Microkernel

1
Microkernel

• L4

2
Whats L4 ?

• Originally, L4 is the name of a second-generation
  microkernel designed and implemented by Jochen
  Liedtke, running on i486 and Pentium CPUs.
• Now, there are numerous implementations of the L4
  API on several hardware architectures.

3
Implementations of the L4 µ-Kernel interface
4
Design paradigm (1/2)

• Jochen Liedtke A microkernel does no real
  work.
• Kernel provides only inevitable mechanisms.
• No policies enforced by the kernel.
• What is inevitable ?

5
Design paradigm (2/2)

• L4 microkernel only provides the four basic
  mechanisms
• a thread model
• a mechanism for synchronous thread communication
• Scheduling
• an address space abstraction

6
L4 architecture
7
L4 architecture (with L4Linux)
8
L4 thread

• Identified by thread id
• Consists of
• Instruction pointer
• Stack
• Registers, flags,
• Etc.
• L4 only manages (preserves) IP, SP and registers.
• Entry point, stack allocation is managed by
  user-level applications

9
L4 address spaces (1/2)

• Protection domain
• Container for code and data
• Constructed of memory pages
• Three operations
• map
• Share page with other address space.
• grant
• Give page to other address space.
• unmap
• Remove page from other address space

10
L4 address spaces (2/2)

• Recursive address Spaces

11
L4 IPC (1/6)

• Synchronous
• sender and receiver have to meet and agree to
  communicate
• Secure
• they do not need to trust each other
• Unbuffered
• no in-kernel message caching
• Blocking
• a partner can wait for another
• Direct
• the partner is directly addressed

12
L4 IPC (2/6)

• Passing Short Messages
• A high percentage of IPC messages are very short.
  Transfer those messages in registers.
• Copying Large Data Messages
• L4 allows for single-copy transfers by
  temporarily sharing the target region with the
  sender.
• Lazy Scheduling

13
L4 IPC (3/6)

• Operations
• Send only
• Receive only (close wait)
• (from a specified thread)
• Wait (open wait)
• (for any thread)
• Call
• (send to specific thread, receive from that
  thread)
• Reply and wait
• (send to specific thread, wait for any)
• call and reply-and-wait are optimizations for
  typical client-server architecture

• Message types
• Register message
• (Short IPC)
• Bulk data
• (direct string)
• Memory buffer references
• (indirect string)
• Mappings
• (flexpages)

14
L4 IPC (4/6)

• Direct string

15
L4 IPC (5/6)

• Indirect string

16
L4 IPC (6/6)

• Others
• IDL compiler
• generate IPC-related code from IDL specification
• DICE (IDL compiler for DROPS)

17
Designing a microkernel

• What is the main goal for this kernel ?
• Performance? Robustness? Simplicity?
• What is the main application will run on this
  kernel ?
• Multimedia? Communication? Editor? or General?
• What special instructions dose hardware support ?
• Branch instructions
• Multimedia related instructions
• What is inevitable in microkernel ?
• (refer to page 4)
• How to improve IPC performance ?
• Reduce overhead
• Ex. security, encoding, etc.
• Make the common case fast
• Short IPC using registers
• How to test the system ?
• Benchmark, simulator, etc.

18
Reference

• L4hq.org
• http//l4hq.org/docs/
• lecture in Dresden
• http//os.inf.tu-dresden.de/Studium/MkK/
• http//os.inf.tu-dresden.de/Studium/KMB/
• http//i30www.ira.uka.de/teaching/courses/lecture.
  php?courseid99
• http//www.cs.cornell.edu/home/ulfar/ukernel/ukern
  el.htmlcurrent-l4.