Re: [PATCH v4 00/29] Add support for Clang LTO

Sami Tolvanen
2020-09-30T22:13:07Z

On Wed, Sep 30, 2020 at 2:58 PM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> On Tue, Sep 29, 2020 at 2:46 PM Sami Tolvanen <samitolvanen@google.com> 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.
>
> Sami, thanks for continuing to drive the series. I encourage you to
> keep resending with fixes accumulated or dropped on a weekly cadence.
>
> The series worked well for me on arm64, but for x86_64 on mainline I
> saw a stream of new objtool warnings:
[...]

Objtool normally won't print out these warnings when run on vmlinux.o,
but we can't pass --vmlinux to objtool as that also implies noinstr
validation right now. I think we'd have to split that from --vmlinux
to avoid these. I can include a patch to add a --noinstr flag in v5.
Peter, any thoughts about this?

> I think those should be resolved before I provide any kind of tested
> by tag. My other piece of feedback was that I like the default
> ThinLTO, but I think the help text in the Kconfig which is visible
> during menuconfig could be improved by informing the user the
> tradeoffs. For example, if CONFIG_THINLTO is disabled, it should be
> noted that full LTO will be used instead. Also, that full LTO may
> produce slightly better optimized binaries than ThinLTO, at the cost
> of not utilizing multiple cores when linking and thus significantly
> slower to link.
>
> Maybe explaining that setting it to "n" implies a full LTO build,
> which will be much slower to link but possibly slightly faster would
> be good? It's not visible unless LTO_CLANG and ARCH_SUPPORTS_THINLTO
> is enabled, so I don't think you need to explain that THINLTO without
> those is *not* full LTO. I'll leave the precise wording to you. WDYT?

Sure, sounds good. I'll update the help text in the next version.

> Also, when I look at your treewide DISABLE_LTO patch, I think "does
> that need to be a part of this series, or is it a cleanup that can
> stand on its own?" I think it may be the latter? Maybe it would help
> shed one more patch than to have to carry it to just send it? Or did
> I miss something as to why it should remain a part of this series?

I suppose it could be stand-alone, but as these patches are also
disabling LTO by filtering out flags in some of the same files,
removing the unused DISABLE_LTO flags first would reduce confusion.
But I'm fine with sending it separately too if that's preferred.

Sami