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Date: Wed, 24 Jun 2020 14:30:14 -0700
From: Sami Tolvanen <samitolvanen@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Masahiro Yamada <masahiroy@...nel.org>, Will Deacon <will@...nel.org>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
"Paul E. McKenney" <paulmck@...nel.org>,
Kees Cook <keescook@...omium.org>,
Nick Desaulniers <ndesaulniers@...gle.com>,
clang-built-linux@...glegroups.com,
kernel-hardening@...ts.openwall.com, linux-arch@...r.kernel.org,
linux-arm-kernel@...ts.infradead.org, linux-kbuild@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
x86@...nel.org
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.
Sami
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