riscv_priv_ext_without_insn_or_csrs

RISC-V: Add privileged extensions without instructions/CSRs

• Branch: riscv-priv-ext-without-insn-or-csrs
• Tracking PR: #48 (view Pull Request and Diff)
• Mailing List:
  • RFC PATCH v1 (2022-09-22)

Conflicts With

• RISC-V: Workaround for CSR implications to the Privileged Architecture
  It also adds Smepmp extension but corresponding change is exactly the same (so resolving the conflict is easy).

Issue Solved

Currently, GNU Binutils does not support following privileged extensions:

• Smepmp
• Svnapot
• Svpbmt

as they do not provide new CSRs or new instructions (Smepmp extends the privileged architecture CSRs but does not define the CSR itself). However, I started considering adding them to GNU Binutils' supported extension list.

One of the reason is simple: if we do that, we no longer have to filter ISA strings just for toolchains (if full ISA string is given by a vendor, we can straightly use it).

And there's a fact that supports this theory: there's already an (unprivileged) extension which does not provide CSRs or instructions (but only an architectural guarantee): Zkt (constant timing guarantee for certain subset of RISC-V instructions).

This simple patchset simply adds three privileged extensions I described above.