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

[Fangrui Song]

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
> 	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>