System requirements
===================

The following extensions are expected to be supported by a system on which
Xen is run:
- Zbb:
  RISC-V doesn't have a CLZ instruction in the base ISA.
  As a consequence, __builtin_ffs() emits a library call to ffs() on GCC,
  or a de Bruijn sequence on Clang.
  Zbb extension adds a CLZ instruction, after which __builtin_ffs() emits
  a very simple sequence.
  The similar issue occurs with other __builtin_<bitop>, so it is needed to
  provide a generic version of bitops in RISC-V bitops.h
- Zihintpause:
  On a system that doesn't have this extension, cpu_relax() should be
  implemented properly.