Enabling Memory Mapped IO > 4GB has issues on R720

Hello,

We have a PCI card which needs to expose 2GB of memory to the host. When we enable the "Memory Mapped IO > 4B"  option in the BIOS, the system goes into continuous reboot cycles. When we disable it, the memory is not exposed as expected. 

Could someone please help me to resolve this issue?

Thanks,

Sarma