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From:   Sedat Dilek <sedat.dilek@gmail.com>
Date:   Sat, 30 Jan 2021 01:08:06 +0100
Message-ID: <CA+icZUXpn_VKePTpnEhcpuSxPkuQTSKYfsVeMbxU9-rBp1ZJXw@mail.gmail.com>
Subject: Re: [PATCH v6 0/2] Kbuild: DWARF v5 support
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     Masahiro Yamada <masahiroy@kernel.org>,
        Nathan Chancellor <natechancellor@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org,
        Clang-Built-Linux ML <clang-built-linux@googlegroups.com>,
        linux-kbuild@vger.kernel.org, linux-arch@vger.kernel.org,
        Jakub Jelinek <jakub@redhat.com>,
        Fangrui Song <maskray@google.com>,
        Caroline Tice <cmtice@google.com>,
        Nick Clifton <nickc@redhat.com>, Yonghong Song <yhs@fb.com>,
        Jiri Olsa <jolsa@kernel.org>,
        Andrii Nakryiko <andrii@kernel.org>,
        Arnaldo Carvalho de Melo <acme@kernel.org>
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On Fri, Jan 29, 2021 at 8:43 PM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> DWARF v5 is the latest standard of the DWARF debug info format.
>
> DWARF5 wins significantly in terms of size and especially so when mixed
> with compression (CONFIG_DEBUG_INFO_COMPRESSED).
>
> Link: http://www.dwarfstd.org/doc/DWARF5.pdf
>
> Patch 1 is a cleanup that lays the ground work and isn't DWARF
> v5 specific.
> Patch 2 implements Kconfig and Kbuild support for DWARFv5.
>

When you will do a v7...

Can you look also at places where we have hardcoded DWARF-2 handling...

For example:

arch/x86/purgatory/Makefile:AFLAGS_REMOVE_setup-x86_$(BITS).o   += -Wa,-gdwarf-2
arch/x86/purgatory/Makefile:AFLAGS_REMOVE_entry64.o
 += -Wa,-gdwarf-2

- Sedat -

> Changes from v5:
> * Drop previous patch 1, it has been accepted into kbuild:
>   https://git.kernel.org/pub/scm/linux/kernel/git/masahiroy/linux-kbuild.git/commit/?h=kbuild&id=3f4d8ce271c7082be75bacbcbd2048aa78ce2b44
> * Trying to set -Wa,-gdwarf-4 in the earlier patch was the source of
>   additional complexity. Drop it that part of the patch. We can revisit
>   clang without the integrated assembler setting -Wa,-gdwarf-4 later.
>   That is a separate problem from generally supporting DWARF v5.
> * Rework the final patch for clang without the integrated assembler.
>   -Wa,-gdwarf-5 is required for DWARF5 in that case otherwise GAS will
>   not accept the assembler directives clang produces from C code when
>   generating asm.
>
> 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.
>
> 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.
>
> Nick Desaulniers (2):
>   Kbuild: make DWARF version a choice
>   Kbuild: implement support for DWARF v5
>
>  Makefile                          | 16 ++++++++++--
>  include/asm-generic/vmlinux.lds.h |  6 ++++-
>  lib/Kconfig.debug                 | 41 ++++++++++++++++++++++++++-----
>  scripts/test_dwarf5_support.sh    |  8 ++++++
>  4 files changed, 62 insertions(+), 9 deletions(-)
>  create mode 100755 scripts/test_dwarf5_support.sh
>
> --
> 2.30.0.365.g02bc693789-goog
>