1. A non-transitory computer readable medium comprising instructions, which when executed in a computer system comprising a virtual machine monitor (VMM) running on system hardware and supporting a virtual machine (VM) having a first application and a guest OS running therein, causes the computer system to carry out a method of processing file I/O operations for files, comprising: a first shim program intercepting file I/O calls from the first application, the first shim program running completely within the same VM as the first application and interfacing with the VMM using one or more hypercalls from the first shim to the VMM; the first shim program emulating file I/O semantics of an intercepted file I/O call by mapping a target file to which the intercepted file I/O call was directed to a first memory region; and the first application accessing a version of the target file from the first memory region using memory-mapped I/O.