Date: Thu, 3 Dec 2020 11:26:23 +0000 From: Will Deacon <will@...nel.org> To: Sami Tolvanen <samitolvanen@...gle.com> Cc: Masahiro Yamada <masahiroy@...nel.org>, Steven Rostedt <rostedt@...dmis.org>, Josh Poimboeuf <jpoimboe@...hat.com>, Peter Zijlstra <peterz@...radead.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, "Paul E. McKenney" <paulmck@...nel.org>, Kees Cook <keescook@...omium.org>, Nick Desaulniers <ndesaulniers@...gle.com>, clang-built-linux@...glegroups.com, kernel-hardening@...ts.openwall.com, linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org Subject: Re: [PATCH v8 00/16] Add support for Clang LTO 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 > . I will post x86_64 patches separately after we have fixed the > remaining objtool warnings . 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
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