Re: [PATCH v8 00/16] Add support for Clang LTO

Sami Tolvanen
2020-12-03T17:08:29Z

On Thu, Dec 3, 2020 at 3:26 AM Will Deacon <will@kernel.org> wrote:
>
> Hi Sami,
>
> On Tue, Dec 01, 2020 at 01:36:51PM -0800, Sami Tolvanen wrote:
> > This patch series adds support for building the kernel 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 arm64 support depends on Will's memory ordering patches
> > [1]. I will post x86_64 patches separately after we have fixed the
> > remaining objtool warnings [2][3].
>
> I took this series for a spin, with my for-next/lto branch merged in but
> I see a failure during the LTO stage with clang 11.0.5 because it doesn't
> understand the '.arch_extension rcpc' directive we throw out in READ_ONCE().

I just tested this with Clang 11.0.0, which I believe is the latest
11.x version, and the current Clang 12 development branch, and both
work for me. Godbolt confirms that '.arch_extension rcpc' is supported
by the integrated assembler starting with Clang 11 (the example fails
with 10.0.1):

https://godbolt.org/z/1csGcT

What does running clang --version and ld.lld --version tell you?

> We actually check that this extension is available before using it in
> the arm64 Kconfig:
>
> config AS_HAS_LDAPR
> def_bool $(as-instr,.arch_extension rcpc)
>
> so this shouldn't happen. I then realised, I wasn't passing LLVM_IAS=1
> on my Make command line; with that, then the detection works correctly
> and the LTO step succeeds.
>
> Why is it necessary to pass LLVM_IAS=1 if LTO is enabled? I think it
> would be _much_ better if this was implicit (or if LTO depended on it).

Without LLVM_IAS=1, Clang uses two different assemblers when LTO is
enabled: the external GNU assembler for stand-alone assembly, and
LLVM's integrated assembler for inline assembly. as-instr tests the
external assembler and makes an admittedly reasonable assumption that
the test is also valid for inline assembly.

I agree that it would reduce confusion in future if we just always
enabled IAS with LTO. Nick, Nathan, any thoughts about this?

Sami