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=-9.0 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 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 5C509C64E8A for ; Thu, 3 Dec 2020 11:27:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0712A22244 for ; Thu, 3 Dec 2020 11:27:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729797AbgLCL1M (ORCPT ); Thu, 3 Dec 2020 06:27:12 -0500 Received: from mail.kernel.org ([198.145.29.99]:52412 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725985AbgLCL1L (ORCPT ); Thu, 3 Dec 2020 06:27:11 -0500 Date: Thu, 3 Dec 2020 11:26:23 +0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1606994790; bh=Z6fRxajiQsjMgIcL2B8U6vdSMQSDFY19TKRTAoS5XwM=; h=From:To:Cc:Subject:References:In-Reply-To:From; b=BmIBlbUTavBbJSA9SMVKb3tOHpXr6pmjMIykJxf8ysFQLdGr4NcG39u+tYjgBM15e ND0gIZQk/Z9Q2lye0p54KpIoxeRbEo86wBhS+sakJ/LTeGH97qgsHZUNuhciXa3sjH Sdap/WsTwkwgmaDLJvxtIbvdHONV85FJ/NQJoTHO+D6r6+nGzh+djD8BmVCZPcr1UA 9KjdS0id2HpEqXYn1f4KtDSfcRnH9mLEjtLYqQpbDQNvRQtx3PhNUaVOjUSpg7p1Fx +aW7XBdMBvjBOcwlwLm9jvoC5FZG3oy1RfqNQMCD7k9DXou+nfkgGL9P6MIG326q2n FxiM1lHhq2TrA== From: Will Deacon To: Sami Tolvanen Cc: Masahiro Yamada , Steven Rostedt , Josh Poimboeuf , Peter Zijlstra , Greg Kroah-Hartman , "Paul E. McKenney" , Kees Cook , Nick Desaulniers , clang-built-linux@googlegroups.com, kernel-hardening@lists.openwall.com, linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org, linux-pci@vger.kernel.org Subject: Re: [PATCH v8 00/16] Add support for Clang LTO Message-ID: <20201203112622.GA31188@willie-the-truck> References: <20201201213707.541432-1-samitolvanen@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201201213707.541432-1-samitolvanen@google.com> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Sami, 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(). 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). Cheers, Will