The interrupt issuing mechanism of PCI Express device

1. Disable MSI and APIC
PCI-e->[INTx msg]->ICH-> [interrupt routing]->8259->CPU

2. Disable MSI and Enable APIC
PCI-e->[INTx msg]->ICH->[interrupt routing]->I/O_APIC->[inter-APIC message]->system_bus->local_APIC->CPU

3. Enable MSI and Enable APIC
PCI-e->[issue the memory write transaction with MSI data]->ICH->[do PCI-e packet routing]->I/O_APIC->[MSI message]->system_bus->local_APIC->CPU