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

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

2. 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

3. 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

4. Techniques to Export and Expose Resources Securely • Secure bindings • Visible revocation • Abort Protocol

5. 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.

6. 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 Aegis Scheduling and Processor Environments

7. 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) Procedure & System callsExceptions

8. 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. Address Translations &Protected Control Transfers

9. 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

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

11. Xok/ExOS Performance

12. Conclusion Questions???