Re: [PATCH v5 0/3] Kbuild: DWARF v5 support

[Yonghong Song]

On 1/15/21 1:53 PM, Sedat Dilek wrote:
> On Fri, Jan 15, 2021 at 10:06 PM Nick Desaulniers
> <ndesaulniers@xxxxxxxxxx> wrote:
> > DWARF v5 is the latest standard of the DWARF debug info format.
> >
> > DWARF5 wins significantly in terms of size when mixed with compression
> > (CONFIG_DEBUG_INFO_COMPRESSED).
> >
> > Link: http://www.dwarfstd.org/doc/DWARF5.pdf
> >
> > Patch 1 is a cleanup from Masahiro and isn't DWARF v5 specific.
> > Patch 2 is a cleanup that lays the ground work and isn't DWARF
> > v5 specific.
> > Patch 3 implements Kconfig and Kbuild support for DWARFv5.
> >
> > Changes from v4:
> > * drop set -e from script as per Nathan.
> > * add dependency on !CONFIG_DEBUG_INFO_BTF for DWARF v5 as per Sedat.
> > * Move LLVM_IAS=1 complexity from patch 2 to patch 3 as per Arvind and
> >    Masahiro. Sorry it took me a few tries to understand the point (I
> >    might still not), but it looks much cleaner this way. Sorry Nathan, I
> >    did not carry forward your previous reviews as a result, but I would
> >    appreciate if you could look again.
> > * Add Nathan's reviewed by tag to patch 1.
> > * Reword commit message for patch 3 to mention LLVM_IAS=1 and -gdwarf-5
> >    binutils addition later, and BTF issue.
> > * I still happen to see a pahole related error spew for the combination
> >    of:
> >    * LLVM=1
> >    * LLVM_IAS=1
> >    * CONFIG_DEBUG_INFO_DWARF4
> >    * CONFIG_DEBUG_INFO_BTF
> >    Though they're non-fatal to the build. I'm not sure yet why removing
> >    any one of the above prevents the warning spew. Maybe we'll need a v6.
>
> En plus, I encountered breakage with GCC v10.2.1 and LLVM=1 and
> CONFIG_DEBUG_INFO_DWARF4.
> So might be good to add a "depends on !DEBUG_INFO_BTF" in this combination.

I suggested not to add !DEBUG_INFO_BTF to CONFIG_DEBUG_INFO_DWARF4.
It is not there before and adding this may suddenly break some users.

If certain combination of gcc/llvm does not work for 
CONFIG_DEBUG_INFO_DWARF4 with pahole, this is a bug bpf community
should fix.

> I had some other small nits commented in the single patches.
>
> As requested in your previous patch-series, feel free to add my:
>
> Tested-by: Sedat Dilek <sedat.dilek@xxxxxxxxx>
>
> - Sedat -
>
> > Changes from v3:
> >
> > Changes as per Arvind:
> > * only add -Wa,-gdwarf-5 for (LLVM=1|CC=clang)+LLVM_IAS=0 builds.
> > * add -gdwarf-5 to Kconfig shell script.
> > * only run Kconfig shell script for Clang.
> >
> > Apologies to Sedat and Nathan; I appreciate previous testing/review, but
> > I did no carry forward your Tested-by and Reviewed-by tags, as the
> > patches have changed too much IMO.
> >
> > Changes from v2:
> > * Drop two of the earlier patches that have been accepted already.
> > * Add measurements with GCC 10.2 to commit message.
> > * Update help text as per Arvind with help from Caroline.
> > * Improve case/wording between DWARF Versions as per Masahiro.
> >
> > Changes from the RFC:
> > * split patch in 3 patch series, include Fangrui's patch, too.
> > * prefer `DWARF vX` format, as per Fangrui.
> > * use spaces between assignment in Makefile as per Masahiro.
> > * simplify setting dwarf-version-y as per Masahiro.
> > * indent `prompt` in Kconfig change as per Masahiro.
> > * remove explicit default in Kconfig as per Masahiro.
> > * add comments to test_dwarf5_support.sh.
> > * change echo in test_dwarf5_support.sh as per Masahiro.
> > * remove -u from test_dwarf5_support.sh as per Masahiro.
> > * add a -gdwarf-5 cc-option check to Kconfig as per Jakub.
> >
> > *** BLURB HERE ***
> >
> > Masahiro Yamada (1):
> >    Remove $(cc-option,-gdwarf-4) dependency from CONFIG_DEBUG_INFO_DWARF4
> >
> > Nick Desaulniers (2):
> >    Kbuild: make DWARF version a choice
> >    Kbuild: implement support for DWARF v5
> >
> >   Makefile                          | 13 +++++++---
> >   include/asm-generic/vmlinux.lds.h |  6 ++++-
> >   lib/Kconfig.debug                 | 42 +++++++++++++++++++++++++------
> >   scripts/test_dwarf5_support.sh    |  8 ++++++
> >   4 files changed, 57 insertions(+), 12 deletions(-)
> >   create mode 100755 scripts/test_dwarf5_support.sh
> >
> > --
> > 2.30.0.284.gd98b1dd5eaa7-goog