Re: [PATCH] riscv: Handle zicsr/zifencei issues between clang and binutils

[Conor Dooley <conor@kernel.org>]

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Hey Nathan,

On Mon, Mar 13, 2023 at 04:00:23PM -0700, Nathan Chancellor wrote:
> There are two related issues that appear in certain combinations with
> clang and GNU binutils.
> 
> The first occurs when a version of clang that supports zicsr or zifencei
> via '-march=' [1] (i.e, >= 17.x) is used in combination with a version
> of GNU binutils that do not recognize zicsr and zifencei in the
> '-march=' value (i.e., < 2.36):
> 
>   riscv64-linux-gnu-ld: -march=rv64i2p0_m2p0_a2p0_c2p0_zicsr2p0_zifencei2p0: Invalid or unknown z ISA extension: 'zifencei'
>   riscv64-linux-gnu-ld: failed to merge target specific data of file fs/efivarfs/file.o
>   riscv64-linux-gnu-ld: -march=rv64i2p0_m2p0_a2p0_c2p0_zicsr2p0_zifencei2p0: Invalid or unknown z ISA extension: 'zifencei'
>   riscv64-linux-gnu-ld: failed to merge target specific data of file fs/efivarfs/super.o
> 
> The second occurs when a version of clang that does not support zicsr or
> zifencei via '-march=' (i.e., <= 16.x) is used in combination with a
> version of GNU as that defaults to a newer ISA base spec, which requires
> specifying zicsr and zifencei in the '-march=' value explicitly (i.e, >=
> 2.38):
> 
>   ../arch/riscv/kernel/kexec_relocate.S: Assembler messages:
>   ../arch/riscv/kernel/kexec_relocate.S:147: Error: unrecognized opcode `fence.i', extension `zifencei' required
>   clang-12: error: assembler command failed with exit code 1 (use -v to see invocation)
> 
> This is the same issue addressed by commit 6df2a016c0c8 ("riscv: fix
> build with binutils 2.38") (see [2] for additional information) but
> older versions of clang miss out on it because the cc-option check
> fails:
> 
>   clang-12: error: invalid arch name 'rv64imac_zicsr_zifencei', unsupported standard user-level extension 'zicsr'
>   clang-12: error: invalid arch name 'rv64imac_zicsr_zifencei', unsupported standard user-level extension 'zicsr'
> 
> To resolve the first issue, only attempt to add zicsr and zifencei to
> the march string when using the GNU assembler 2.38 or newer, which is
> when the default ISA spec was updated, requiring these extensions to be
> specified explicitly. LLVM implements an older version of the base
> specification for all currently released versions, so these instructions
> are available as part of the 'i' extension. If LLVM's implementation is
> updated in the future, a CONFIG_AS_IS_LLVM condition can be added to
> CONFIG_TOOLCHAIN_NEEDS_EXPLICIT_ZICSR_ZIFENCEI.
> 
> To resolve the second issue, use version 2.2 of the base ISA spec when
> using an older version of clang that does not support zicsr or zifencei
> via '-march=', as that is the spec version most compatible with the one
> clang/LLVM implements and avoids the need to specify zicsr and zifencei
> explicitly due to still being a part of 'i'.
> 
> [1]: https://github.com/llvm/llvm-project/commit/22e199e6afb1263c943c0c0d4498694e15bf8a16
> [2]: https://lore.kernel.org/ZAxT7T9Xy1Fo3d5W@aurel32.net/
> 
> Cc: stable@vger.kernel.org
> Link: https://github.com/ClangBuiltLinux/linux/issues/1808
> Co-developed-by: Conor Dooley <conor.dooley@microchip.com>
> Signed-off-by: Conor Dooley <conor.dooley@microchip.com>

TBH, barely identifiable as having much to do with my V1 anymore, so
could easily have dropped those.
Either way, thanks for sorting this out while I've been sick :)

Acked-by: Conor Dooley <conor.dooley@microchip.com>

> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> ---
> This is essentially a v3 of Conor's v1 and v2 but since I am sending the
> patch after finding a separate but related issue, I left it at v1:
> 
> - v1: https://lore.kernel.org/20230223220546.52879-1-conor@kernel.org/
> - v2: https://lore.kernel.org/20230308220842.1231003-1-conor@kernel.org/
> 
> I have built allmodconfig with the following toolchain combinations to
> confirm this problem is resolved:
> 
> - clang 12/17 + GNU as and ld 2.35/2.39
> - clang 12/17 with the integrated assembler + GNU ld 2.35/2.39
> - clang 12/17 with the integrated assembler + ld.lld
> 
> There are a couple of other incompatibilities between clang-17 and GNU
> binutils that I had to patch to get allmodconfig to build successfully
> but those are less likely to be hit in practice because the full LLVM
> stack can be used with LLVM versions 13.x and newer.

> I will follow up
> with separate issues and patches.

Cool, I'll "look forward" to those...

Thanks,
Conor.

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