From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from lindbergh.monkeyblade.net ([23.128.96.19]:43698 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2390541AbgFYIDf (ORCPT ); Thu, 25 Jun 2020 04:03:35 -0400 Date: Thu, 25 Jun 2020 10:03:13 +0200 From: Peter Zijlstra Subject: Re: [PATCH 00/22] add support for Clang LTO Message-ID: <20200625080313.GY4817@hirez.programming.kicks-ass.net> 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: Sender: linux-kbuild-owner@vger.kernel.org List-ID: To: Nick Desaulniers Cc: Sami Tolvanen , Masahiro Yamada , Will Deacon , Greg Kroah-Hartman , "Paul E. McKenney" , Kees Cook , clang-built-linux , Kernel Hardening , linux-arch , Linux ARM , Linux Kbuild mailing list , LKML , linux-pci@vger.kernel.org, "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" On Wed, Jun 24, 2020 at 02:31:36PM -0700, Nick Desaulniers wrote: > On Wed, Jun 24, 2020 at 2:15 PM 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. > > Hi Peter, could you expand on the issue for the folks on the thread? > I'm happy to try to hack something up in LLVM if we check that X does > or does not happen; maybe we can even come up with some concrete test > cases that can be added to LLVM's codebase? I'm sure Will will respond, but the basic issue is the trainwreck C11 made of dependent loads. Anyway, here's a link to the last time this came up: https://lore.kernel.org/linux-arm-kernel/20171116174830.GX3624@linux.vnet.ibm.com/