It's basically really just a fixed address memory space of a certain fixed size that needs to be kept free by the resource allocator, very similar to the way it needs to be done for the IOAPIC and Local APIC.
Great. That makes it easy.
Since the PCIe BAR is part of the northbridge, I tried to tell the resource allocator to do this in the northbridge.c code, but I couldn't seem to figure out a way to make the (old) allocator honor it.
There was no check for fixed resources before. There was a #warning in compute_allocate_resources about it. It just set the limits below 0xfec00000, and as long as your fixed resources were above that you were fine.
The gotcha here is that I haven't implemented fixed resources in the middle of the rest of the PCI memory address space. If you declare a fixed region 0xf4000000-0xf4000007 you can no longer use 0xf4000008-0xfec00000. All of the PCI BARs have to fit on one side (above or below) the fixed regions.
I don't think that's much of an issue, but it is there.