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

Conor Dooley
2023-03-10T16:40:21Z

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On Fri, Mar 10, 2023 at 11:35:57PM +0800, Bin Meng wrote:
> On Fri, Mar 10, 2023 at 11:07 PM Bin Meng <bmeng.cn@gmail.com> 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...
> > >
> > > 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.
> >
> > I am unable to reproduce the build failure as reported by commit 6df2a016c0c8
> > ("riscv: fix build with binutils 2.38") by Aurelien Jarno using kernel.org
> > pre-built GCC 11.3.0 [1] which includes binutils 2.38.
> >
> > [1] https://mirrors.edge.kernel.org/pub/tools/crosstool/files/bin/x86_64/11.3.0/x86_64-gcc-11.3.0-nolibc-x86_64-linux.tar.xz
> >
> > The defconfig of v5.16 kernel (commit 6df2a016c0c8 lands in v5.17) builds fine
> > for me. Anything I am missing?
> >
>
> Some further note:
>
> After I switched to kernel.org pre-built GCC 12.2.0 [1] which includes
> binutils 2.39, I was able to reproduce the exact same build failure of
> v5.16 kernel as described in the commit 6df2a016c0c8 ("riscv: fix
> build with binutils 2.38") by Aurelien Jarno.
>
> To verify the commit message of 6df2a016c0c8 is accurate or not, I
> built a GAS from binutils 2.37 and replaced the GAS 2.39 in the
> kernel.org package, surprisingly kernel v5.16 did not build with the
> same build failure.
>
> So it seems that it's GCC that caused the build failure instead of GAS
> from binutils??

Right, that's what I was getting at in the bit below the --- line in my
patch. I think Aurelien was misled by the failure message and your email
([1] in my links above) which claimed that binutils would default to
the 20191213 spec.
It appears (and I'm not a tc person) that GCC must call GAS with the
--misa-spec argument, and in GCC 12 the value used is 20191213.
Either GCC 11 must pass --misa-spec=2.2 to binutils, or it passes
nothing, leading binutils to be permissive about what -march=rv64i
means.

The permissive option would "seem" to be correct, as building with clang
(that to my knowledge doesn't pass --misa-spec to GAS) and with
-march=rv64i has no issues assembling.

It'd appear to me that binutils is a *player* in this issue, but is not
the culprit of the issue Aurelien sought to fix.

I dunno what I am talking about though, this is all from playing around
with many tc variants and see how it goes!

Cheers,
Conor.

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