When reviewing the RISC-V port of the change, I noticed the comment in the x86 code is worded incorrectly:

  // Record a new watermark, unless the update is above the safe limit.
  __ cmpptr(rsp, Address(thread, JavaThread::shadow_zone_safe_limit()));
  __ jccb(Assembler::belowEqual, L_done);

Stacks grow downwards, so we are recording a new watermark *when* update is above the safe limit.