Documentation is now handled by the same processes we use for code: Add something to the Documentation/ directory in the coreboot repo, and it will be rendered to https://doc.coreboot.org/. Contributions welcome!
The MPTABLE can be autogenerated from devicetree.cb. This is done by a write_smp_table() declared weak in mpspec.c. If the mainboard doesn't provide it's own function, this generic implementation is called.
Syntax in devicetree.cb:
ioapic_irq <APICID> <INTA|INTB|INTC|INTD> <INTPIN>
The ioapic_irq directive can be used in pci and pci_domain devices. If there's no directive, the autogen code traverses the tree back to the pci_domain and stops at the first device which such a directive, and use that information to generate the entry according to PCI IRQ routing rules.