On Fri, Oct 09, 2020 at 03:35:12PM -0400, Steven Rostedt wrote:

On Fri,  9 Oct 2020 09:13:09 -0700
Sami Tolvanen [off-list ref] wrote:

quoted

This patch series adds support for building x86_64 and arm64 kernels
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 this version is based on tip/master to reduce the number
of prerequisite patches, and to make it easier to manage changes to
objtool. Patch 1 is from Masahiro's kbuild tree, and while it's not
directly related to LTO, it makes the module linker script changes
cleaner.

I went to test this, but it appears that the latest tip/master fails to
build for me. This error is on tip/master, before I even applied a single
patch.

(config attached)

Ah yes, X86_DECODER_SELFTEST seems to be broken in tip/master. If you
prefer, I have these patches on top of mainline here:

  https://github.com/samitolvanen/linux/tree/clang-lto

Testing your config with LTO on this tree, it does build and boot for
me, although I saw a couple of new objtool warnings, and with LLVM=1,
one warning from llvm-objdump.

Sami

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