riscv: Handle zicsr/zifencei issues between clang and binutils

riscv: Handle zicsr/zifencei issues between clang and binutils

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>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230313-riscv-zicsr-zifencei-fiasco-v1-1-dd1b7840a551@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>