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

Aurelien Jarno
2023-03-11T10:54:35Z

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On 2023-03-11 10:40, Conor Dooley wrote:
> On Sat, Mar 11, 2023 at 11:11:57AM +0100, Aurelien Jarno wrote:
> > On 2023-03-10 16:40, Conor Dooley wrote:
> > > 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.
> >
> > No I was not misled by that, at that time GCC 11.3 and GCC 12 were not
> > released.
> >
> > binutils definitely defaults to 20191213 if you do not use the
> > --with-isa-spec with a different value when configuring it.
>
> I specifically built binutils without that flag, and had no issues
> building the kernel with clang, which is the source of my confusion here
> really.
> I'd have expected that to fail
>
> > > 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.
> >
> > GCC 11.1 and 11.2 pass nothing to binutils, hence the issue I reported
> > and the patch fixing that. Basic support for misa-spec has been
> > backported to GCC 11.3, with a default to --misa-spec=2.2, hence the
> > "permissive" behaviour you observe.
> >
> > > 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.
> >
> > The "permissive" way only work if we drop support for GCC 11.1 and 11.2,
> > which sounds acceptable to me.
>
> I don't think that we can do that.
>
> > > 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 *disagree*. At the time the commit has been merged, binutils was the
> > only culprit.
>
> Okay, fair enough! I was only going off my observations with
> current-day, and clearly you remember well the situation that you
> encountered this with!
>
> > With more recent versions of GCC 11.3 and GCC 12 released
> > in the meantime, the situation is way more complex.
>
> This patch is going to be problematic w/ versions of gcc that do not
> understand -misa-spec + binutils 2.38 then, isn't it.
> Based on what Jess has said, pursuing this approach seems ill-advised
> anyway.

It appears that while GCC 11.1 and GCC 11.2 do not pass -misa-spec=2.2
by default to as (contrary to GCC 11.3), they do accept getting called
with that option and correctly pass it to as. That way your patch works
fine with all GCC 11 and GCC 12 versions.

That said I found various discussions on IRC from that time, and forcing
an old ISA instead of enabling additional extensions was discouraged.
From what I understand, it might create some incompatibilities in the
future when new extensions are added (IOW enabling an extension could be
incompatible with an old ISA spec).

--
Aurelien Jarno GPG: 4096R/1DDD8C9B
aurelien@aurel32.net http://www.aurel32.net

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