* RET_MMIO_PF_EMULATE: it is a real mmio page fault, emulate the instruction
* directly.
* RET_MMIO_PF_INVALID: invalid spte is detected then let the real page
* fault path update the mmio spte.
* RET_MMIO_PF_RETRY: let CPU fault again on the address.
* RET_MMIO_PF_EMULATE: it is a real mmio page fault, emulate the instruction
* directly.
* RET_MMIO_PF_INVALID: invalid spte is detected then let the real page
* fault path update the mmio spte.
* RET_MMIO_PF_RETRY: let CPU fault again on the address.