Originally, this patch is target to get rid of ioapic changes in dom0. Before this patch, GSI irq should be mapped and setup through dom0 programming ioapic entries, but it depends on using ioapic logic in dom0. And if we remove ioapic logic from dom0, we need to find new way how to setup GSI irq. And this patch comes out under this situation. The idea is from that in Xen the interface MAP_PIRQ_TYPE_MSI is used to build the pirq and irq mapping for MSI IRQ for each domain. Since MSI IRQ can be setup through this hypercall, and I think we also can leverage the interface MAP_PIRQ_TYPE _GSI to build the mapping for GSI irq. Further analysis showes that this interface is only used for assigning devices to HVM domain in qemu, and I think it should be Okay for dom0 building the mapping between its pirq and irq. One different thing for GSI irq is that more info should be provided in the call, since GSI IRQ has different trigger-mode and polarity (originally it is provided by ioapic write in dom0). Certainly, I also think we need to document the related info, and if you agree to the change, I am happy to add it.