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Date:   Thu, 25 Jun 2020 10:03:13 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     Sami Tolvanen <samitolvanen@google.com>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Will Deacon <will@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        linux-arch <linux-arch@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, linux-pci@vger.kernel.org,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>
Subject: Re: [PATCH 00/22] add support for Clang LTO
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On Wed, Jun 24, 2020 at 02:31:36PM -0700, Nick Desaulniers wrote:
> On Wed, Jun 24, 2020 at 2:15 PM Peter Zijlstra <peterz@infradead.org> 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.
> 
> Hi Peter, could you expand on the issue for the folks on the thread?
> I'm happy to try to hack something up in LLVM if we check that X does
> or does not happen; maybe we can even come up with some concrete test
> cases that can be added to LLVM's codebase?

I'm sure Will will respond, but the basic issue is the trainwreck C11
made of dependent loads.

Anyway, here's a link to the last time this came up:

  https://lore.kernel.org/linux-arm-kernel/20171116174830.GX3624@linux.vnet.ibm.com/