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 > [1]. I will post x86_64 patches separately after we have fixed the > remaining objtool warnings [2][3]. 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
