I found a similar issue here: Pcie ethernet i210 flash failed. From this I believe the driver is actually reading the bus address (0x4000_0000) rather than the system address (0x1f_4000_0000). The fix from that link seems to be to manually change the read address, rather than moving the BAR itself. Unfortunately the driver is still expecting a 32-bit value, so it won’t be able to store 0x1f_4000_0000. This is why I am trying to change the BAR to something with a 32-bit address.