Margo Seltzer, Yasuhiro Endo, Chris Small, and Keith Smith

Harvard University

Division of Engineering and Applied Sciences

October 31, 1996

Outline

• Why extensibility?
• Extensibility in VINO.
• Challenges in extensibility.
• Performance.

Why Extensibility?

• Systems optimize for the common case.
• Some important cases are uncommon.
• Phenomenon appears in many places.
  • Database servers.
    • Download queries.
    • Download new data types.
  • Web browsers.
    • Download applets.
  • Operating systems.
    • Download drivers.
    • Download entire subsystems.
    • Download minor modifications.

Extensibility in VINO

• Working assumptions
  • The OS frequently does almost the correct thing.
  • Often minor tweaks can fix major problems.
  • Minimize effort to modify kernel behavior.
• Design principles
  • Extensibility should be fine-grain (e.g., function call).
  • Extensions should look just like kernel code.
  • Extensions should be able to call kernel functions.

VINO Implementation

• New kernel design and implementation.
• Use NetBSD device drivers and locore.
• Object-oriented design (C++).
• Design for per-method extensibility.
  • Highly (overly?) modularized.
  • Encapsulate every policy decision in a method.
  • Two extension techniques:
    • Replace or extend methods.
    • Specify event handler.

Extensibility Challenges

• Three interfaces between extension and kernel.
• All three interfaces can be abused.
  • Interface: Kernel and extension share memory.
  • Problem: Extension reads/writes private kernel memory.

  • Interface: Kernel calls extension.
  • Problem: Extension returns invalid data (or doesn't return).

  • Interface: Extension can call other kernel functions.
  • Problem: Extension calls forbidden kernel functions.

Protecting the Kernel

• Extension accesses forbidden memory.
  • Software fault isolation (VINO).
  • Safe language (e.g., Java, Modula-3 [SPIN]).
• Extension returns invalid data.
  • Validate return values.
  • Time-out long running extensions.
• Extension calls forbidden functions.
  • Static check at download time.
  • Software fault isolation checks indirect jumps.
  • Check security--extensions have privileges of application that installed them.

Interface Abuse

• Misusing legal interface functions.
  • Fail to release locks.
  • Fail to free resources (e.g., memory).
• Operating system must detect these problems.
  • Time-out contested locks.
  • Resource limits.
• Trade-off between interface flexibility and potential for abuse.
  • Disallow locks; require lock-do-unlock interface.
  • Allow locks; support lock, do, ..., do, unlock interface.

Handling Failure

• Remove extension from kernel.
• Undo changes to kernel state made by extension.
  • Free memory.
  • Release locks.

Transactions

• Why?
  • Guarantee atomicity.
  • Single mechanism to enforce consistency.
  • Generally useful tool.
  • Allows nested extension calls.
• How?
  • Returns kernel to pre-extension state on failure.
  • Ensures that other threads do not depend on interim extension state.

Transaction Implementation

• Extensions invoked through wrapper.
  • Begin a transaction.
  • Switch stacks.
  • Calls extension.
  • Commits transaction.
• State changes must be logged.
  • State changes made by accessor methods.
  • Accessor methods write log records.
  • Log can be transient.
  • Implemented as a call stack of undo functions.
• If extension fails, abort transaction.
  • Jump to abort call stack.
  • Return through each "undo" function.

Performance

• Allowing extensibility has costs.
  • Extra levels of indirection.
  • Transaction overhead.
  • Validation of return value(s).
  • Cost of graft code.
  • Software fault isolation.
  • Abort cost.

Measuring Performance

Sample Grafts

• Measured costs on sample extensions.
  • VM Page eviction.
    • Keep important pages in memory.
  • File read ahead.
    • Support non-sequential, but known access.
  • Process scheduling.
    • Allows group scheduling.
  • Data encryption.
    • Adds new functionality.
    • Filter between user and file system.

Performance Overhead

Performance Summary

• 100-450ms total overhead.
  • Not cheap.
  • Negligible when savings is disk I/O.
  • Untuned implementation.
  • Not feasible for tiny performance improvement.

Conclusions

• Possible to build extensible OS.
• Extensible OS is a good idea.
• Performance trade-off is critical.
• Applicable beyond field of operating systems (e.g., to web browsers).

Future Work

• Finish building VINO.
  • Networking.
  • Naming.
• Build applications that use extensions to optimize performance.
• Interface design.
  • What types of extensions actually get used?
  • Revisit flexibility vs. performance trade-off.

E-mail:{chris,keith,margo,yaz}@eecs.harvard.edu

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