ACPI or for when ACPI support is disabled or not present in the kernel.
Basically, the nexus device is now split into two with some parts
(such as adding default ISA, MCA, and EISA busses if they aren't found
as well as support for PCI bus device ivars) being moved to the legacy
driver.
i386/isa/pcibus.c. This gets -current running again on multiple host->pci
machines after the most recent nexus commits. I had discussed this with
Mike Smith, but ended up doing it slightly differently to what we
discussed as it turned out cleaner this way. Mike was suggesting creating
a new resource (SYS_RES_PCIBUS) or something and using *_[gs]et_resource(),
but IMHO that wasn't ideal as SYS_RES_* is meant to be a global platform
property, not a quirk of a given implementation. This does use the ivar
methods but does so properly. It also now prints the physical pci bus that
a host->pci bridge (pcib) corresponds to.
