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

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Date:   Fri, 15 Jan 2021 22:53:21 +0100
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        Clang-Built-Linux ML <clang-built-linux@...glegroups.com>,
        linux-kbuild@...r.kernel.org, linux-arch@...r.kernel.org,
        Jakub Jelinek <jakub@...hat.com>,
        Fangrui Song <maskray@...gle.com>,
        Caroline Tice <cmtice@...gle.com>,
        Nick Clifton <nickc@...hat.com>, Yonghong Song <yhs@...com>,
        Jiri Olsa <jolsa@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: Re: [PATCH v5 0/3] Kbuild: DWARF v5 support

On Fri, Jan 15, 2021 at 10:06 PM Nick Desaulniers
<ndesaulniers@...gle.com> 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 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@...il.com>

- 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
>