optimization (LTO). In addition to performance, the primary > > > motivation for LTO is to allow Clang's Control-Flow Integrity (CFI) > > > to be used in the kernel. Google has shipped millions of Pixel > > > devices running three major kernel versions with LTO+CFI since 2018. > > > > > > Most of the patches are build system changes for handling LLVM > > > bitcode, which Clang produces with LTO instead of ELF object files, > > > postponing ELF processing until a later stage, and ensuring initcall > > > ordering. > > > > > > Note that arm64 support depends on Will's memory ordering patches > > > [1]. I will post x86_64 patches separately after we have fixed the > > > remaining objtool warnings [2][3]. > > > > I took this series for a spin, with my for-next/lto branch merged in but > > I see a failure during the LTO stage with clang 11.0.5 because it doesn't > > understand the '.arch_extension rcpc' directive we throw out in READ_ONCE(). > > I just tested this with Clang 11.0.0, which I believe is the latest > 11.x version, and the current Clang 12 development branch, and both > work for me. Godbolt confirms that '.arch_extension rcpc' is supported > by the integrated assembler starting with Clang 11 (the example fails > with 10.0.1): > > https://godbolt.org/z/1csGcT > > What does running clang --version and ld.lld --version tell you? I'm using some Android prebuilts I had kicking around: Android (6875598, based on r399163b) clang version 11.0.5 (https://android.googlesource.com/toolchain/llvm-project 87f1315dfbea7c137aa2e6d362dbb457e388158d) Target: x86_64-unknown-linux-gnu Thread model: posix InstalledDir: /usr/local/google/home/willdeacon/work/android/repo/android-kernel/prebuilts-master/clang/host/linux-x86/clang-r399163b/bin and: LLD 11.0.5 (/buildbot/tmp/tmpx1DlI_ 87f1315dfbea7c137aa2e6d362dbb457e388158d) (compatible with GNU linkers) > > We actually check that this extension is available before using it in > > the arm64 Kconfig: > > > > config AS_HAS_LDAPR > > def_bool $(as-instr,.arch_extension rcpc) > > > > so this shouldn't happen. I then realised, I wasn't passing LLVM_IAS=1 > > on my Make command line; with that, then the detection works correctly > > and the LTO step succeeds. > > > > Why is it necessary to pass LLVM_IAS=1 if LTO is enabled? I think it > > would be _much_ better if this was implicit (or if LTO depended on it). > > Without LLVM_IAS=1, Clang uses two different assemblers when LTO is > enabled: the external GNU assembler for stand-alone assembly, and > LLVM's integrated assembler for inline assembly. as-instr tests the > external assembler and makes an admittedly reasonable assumption that > the test is also valid for inline assembly. > > I agree that it would reduce confusion in future if we just always > enabled IAS with LTO. Nick, Nathan, any thoughts about this? That works for me, although I'm happy with anything which means that the assembler checks via as-instr apply to the assembler which will ultimately be used. Will _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel