- kernel turns on IO APICs if no_apic is _not_ set or is equal 0
- pci driver must use the acpi driver to setup IRQ routing otherwise
the system cannot work correctly except systems like KVM that use
only legacy (E)ISA IRQs 0-15
- the ability for kernel to use ACPI tables to detect IO APICs. It is
the bare minimum the kernel needs to know about ACPI tables.
- it will be used to find out about processors as the MPS tables are
deprecated by ACPI and not all vendorsprovide them.