[PATCH kernel 2/3] powerpc/llvm: Sample config for LLVM LTO

[Nick Desaulniers]

On Thu, Feb 10, 2022 at 6:31 PM Alexey Kardashevskiy <aik at ozlabs.ru> wrote:
>
> The config is a copy of ppc64_defconfig with a few tweaks. This could be
> a smaller config to merge into ppc64_defconfig but unfortunately
> merger does not allow disabling already enabled options.

Cool series!

>
> This is a command line to compile the kernel using the upstream llvm:
>
> make -j64 O=/home/aik/pbuild/kernels-llvm/ \
>  "KCFLAGS=-Wmissing-braces -Wno-array-bounds" \
>  ARCH=powerpc LLVM_IAS=1 ppc64le_lto_defconfig CC=clang LLVM=1

That command line invocation is kind of a mess, and many things
shouldn't be necessary.

O= is just noise; if folks are doing in tree builds then that doesn't
add anything meaningful.
KCFLAGS= why? I know -Warray-bounds is being worked on actively, but
do we have instances of -Wmissing-braces at the moment? Let's get
those fixed up.
LLVM_IAS=1 is implied by LLVM=1.
CC=clang is implied by LLVM=1

why add a new config? I think it would be simpler to just show command
line invocations of `./scripts/config -e` and `make`. No new config
required.

>
> Forces CONFIG_BTRFS_FS=y to make CONFIG_ZSTD_COMPRESS=y to fix:
> ld.lld: error: linking module flags 'Code Model': IDs have conflicting values in 'lib/built-in.a(entropy_common.o at 5332)' and 'ld-temp.o'
>
> because modules are linked with -mcmodel=large but the kernel uses -mcmodel=medium

Please file a bug about this.
https://github.com/ClangBuiltLinux/linux/issues

>
> Enables CONFIG_USERFAULTFD=y as otherwise vm_userfaultfd_ctx becomes
> 0 bytes long and clang sanitizer crashes as
> https://bugs.llvm.org/show_bug.cgi?id=500375

The above hyperlink doesn't work for me. Upstream llvm just moved from
bugzilla to github issue tracker.

https://github.com/llvm/llvm-project/issues

>
> Disables CONFIG_FTR_FIXUP_SELFTEST as it uses FTR_SECTION_ELSE with
> conditional branches. There are other places like this and the following
> patches address that.
>
> Disables CONFIG_FTRACE_MCOUNT_USE_RECORDMCOUNT as CONFIG_HAS_LTO_CLANG
> depends on it being disabled. In order to avoid disabling way too many
> options (like DYNAMIC_FTRACE/FUNCTION_TRACER), this converts
> FTRACE_MCOUNT_USE_RECORDMCOUNT from def_bool to bool.
>
> Note that even with this config there is a good chance that LTO
> is going to fail linking vmlinux because of the "bc" problem.

I think rather than adding a new config with LTO enabled and a few
things turned off, it would be better to not allow LTO to be
selectable if those things are turned on, until the combination of the
two are fixed.

-- 
Thanks,
~Nick Desaulniers