kernel/doc/x86/irq.md

Full IRQ routing?

External IRQ sources:

• Legacy IRQ lines
• PCI IRQ pins (INTA#/INTB#/INTC#/INTD#)

Each of these is routed to a single GSI (IRQ of I/O APIC), which in turn is mapped to a single interrupt vector in any of the Local APICs

Mapping legacy IRQ to a handler: If I/O APIC is available

1. Get 8259 IRQ -> I/O APIC Line mapping
2. Allocate an interrupt vector/reuse shared vector
3. Add a mapping in I/O APIC
4. Add entry to IRQ handlers list
5. Unmask the GSI Otherwise:
6. Add entry irq_number to IRQ handlers list

I/O APIC init:

1. Assign I/O APIC address variable
2. Relocate existing legacy handlers to I/O APIC