External interrupts are passed through to the guest by setting `hip.VSEIP`, which the guest OS sees as `sip.SEIP`.
The hypervisor software decides which interrupts should be passed through to which guest OS.
If the hardware has GEILEN>0 then the hypervisor software can configure `hstatus.VGEIN` and `hgeie` such that `hip.VSEIP` can be set by the external interrupt directly.
If the hardware has GEILEN=0 then the external interrupt will first go to the hypervisor software, which can pass through the interrupt to the guest by setting `hvip.VSEIP`.
