[PATCH kbuild v3 0/2] add an elfnote with type BUILD_COMPILER_LTO_INFO

[Yonghong Song <yhs@fb.com>]

Currently, clang LTO built vmlinux won't work with pahole.
LTO introduced cross-cu dwarf tag references and broke
current pahole model which handles one cu as a time.
The solution is to merge all cu's as one pahole cu as in [1].
We would like to do this merging only if cross-cu dwarf
references happens. The LTO build mode is a pretty good
indication for that.

In earlier version of this patch ([2]), clang flag
-grecord-gcc-switches is proposed to add to compilation flags
so pahole could detect "-flto" and then merging cu's.
This will increate the binary size of 1% without LTO though.

Arnaldo suggested to use a note to indicate the vmlinux
is built with LTO. Such a cheap way to get whether the vmlinux
is built with LTO or not helps pahole but is also useful
for tracing as LTO may inline/delete/demote global functions,
promote static functions, etc.

This patch set added an elfnote with type BUILD_COMPILER_LTO_INFO.
The owner of the note is "Linux". Patch #1 did some refactoring
and Patch #2 added the elfnote with BUILD_COMPILER_LTO_INFO to
indicate whether vmlinux is built with LTO or not.

 [1] https://lore.kernel.org/bpf/20210325065316.3121287-1-yhs@fb.com/
 [2] https://lore.kernel.org/bpf/20210331001623.2778934-1-yhs@fb.com/

Yonghong Song (2):
  kbuild: move LINUX_ELFNOTE_BUILD_SALT to elfnote.h
  kbuild: add an elfnote with type BUILD_COMPILER_LTO_INFO

 include/linux/build-salt.h | 2 --
 include/linux/compiler.h   | 8 ++++++++
 include/linux/elfnote.h    | 6 ++++++
 init/version.c             | 2 ++
 scripts/mod/modpost.c      | 1 +
 5 files changed, 17 insertions(+), 2 deletions(-)

-- 
2.30.2