Re: [PATCH v7 00/17] Add support for Clang LTO

Nathan Chancellor
2020-11-21T03:14:34Z

On Fri, Nov 20, 2020 at 11:29:51AM +0100, Ard Biesheuvel wrote:
> On Thu, 19 Nov 2020 at 00:42, Nick Desaulniers <ndesaulniers@google.com> wrote:
> >
> > On Wed, Nov 18, 2020 at 2:07 PM Sami Tolvanen <samitolvanen@google.com> 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 v7 brings back arm64 support as Will has now staged the
> > > prerequisite memory ordering patches [1], and drops x86_64 while we work
> > > on fixing the remaining objtool warnings [2].
> > >
> > > [1] https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git/log/?h=for-next/lto
> > > [2] https://lore.kernel.org/lkml/20201114004911.aip52eimk6c2uxd4@treble/
> > >
> > > You can also pull this series from
> > >
> > > https://github.com/samitolvanen/linux.git lto-v7
> >
> > Thanks for continuing to drive this series Sami. For the series,
> >
> > Tested-by: Nick Desaulniers <ndesaulniers@google.com>
> >
> > I did virtualized boot tests with the series applied to aarch64
> > defconfig without CONFIG_LTO, with CONFIG_LTO_CLANG, and a third time
> > with CONFIG_THINLTO. If you make changes to the series in follow ups,
> > please drop my tested by tag from the modified patches and I'll help
> > re-test. Some minor feedback on the Kconfig change, but I'll post it
> > off of that patch.
> >
>
> When you say 'virtualized" do you mean QEMU on x86? Or actual
> virtualization on an AArch64 KVM host?
>
> The distinction is important here, given the potential impact of LTO
> on things that QEMU simply does not model when it runs in TCG mode on
> a foreign host architecture.

I have booted this series on my Raspberry Pi 4 (ARCH=arm64 defconfig).

$ uname -r
5.10.0-rc4-00108-g830200082c74

$ zgrep LTO /proc/config.gz
CONFIG_LTO=y
CONFIG_ARCH_SUPPORTS_LTO_CLANG=y
CONFIG_ARCH_SUPPORTS_THINLTO=y
CONFIG_THINLTO=y
# CONFIG_LTO_NONE is not set
CONFIG_LTO_CLANG=y
# CONFIG_HID_WALTOP is not set

and I have taken that same kernel and booted it under QEMU with
'-enable-kvm' without any visible issues.

I have tested four combinations:

clang 12 @ f9f0a4046e11c2b4c130640f343e3b2b5db08c1:
* CONFIG_THINLTO=y
* CONFIG_THINLTO=n

clang 11.0.0
* CONFIG_THINLTO=y
* CONFIG_THINLTO=n

Tested-by: Nathan Chancellor <natechancellor@gmail.com>

Cheers,
Nathan