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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???
