[PATCH v3] RISC-V: build: Allow LTO to be selected

[Nick Desaulniers]

On Tue, Oct 3, 2023 at 12:17 PM Nathan Chancellor <nathan at kernel.org> wrote:
>
> From: Wende Tan <twd2.me at gmail.com>
>
> Allow LTO to be selected for RISC-V, only when LLD >= 14, since there is an
> issue [1] in prior LLD versions that prevents LLD to generate proper
> machine code for RISC-V when writing `nop`s.
>
> I have tested enabling LTO for `defconfig`. The LLD took ~2m21s and ~3GiB
> on our Intel Xeon Gold 6140 server and produced an 18MiB Image. The image
> can boot to shell using an archriscv rootfs on QEMU.
>
> I have also tested it for `allyesconfig` without COMPILE_TEST, FTRACE,
> KASAN, and GCOV. The LLD took ~7h03m and ~335GiB on the server,
> successfully producing a 1.7GiB Image. Unfortunately, we cannot boot this
> image because the `create_kernel_page_table()` -> `alloc_pmd_early()` ->
> `BUG_ON()` logic limits the image to be < 1GiB. Maybe we can fix it in a
> separate patch further.
>
> Disable LTO for arch/riscv/kernel/pi, as llvm-objcopy expects an ELF
> object file when manipulating the files in that subfolder, rather than
> LLVM bitcode.
>
> [1] https://github.com/llvm/llvm-project/issues/50505, resolved by LLVM
>     commit e63455d5e0e5 ("[MC] Use local MCSubtargetInfo in writeNops")
>
> Tested-by: Wende Tan <twd2.me at gmail.com>
> Signed-off-by: Wende Tan <twd2.me at gmail.com>
> Co-developed-by: Nathan Chancellor <nathan at kernel.org>
> Signed-off-by: Nathan Chancellor <nathan at kernel.org>

Perhaps worth a v4 due to linker relaxation resulting in boot failures
you observed:
https://github.com/ClangBuiltLinux/linux/issues/1942 ?

-- 
Thanks,
~Nick Desaulniers