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Date: Fri, 9 Oct 2020 14:05:48 -0700
From: Sami Tolvanen <>
To: Steven Rostedt <>
Cc: Masahiro Yamada <>, Will Deacon <>,
	Peter Zijlstra <>,
	Greg Kroah-Hartman <>,
	"Paul E. McKenney" <>,
	Kees Cook <>,
	Nick Desaulniers <>,,,,,,,,
Subject: Re: [PATCH v5 00/29] Add support for Clang LTO

On Fri, Oct 09, 2020 at 03:35:12PM -0400, Steven Rostedt wrote:
> On Fri,  9 Oct 2020 09:13:09 -0700
> Sami Tolvanen <> wrote:
> > 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:

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.


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