* linux drivers really use it

* 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.

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

* They are really the subset of mstatus and mie. We should refactor the
  access of CSR to support this.

* fix the cause number, the interrupt bit should set with
  0x8000000000000000, instead of 0x80000000