Re: [PATCH v2] RISC-V: remove I-extension ISA spec version dance

Jessica Clarke
2023-03-09T02:20:07Z

On 8 Mar 2023, at 22:08, Conor Dooley <conor@kernel.org> wrote:
>
> From: Conor Dooley <conor.dooley@microchip.com>
>
> The spec folk, in their infinite wisdom, moved both control and status
> registers & the FENCE.I instructions out of the I extension into their
> own extensions (Zicsr, Zifencei) in the 20190608 version of the ISA
> spec [0].
> The GCC/binutils crew decided [1] to move their default version of the
> ISA spec to the 20191213 version of the ISA spec, which came into being
> for version 2.38 of binutils and GCC 12. Building with this toolchain
> configuration would result in assembler issues:
> CC arch/riscv/kernel/vdso/vgettimeofday.o
> <<BUILDDIR>>/arch/riscv/include/asm/vdso/gettimeofday.h: Assembler messages:
> <<BUILDDIR>>/arch/riscv/include/asm/vdso/gettimeofday.h:71: Error: unrecognized opcode `csrr a5,0xc01'
> <<BUILDDIR>>/arch/riscv/include/asm/vdso/gettimeofday.h:71: Error: unrecognized opcode `csrr a5,0xc01'
> <<BUILDDIR>>/arch/riscv/include/asm/vdso/gettimeofday.h:71: Error: unrecognized opcode `csrr a5,0xc01'
> <<BUILDDIR>>/arch/riscv/include/asm/vdso/gettimeofday.h:71: Error: unrecognized opcode `csrr a5,0xc01'
> This was fixed in commit 6df2a016c0c8 ("riscv: fix build with binutils
> 2.38") by Aurelien Jarno, but has proven fragile.
>
> Before LLVM 17, LLVM did not support these extensions and, as such, the
> cc-option check added by Aurelien worked. Since commit 22e199e6afb1
> ("[RISCV] Accept zicsr and zifencei command line options") however, LLVM
> *does* support them and the cc-option check passes.
>
> This surfaced as a problem while building the 5.10 stable kernel using
> the default Tuxmake Debian image [2], as 5.10 did not yet support ld.lld,
> and uses the Debian provided binutils 2.35.
> Versions of ld prior to 2.38 will refuse to link if they encounter
> unknown ISA extensions, and unfortunately Zifencei is not supported by
> bintuils 2.35.
>
> Instead of dancing around with adding these extensions to march, as we
> currently do, Palmer suggested locking GCC builds to the same version of
> the ISA spec that is used by LLVM. As far as I can tell, that is 2.2,
> with, apparently [3], a lack of interest in implementing a flag like
> GCC's -misa-spec at present.
>
> Add {cc,as}-option checks to add -misa-spec to KBUILD_{A,C}FLAGS when
> GCC is used & remove the march dance.
>
> As clang does not accept this argument, I had expected to encounter
> issues with the assembler, as neither zicsr nor zifencei are present in
> the ISA string and the spec version *should* be defaulting to a version
> that requires them to be present. The build passed however and the
> resulting kernel worked perfectly fine for me on a PolarFire SoC...

For what it’s worth, LLVM is likely to move from only supporting the
old ratified spec to only supporting the latest one, with no ugly
-misa-spec like the GNU world. You may therefore wish to reconsider
this...

Jess

> Link: https://riscv.org/wp-content/uploads/2019/06/riscv-spec.pdf [0]
> Link: https://groups.google.com/a/groups.riscv.org/g/sw-dev/c/aE1ZeHHCYf4 [1]
> Link: https://lore.kernel.org/all/CA+G9fYt9T=ELCLaB9byxaLW2Qf4pZcDO=huCA0D8ug2V2+irJQ@mail.gmail.com/ [2]
> Link: https://discourse.llvm.org/t/specifying-unpriviledge-spec-version-misa-spec-gcc-flag-equivalent/66935 [3]
> CC: stable@vger.kernel.org
> Suggested-by: Palmer Dabbelt <palmer@rivosinc.com>
> Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
> Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
> ---
> I think Aurelien's original commit message might actually not be quite
> correct? I found, in my limited testing, that it is not the default
> behaviour of gas that matters, but rather the toolchain itself?
> My binutils versions (both those built using the clang-built-linux
> tc-build scripts which do not set an ISA spec version, and one built
> using the riscv-gnu-toolchain infra w/ an explicit 20191213 spec version
> set) do not encounter these issues.
> From *my* testing, I was only able to reproduce the above build failures
> because of *GCC* defaulting to a newer ISA spec version, and saw no
> issues with CC=clang builds, where -misa-spec is not (AFAICT) passed to
> gas.
> I'm far from a toolchain person, so I am very very happy to have the
> reason for that explained to me, as I've been scratching my head about
> it all evening.
>
> I'm also not super confident in my "as-option"ing, but it worked for me,
> so it's gotta be perfect, right? riiight??
>
> Changes from v1:
> - entirely new approach to the issue
> ---
> arch/riscv/Makefile | 6 ++----
> 1 file changed, 2 insertions(+), 4 deletions(-)
>
> diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
> index 6203c3378922..2df7a5dc071c 100644
> --- a/arch/riscv/Makefile
> +++ b/arch/riscv/Makefile
> @@ -57,10 +57,8 @@ riscv-march-$(CONFIG_ARCH_RV64I) := rv64ima
> riscv-march-$(CONFIG_FPU) := $(riscv-march-y)fd
> riscv-march-$(CONFIG_RISCV_ISA_C) := $(riscv-march-y)c
>
> -# Newer binutils versions default to ISA spec version 20191213 which moves some
> -# instructions from the I extension to the Zicsr and Zifencei extensions.
> -toolchain-need-zicsr-zifencei := $(call cc-option-yn, -march=$(riscv-march-y)_zicsr_zifencei)
> -riscv-march-$(toolchain-need-zicsr-zifencei) := $(riscv-march-y)_zicsr_zifencei
> +KBUILD_CFLAGS += $(call cc-option,-misa-spec=2.2)
> +KBUILD_AFLAGS += $(call as-option,-Wa$(comma)-misa-spec=2.2)
>
> # Check if the toolchain supports Zihintpause extension
> riscv-march-$(CONFIG_TOOLCHAIN_HAS_ZIHINTPAUSE) := $(riscv-march-y)_zihintpause
> --
> 2.39.2
>
>
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