Date: Thu, 3 Dec 2020 09:07:30 -0800 From: Sami Tolvanen <samitolvanen@...gle.com> To: Will Deacon <will@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>, Nathan Chancellor <natechancellor@...il.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>, clang-built-linux <clang-built-linux@...glegroups.com>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, linux-arch <linux-arch@...r.kernel.org>, linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>, linux-kbuild <linux-kbuild@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, PCI <linux-pci@...r.kernel.org> Subject: Re: [PATCH v8 00/16] Add support for Clang LTO On Thu, Dec 3, 2020 at 3:26 AM Will Deacon <will@...nel.org> wrote: > > 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(). 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? > 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? Sami
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