The VT-d spec defines the BIOS considerations about RMRR in section 8.4:
BIOS must report the RMRR reported memory addresses as reserved (or as
EFI runtime) in the system memory map returned through methods such as
INT15, EFI GetMemoryMap etc.
So we should treat it as firmware bug if the RMRR range is not mapped as
RESERVED in the system memory map table.
As for how should the driver handle this case, ignoring buggy RMRR with
a warning message might be a possible choice.