From mboxrd@z Thu Jan 1 00:00:00 1970 From: Sami Tolvanen Subject: Re: [PATCH 00/22] add support for Clang LTO Date: Wed, 24 Jun 2020 14:30:14 -0700 Message-ID: <20200624213014.GB26253@google.com> References: <20200624203200.78870-1-samitolvanen@google.com> <20200624211540.GS4817@hirez.programming.kicks-ass.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20200624211540.GS4817@hirez.programming.kicks-ass.net> Sender: linux-kbuild-owner@vger.kernel.org To: Peter Zijlstra Cc: Masahiro Yamada , Will Deacon , 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, x86@kernel.org List-Id: linux-arch.vger.kernel.org On Wed, Jun 24, 2020 at 11:15:40PM +0200, Peter Zijlstra wrote: > On Wed, Jun 24, 2020 at 01:31:38PM -0700, Sami Tolvanen wrote: > > This patch series adds support for building x86_64 and arm64 kernels > > 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's > > Pixel devices have shipped with LTO+CFI kernels 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 first objtool patch in the series is already in linux-next, > > but as it's needed with LTO, I'm including it also here to make testing > > easier. > > I'm very sad that yet again, memory ordering isn't addressed. LTO vastly > increases the range of the optimizer to wreck things. I believe Will has some thoughts about this, and patches, but I'll let him talk about it. Sami From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59498 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2403939AbgFXVaW (ORCPT ); Wed, 24 Jun 2020 17:30:22 -0400 Received: from mail-pg1-x543.google.com (mail-pg1-x543.google.com [IPv6:2607:f8b0:4864:20::543]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 9FA69C061796 for ; Wed, 24 Jun 2020 14:30:22 -0700 (PDT) Received: by mail-pg1-x543.google.com with SMTP id d4so2090588pgk.4 for ; Wed, 24 Jun 2020 14:30:22 -0700 (PDT) Date: Wed, 24 Jun 2020 14:30:14 -0700 From: Sami Tolvanen Subject: Re: [PATCH 00/22] add support for Clang LTO Message-ID: <20200624213014.GB26253@google.com> References: <20200624203200.78870-1-samitolvanen@google.com> <20200624211540.GS4817@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200624211540.GS4817@hirez.programming.kicks-ass.net> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Peter Zijlstra Cc: Masahiro Yamada , Will Deacon , 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, x86@kernel.org Message-ID: <20200624213014.LBeP4NxM9DNTCNs8ranz9UOEg1lhtXIxF5bvR4J7VC4@z> On Wed, Jun 24, 2020 at 11:15:40PM +0200, Peter Zijlstra wrote: > On Wed, Jun 24, 2020 at 01:31:38PM -0700, Sami Tolvanen wrote: > > This patch series adds support for building x86_64 and arm64 kernels > > 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's > > Pixel devices have shipped with LTO+CFI kernels 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 first objtool patch in the series is already in linux-next, > > but as it's needed with LTO, I'm including it also here to make testing > > easier. > > I'm very sad that yet again, memory ordering isn't addressed. LTO vastly > increases the range of the optimizer to wreck things. I believe Will has some thoughts about this, and patches, but I'll let him talk about it. Sami