Userkernel

Userkernel is a term used in computer science to describe an architectural approach in which kernel functionality is implemented or exposed from within user space rather than solely in a traditional privileged kernel. In such designs, the boundary between user applications and kernel services is reimagined, with some kernel-like services provided by a user-space component that offers the same interfaces as a kernel, while access to hardware is mediated by a small trusted layer such as a hypervisor, a minimal kernel, or a hardware abstraction layer.

In practice, userkernel concepts appear in two related forms. One is a user-space kernel library or library operating system that delivers OS abstractions—such as process management, memory management, and interprocess communication—to applications within a user-space environment. Another form aligns with philosophies like microkernels or exokernels, where core services run in user space and communicate through message passing with a small, trusted kernel or with architectural support for safety and isolation.

Advantages of a userkernel approach include easier debugging and higher isolation, as failures can be confined to user-space components; potential for greater portability and experimental exploration of OS interfaces; and the possibility of reducing the risk of catastrophic kernel failures. Drawbacks include potential performance overhead from user-space mediation, increased complexity in ensuring security and correctness across boundary crossings, and heavy reliance on a robust boundary to hardware and privileged resources.

Userkernel is primarily discussed in academic and research contexts as a platform to explore OS design trade-offs relative to monolithic kernels, microkernels, exokernels, and unikernel configurations. Related concepts include microkernel, exokernel, and library operating systems.