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

[Nathan Chancellor]

On Tue, Oct 10, 2023 at 10:03:15AM -0700, Nick Desaulniers wrote:
> 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 ?

Yes, agreed. RISC-V patchwork folks, consider marking this as "Changes
Requested" or whatever other status there is that indicates that I need
to send a v4 working around that issue, at least until we better
understand the issue.

Cheers,
Natha