UML has a new IO memory interface. This allows a file on the host to be mapped into UML as a separate, named region of memory. A driver would request the region and then provide some appropriate userspace access to it.
The demo driver provided with UML allows a process to mmap the region into its own address space. The process can then read and write the file through memory accesses in its own address space.
The purpose of this interface is to allow UML to access simulated hardware. So, on the host, there would be a process emulating a device which might be treating the file as a set of device registers, watching for changes, interpreting them as commands, simulating those commands, and writing results back into the file for the UML driver to see.