Date: Thu, 3 Dec 2020 18:22:53 +0000 From: Will Deacon <will@...nel.org> To: Sami Tolvanen <samitolvanen@...gle.com> Cc: Nick Desaulniers <ndesaulniers@...gle.com>, Nathan Chancellor <natechancellor@...il.com>, 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 03, 2020 at 09:07:30AM -0800, Sami Tolvanen wrote: > On Thu, Dec 3, 2020 at 3:26 AM Will Deacon <will@...nel.org> wrote: > > 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? I'm using some Android prebuilts I had kicking around: Android (6875598, based on r399163b) clang version 11.0.5 (https://android.googlesource.com/toolchain/llvm-project 87f1315dfbea7c137aa2e6d362dbb457e388158d) Target: x86_64-unknown-linux-gnu Thread model: posix InstalledDir: /usr/local/google/home/willdeacon/work/android/repo/android-kernel/prebuilts-master/clang/host/linux-x86/clang-r399163b/bin and: LLD 11.0.5 (/buildbot/tmp/tmpx1DlI_ 87f1315dfbea7c137aa2e6d362dbb457e388158d) (compatible with GNU linkers) > > 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? That works for me, although I'm happy with anything which means that the assembler checks via as-instr apply to the assembler which will ultimately be used. Will
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