I don't understand: " I really like Azul Systems' hack of using the EPT/RVI virtualized page tables as a hardware read barrier for real-time garbage collection,

I don't understand: " I really like Azul Systems' hack of using the EPT/RVI virtualized page tables as a hardware read barrier for real-time garbage collection, and with a single-address space operating system I think you can do the same thing on hardware with a regular MMU." I understand real-time garbage collection, but what is EPT/RVI virtualized page?