1. Exokernel: An Operating System Architecture for Application-Level Resource Management
Dawson R. Engler, M. Frans Kaashoek, and James O'Tool Jr.
M.I.T Laboratory for Computer Science
Cambridge, MA 02139, U.S.A
Presented by Jennifer Minor

2. What is a Kernel? So what is an Exokernel?
Definition from wiktionary.org:
The core, center, or essence of an object or system.
(computing) The central part of many computer operating systems which manages the system's resources and the communication between hardware and software components.
So what is an Exokernel?

3. A Monolithic Kernel is...
What is this?
All operating system services run in kernel mode.
Single address space.
High level abstractions given to application.
Must support a wide range of applications.
Slow to change.
System Calls are expensive.
This is a note.

4. A Microkernel is... Separate mechanism from policy.
Only lower level mechanisms are supported in kernel mode. (Address space management, scheduling and basic IPC)‏
Policies are implemented in user level which are easier to change.
Kernel must protect servers from each other.
Good protection but has to use IPC to communicate.

5. So an Exokernel is...
Similar to microkernel in that only minimum functionality is in the kernel.
Unlike the microkernel it exports hardware resources rather than emulating them.
Physical resources are safely allocated to the application, where it can be managed.
All abstractions are implemented in application-level or as part of a library OS that is part of the application address space.

6. Exokernel Architecture Goal: Separate protection from management.
1. Low-level interface:
Provide simple and efficient primitives.
2. Multiplex resources:
Securely and fine-grained.
3. Limit management to protection:
Protect without specific usage knowledge of resource.
4. Export hardware resources:
Expose hardware and kernel data structures.
5. Notify Application:
Event notifications and visible resource revocation.

7. Exporting Resources Securely
1. Secure Bindings
Hardware mechanisms
Software caching
Downloading application code
2. Visible Resource Revocation
Application level guided deallocation
Application specific knowledge of state needed to be saved
Application notification that resources are scarce
3. Abort Protocol
Mechanism for kernel to force-ably take back resources.
Still notifies application after the fact.

8. Aegis: an Exokernel Processor Time Slicing Timer Interrupts
Represents CPU as a linear vector partitioned time slices that can be allocated by the application.
Timer Interrupts
Denote the beginning and end of a time slice to the user-level code where scheduler activations can be implemented.
Processor Environments
Structures that store information needed to deliver events to applications. (Upcalls)‏
STLB
A large software TLB is over the hardware TLB and can be used on a cache miss to map address.
Guaranteed Mappings
Holds application data and code in memory. Also allows each application a small number of pinned virtual addresses.
Dynamic Code Generation
Creation of executable code at runtime. Used by the network subsystem to download filters for demultiplexing messages.
Protected Control Transfers
Changes the program counter to callee, donates current time slice to callee's processor environment and switches to the callee's context.
User level efficient IPC abstraction can easily be built on top of PCT's.

9. Aegis: Events Event Handler Contexts Include:
Four Types: Exceptions, Interrupts, Protect Control Transfers and Address Translations
Event Handler Contexts Include:
Program counter to jump to on event.
Memory location to save registers.
Additional status registers are needed for timer interrupts and tlb misses.
What happens on a hardware exception?
Aegis saves three scratch registers into the “save-area”.
Loads the exception program counter, the last virtual address translation and cause.
Performs a indirect jump into an applications-specified program counter.
Note: After handling the exception the application can resume execution without going back to the kernel.
Special event handlers have to be defined for start-time-slice, end-time-slice, asynchronous control transfers, and synchronous control transfers.

10. Aegis: Performance Why is performance so much better on Aegis?
Kernel data structures are not mapped. No need to worry about a interrupted TLB miss.
Two paths for system calls, one for calls that require a stack and a second for ones that do.

11. ExOS: a Library Operating System
Implements traditional operating system abstractions at the application level, since it runs in the applications address space.
Fault Isolation
Each application runs in it's own address space.
Efficient
No protection domain crossing to manage resources after they have been allocated.
System calls are near procedure call speed.
Extensible
Policies can be altered at application level.
IPC abstraction
Built on top of protected control transfers.
Virtual Memory
Using low-level hardware abstractions ExOS provides a rudimentary VM system.
Remote Communications
Downloading code into the kernel allows the demultiplexing of the messages without a context switch.

12. ExOS: IPC Performance
ExOS built a lrpc abstraction on top of the low-level protected procedure call interface given by Aegis.
Ultrix does not currently have a lrpc implementation to add new functionality it would need to build on top of one of the existing high-level abstractions such pipes.

13. ExOS: VM Performance
Kernel transitions can be eliminated by implementing abstractions at application level.
Application-level software can implement functionality that is frequently not provided by traditional operating system.

14. ExOS: Application-Specific Safe Handlers
ASH: Untrusted application- level message-handlers that are downloaded into the kernel, made safe with code inspection and sand boxing.
Reduces intermediate copies of message.
Can integrate check summing in transfer mechanism.
Low-latency message replies
Control initiation
Roundtrip Latency (microseconds)‏
Number of Processes

15. Why are Exokernels important?
Fixed high level abstractions hurt application performance
Fixed high level abstractions hide information
Fixed high level abstractions limit the functionality
"Because all applications must share the core abstractions, changes to core abstractions occur rarely, if ever. This is perhaps why few good ideas from the last decade of operating systems research have been adopted into widespread use. What operating systems support scheduler activations [3], multiple protection domains within a single address space [10], efficient IPC [29], or efficient and flexible virtual memory
primitives [4, 21, 25]?”

16. Exokernel Design Proves:
Resources can be securely partitioned with low overhead
Low-level interfaces and exposed kernel data structure can produce efficient implementation due to simplicity
Downloadable application code into the kernel increase performance and responsiveness
Library Operating Systems provide extensible and customizable services at application level.

17. References
MIT Exokernel Operating System
Wikipedia: Exokernel
Wikipedia: Kernel (computer science)
Wikipedia: MicroKernel
Wikipedia: Monolithic Kernel
Wiktionary: kernel