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

On 2/12/22 11:05, Nick Desaulniers wrote:

On Thu, Feb 10, 2022 at 6:31 PM Alexey Kardashevskiy [off-list ref] wrote:

quoted

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!

quoted

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.

I should have added "RFC" in this one as the purpose of the patch is to 
show what works right now and not for actual submission.

quoted

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

quoted

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.

aah this is the correct one:
https://bugs.llvm.org/show_bug.cgi?id=50037

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

oh ok.

quoted

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.

Well, if I want people to try this thing, I kinda need to provide an 
easy way to allow LTO. The new config seemed the easiest (== the 
shortest) :)