1. A method of controlling access to shared virtual address space within a data processing system, the method comprising: associating, by a hypervisor of the data processing system, a memory protection key with a portion of a single logical partition's defined virtual address space, and allowing the single logical partition's defined virtual address space to be shared by multiple partitions of the data processing system, the memory protection key preventing access by a first partition of the multiple partitions to the portion of the single logical partition's virtual address space, and allowing access by a second partition of the multiple partitions to the portion of the single logical partition's virtual address space; and locking by the hypervisor the memory protection key from modification by the first partition of the multiple partitions sharing the single logical partition's virtual address space, wherein the locking prevents the first partition from modifying the memory protection key and thereby gaining access to the portion of the single logical partition's defined virtual address space with the associated memory protection key.