* QEMU does not use an LR bit. Is it safe enough?

* An AMO instruction may trigger page fault when storing the result back
  to memory. In such a case, we should not update the dest register. To
  fix this issue, we delay the register writeback after the store
  operation.

* it seems that qemu initialize mstatus with all 0s, which is wrong

Xv6

See merge request projectn/nemu!24

* do not call page_translate() in M mode

* With RVC, an instruction may cross page boundary. To fix this, we
  should fetch twice.

* machine level interrupts can not be delegated