mapping "random" physical memory into kernel

Can somebody please remind me what the supported way to establish a
kernel virtual address for a "random" chunk of physical memory is?
I'm currently using pmap_mapdev() directly, but that just feels
dirty...

I need to twiddle the settings of an Nvidia NF4 bridge in extended
PCI config space (offset 0x178).  These offsets are not accessible
via normal pci config space writes, but are doable when you use the
0xe000000 mapping.

The problem is that this memory is not really associated with
anything, so a normal bus_alloc_resource() allocation doesn't
seem like it would work.

Thanks,

Drew