[PATCH] lib/Kconfig.debug: Restrict DEBUG_INFO_SPLIT for RISC-V

[Fangrui Song]

On Wed, Aug 16, 2023 at 3:35 PM Nick Desaulniers
<ndesaulniers at google.com> wrote:
>
> On Wed, Aug 16, 2023 at 3:05 PM Fangrui Song <maskray at google.com> wrote:
> >
> > On 2023-08-16, Nathan Chancellor wrote:
> > >When building for ARCH=riscv using LLVM < 14, there is an error with
> > >CONFIG_DEBUG_INFO_SPLIT=y:
> > >
> > >  error: A dwo section may not contain relocations
> > >
> > >This was worked around in LLVM 15 by disallowing '-gsplit-dwarf' with
> > >'-mrelax' (the default), so CONFIG_DEBUG_INFO_SPLIT is not selectable
> > >with newer versions of LLVM:
> > >
> > >  $ clang --target=riscv64-linux-gnu -gsplit-dwarf -c -o /dev/null -x c /dev/null
> > >  clang: error: -gsplit-dwarf is unsupported with RISC-V linker relaxation (-mrelax)
> > >
> > >GCC silently had a similar issue that was resolved with GCC 12.x.
> > >Restrict CONFIG_DEBUG_INFO_SPLIT for RISC-V when using LLVM or GCC <
> > >12.x to avoid these known issues.
> > >
> > >Link: https://github.com/ClangBuiltLinux/linux/issues/1914
> > >Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99090
> > >Reported-by: kernel test robot <lkp at intel.com>
> > >Closes: https://lore.kernel.org/all/202308090204.9yZffBWo-lkp@intel.com/
> > >Signed-off-by: Nathan Chancellor <nathan at kernel.org>
> > >---
> > >Since this only impacts RISC-V, it seems reasonable that this would go
> > >in via their tree but I have added Andrew in case he wants to take it.
> > >---
> > > lib/Kconfig.debug | 5 +++++
> > > 1 file changed, 5 insertions(+)
> > >
> > >diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> > >index d6798513a8c2..bd51274f4771 100644
> > >--- a/lib/Kconfig.debug
> > >+++ b/lib/Kconfig.debug
> > >@@ -355,6 +355,11 @@ endchoice # "Compressed Debug information"
> > > config DEBUG_INFO_SPLIT
> > >       bool "Produce split debuginfo in .dwo files"
> > >       depends on $(cc-option,-gsplit-dwarf)
> > >+      # RISC-V linker relaxation + -gsplit-dwarf has issues with LLVM and GCC
> > >+      # prior to 12.x:
> > >+      # https://github.com/llvm/llvm-project/issues/56642
> > >+      # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99090
> > >+      depends on !RISCV || GCC_VERSION >= 120000
>
> Thanks for the patch. I was initially surprised to see no explicit
> check for LLVM version.  Rereading the commit description a few times,
> I think it makes sense, but I'm guessing we may need to rework this
> condition again should we eventually get split debug info working for
> riscv for llvm.  I am looking to enable split debug info for kernels
> for Android's next release.
>
> Fangrui, any thoughts on feasibility for `-mrelax` and `-gsplit-dwarf`
> composability for llvm?

Yes, this should be feasible and is on my radar
(https://github.com/llvm/llvm-project/issues/56642#issuecomment-1548421026).

It's the next thing I'll do in my spare time after DWARF v5 & linker
relaxation interop: https://reviews.llvm.org/D157657

> Link: https://github.com/llvm/llvm-project/issues/56642
> Reviewed-by: Nick Desaulniers <ndesaulniers at google.com>
>
> > >       help
> > >         Generate debug info into separate .dwo files. This significantly
> > >         reduces the build directory size for builds with DEBUG_INFO,
> > >
> > >---
> > >base-commit: 2ccdd1b13c591d306f0401d98dedc4bdcd02b421
> > >change-id: 20230816-riscv-debug_info_split-0713571a1ac2
> > >
> > >Best regards,
> > >--
> > >Nathan Chancellor <nathan at kernel.org>
> > >
> >
> > LGTM as the author of the Clang mitigation
> > (https://github.com/llvm/llvm-project/issues/56642) and several recent
> > LLVM assembler side linker relaxation improvement (I continue updating
> > https://maskray.me/blog/2021-03-14-the-dark-side-of-riscv-linker-relaxation
> > about the complication of linker relaxation).
> >
> > Reviewed-by: Fangrui Song <maskray at google.com>
>
>
>
> --
> Thanks,
> ~Nick Desaulniers



-- 
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