Re: [PATCH v8 00/16] Add support for Clang LTO

Will Deacon
2020-12-03T11:27:28Z

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