[RISCV][docs] Add some wording around vendor extensions

[reames]

Summary

This adds an initial bit of policy around inclusion of vendor extensions. My intention here is to leave all of the actual decision making to a case by case decision on the regular sync calls, but to spell out some of the pieces we've discussed and (I think) have general agreement on.

[craig.topper]

llvm/docs/RISCVUsage.rst

154

I think it's hypenated as "Non-Standard" in the spec.

[reames]

Match standard usage of hyphen and capitalization.

[craig.topper]

LGTM

[asb]

Two suggestions, but otherwise looks good to me. Thanks!

llvm/docs/RISCVUsage.rst

152

This is the first and only reference to "RVI" in this document. It would be better to spell it out as "RISC-V International"

156

"for discussion" -> "for initial discussion, which may lead to a request to write up an RFC on LLVM's Discourse".

We don't have a policy here yet - I suspect it might not be the case that _every_ vendor extension needs an RFC (e.g. cases where there's a small number of instructions supported only at the MC layer), but I think it would be good to flag that there are cases where broader LLVM developer buy-in might be necessary, and also make it clear that nothing in this document implies discussion on the sync-up calls bypasses that.

[reames]

llvm/docs/RISCVUsage.rst

152

Good catch, done.

156

I landed without this change because I didn't feel the wording you suggested clarified meaning. I understand your point, but "for discussion" doesn't imply anything to me about it being a final discussion or otherwise bypassing usual decision making.

I'm open to trying to clarify this if you think it's needed, but I think it probably needs phrased differently.

[asb]

llvm/docs/RISCVUsage.rst

156

I can't immediately think of better phrasing and it's not a big deal either way. I think you see what I'm trying to clarify but I agree attempts to do so perhaps just make it more verbose and less clear.

llvm/docs/RISCVUsage.rst

 ``experimental-ztso``
   LLVM implements the `v0.1 proposed specification <https://github.com/riscv/riscv-isa-manual/releases/download/draft-20220723-10eea63/riscv-spec.pdf>`_ (see Chapter 25).  Using will set appropriate ELF flags and attributes, but does not yet change code generation.
 
 ``experimental-zvfh``
   LLVM implements `this draft text <https://github.com/riscv/riscv-v-spec/pull/780>`_.
 
 To use an experimental extension from `clang`, you must add `-menable-experimental-extensions` to the command line, and specify the exact version of the experimental extension you are using.  To use an experimental extension with LLVM's internal developer tools (e.g. `llc`, `llvm-objdump`, `llvm-mc`), you must prefix the extension name with `experimental-`.  Note that you don't need to specify the version with internal tools, and shouldn't include the `experimental-` prefix with `clang`.
 
+Vendor Extensions
+=================
+
+Vendor extensions are extensions which are not standardized by RISC-V International, and are instead defined by a hardware vendor.  At the moment, LLVM does not support any vendor extensions for RISC-V, but we expect this to change in the future.
+
+The term vendor extension roughly parallels the definition of a `non-standard` extension from Section 1.3 of the Volume I: RISC-V Unprivileged ISA specification.  In particular, we expect to eventually accept both `custom` extensions and `non-conforming` extensions.
+
+Inclusion of a vendor extension will be considered on a case by case basis.  All proposals should be brought to the bi-weekly RISCV sync calls for discussion.  For a general idea of the factors likely to be considered, please see the `Clang documentation <https://clang.llvm.org/get_involved.html>`_.
+
+It is our intention to follow the naming conventions described in `riscv-non-isa/riscv-toolchain-conventions <https://github.com/riscv-non-isa/riscv-toolchain-conventions#conventions-for-vendor-extensions>`_.  Exceptions to this naming will need to be strongly motivated.
+
+
 Specification Documents
 =======================
 For ratified specifications, please refer to the `official RISC-V International
 page <https://riscv.org/technical/specifications/>`_.  Make sure to check the
 `wiki for not yet integrated extensions
 <https://wiki.riscv.org/display/HOME/Recently+Ratified+Extensions>`_.