https://github.com/riscv/riscv-isa-manual/commit/f2ed45b1791bb602657adc2ea9ab5fc409c62542

https://github.com/riscv/riscv-isa-manual/issues/10

f81b722b is a regression.  If a
read-only segment does not begin on a page boundary, it would cause
the ELF loader to blow up.

* Before waiting for IPI by executing wfi instruction, MSIE bit
  should be set in mie CSR for other harts, else they will get
  stuck at the wfi instruction.

The time counter is enabled with the bit field set in `counteren` CSR.

add htif section in linker script, with ALIGNs to prevent MMIO and data from being placed on same page (#32)

Use IPIs to signal them.