Systems and methods are described for communications across privilege domains within a central processing unit (“CPU”) core. The CPU core can store a kernel context associated with an operating system within the CPU. An application can request access to the CPU, and the CPU can load a user context associated with the application into the CPU. The CPU can execute instructions from the application while both the kernel context and the user context persist in the CPU. Because both contexts are stored on the CPU, the CPU can switch contexts without loading or unloading context data from memory.