MIT’s Exokernel Presented by Victoria Barrow Kyle Safford Sean Sommers

Traditional Operating Systems –Overly Complicated More included than necessary –Inflexible Abstractions are hardcoded and hard to change –Inefficient Application programmers have to work around the hardcoded abstractions

The Exokernel’s Design Principles –Separation of protection from management Library Operating Systems manage resources Exokernel ensures protection of resources –Expose as much of the resources as possible while still ensuring security

Techniques to Export and Expose Resources Securely –Secure bindings –Visible revocation –Abort Protocol

The Aegis Exokernel Represents CPU as linear vector of time slices. Can be allocated similarly to physical memory. Beginning and end time slices denoted by timer interrupts. (similar deliver to exceptions) General purpose context switching done through the applications’ handlers.

Aegis Scheduling and Processor Environments Scheduling of the time slices is done Round Robin style. Scheduling of Library OSes, not of processes and threads. Fairness achieved through binding the time an application takes to save its context. Processor Environments store information needed to deliver events. Contain four contexts to support four kinds of events. –Exception context –Interrupt context –Protected Entry context –Addressing context

Procedure & System calls Exceptions Aegis has two system call paths –do not require stack/ require stack Exceptions –unalign (unaligned pointer access) –overflow –coproc (floating point) –prot (access to protected pages)

Address Translations &Protected Control Transfers Address Translations –Two problems with supporting application level VM. Must provide bootstrapping for virtual naming system Must support virtual memory efficiently Protected Control Transfers –Provides synchronous & asynchronous for efficient implementations of IPC abstractions. –Both guaranty two important properties An atomic protected control transfer. Aegis will not overwrite any application visible register.

Xok/ExOS Xok –Utilized for x86-based computers –Provides protection for accessing higher-level objects –Multiplexes like Aegis and uses round-robin-scheduling –Implements a low-level interface and allows versatility –Implements predicates –Requires explicit credentials to allow access

Xok/ExOS ExOS –LibOS that supports abstractions –Utilizes a process map –Utilizes a global file descriptor table –Must use shared libraries

Xok/ExOS Performance

Conclusion Questions???