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

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