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Date: Wed, 24 Jun 2020 14:30:14 -0700
From: Sami Tolvanen <>
To: Peter Zijlstra <>
Cc: Masahiro Yamada <>, Will Deacon <>,
	Greg Kroah-Hartman <>,
	"Paul E. McKenney" <>,
	Kees Cook <>,
	Nick Desaulniers <>,,,,,,,,
Subject: Re: [PATCH 00/22] add support for Clang LTO

On Wed, Jun 24, 2020 at 11:15:40PM +0200, Peter Zijlstra wrote:
> On Wed, Jun 24, 2020 at 01:31:38PM -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's
> > Pixel devices have shipped with LTO+CFI kernels 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 first objtool patch in the series is already in linux-next,
> > but as it's needed with LTO, I'm including it also here to make testing
> > easier.
> I'm very sad that yet again, memory ordering isn't addressed. LTO vastly
> increases the range of the optimizer to wreck things.

I believe Will has some thoughts about this, and patches, but I'll let
him talk about it.


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