Re: [RFC bpf-next 1/2] bpf, mips: Factor register moves into helpers

Thanks for the review.

> This is a pre-existing issue, but does this switch case in build_insn()
> silently miscompile BPF_MOVSX (move with sign extension) instructions?

Yes.  Patch 1/2 is a behavior-preserving helper extraction, so that
intermediate retains the existing behavior.

> Since you are refactoring the MOV handlers here, should emit_mov_r32() be
> updated to take insn->off and implement the proper sign extension logic?

Patch 2/2 does that for both emit_mov_r32() and emit_mov_r64().  It passes
insn->off to the move helpers.  They handle these verifier-valid widths:
8 and 16 bits for ALU32, and 8, 16, and 32 bits for ALU64.  An offset of
zero remains an ordinary move.  The verifier-inserted zero-extension move
stays on its separate path.