One of the reasons why hardware vendors want to move away from
traditional accesses is to be able to use the larger config space in
PCI-Express, rather than being locked into the 256-byte legacy PCI
Several modern PCI-Express devices utilize the upper config space, but
due to legacy reasons the registers are usually ones that do not require
OS drivers to know about (like BIST stuff or diagnostic registers).
Expect that to change, as MS shakes out the bugs (or maybe we are doing
their job for them?).