1. Operating System Support for Virtual Machines
Sam King
George Dunlap
Peter Chen
CoVirt Project, University of Michigan

2. Operating System Support for Virtual Machines
Software abstraction of a machine
Software backward compatibility
Convenient place for new services
Strong isolation
Sam King
Operating System Support for Virtual Machines

3. Virtual Machine Configurations
Type I VMM
Hardware
Virtual Machine Monitor
Guest Operating System
Guest Process
Guest Process
Guest Process
Guest Operating System
Virtual Machine Monitor
Host Operating System
Hardware
Type II VMM
Runs directly on hardware
Good performance
Uses existing host OS abstractions to implement services
Poor performance
Sam King
Operating System Support for Virtual Machines

4. Can we have Type I performance with Type II elegance?
Yes!
Only 510 lines of code added to Linux
Type II VMM attain performance that approaches type I VMM
Sam King
Operating System Support for Virtual Machines

5. Operating System Support for Virtual Machines
Presentation Outline
Background on UMLinux
Discussion of optimizations
Conclusions
Sam King
Operating System Support for Virtual Machines

6. UMLinux Architecture Linux on top of Linux
Port of Linux to run in UMLinux
ptrace used for virtualization
intercept guest system calls
track guest user / kernel mode transitions
Guest Process
UMLinux VMM
Guest OS
Linux Operating System
PC Hardware
Sam King
Operating System Support for Virtual Machines

7. Operating System Support for Virtual Machines
Hardware Equivalents
Sam King
Operating System Support for Virtual Machines

8. Operating System Support for Virtual Machines
Performance
Sam King
Operating System Support for Virtual Machines

9. Original UMLinux Guest System Call
Guest application
VMM Process
Guest operating system
Host operating system
Sam King
Operating System Support for Virtual Machines

10. Move VMM to Host Kernel Guest application VMM Process
Guest operating system
VMM Process
Host operating system
Sam King
Operating System Support for Virtual Machines

11. Operating System Support for Virtual Machines
Performance
Sam King
Operating System Support for Virtual Machines

12. Operating System Support for Virtual Machines
Switching Guest Modes
Significant number of guest kernel / user mode crossings
Kernel compile benchmark:
22 million guest memory exceptions
1.4 million guest system calls
Protection provided using mprotect system calls
Instead, use x86 segmentation hardware
Sam King
Operating System Support for Virtual Machines

13. Use Segmentation Bounds for Kernel Protection
Guest OS 0x
Guest
Apps 0x
guest kernel-mode
segment bound
Host OS
0xffffffff
0xc
Accessible
Memory
Guest OS 0x
Guest
Apps 0x
guest user-mode
segment bound
Host OS
0xffffffff
0xc
Accessible
Memory
Sam King
Operating System Support for Virtual Machines

14. Operating System Support for Virtual Machines
Performance
Sam King
Operating System Support for Virtual Machines

15. Original UMLinux Guest Application Context Switching
RAM
File
RAM
File
Guest Application Memory
Guest Application Memory
proc_a
mem
proc_a
mem
0x8000
0x8000
proc_b
mem
proc_b
mem
process_a running
process_b page at 0x8000 mapped in, process_b executes
Sam King
Operating System Support for Virtual Machines

16. Multiple Address Space Definitions
Instead, modify hardware page table pointer
guest proc a
guest proc b
Guest OS
switchguest syscall
Page Table Ptr
Host operating system
Sam King
Operating System Support for Virtual Machines

17. Final Performance Results
Sam King
Operating System Support for Virtual Machines

18. Operating System Support for Virtual Machines
Conclusion
Existing OS abstractions sufficient, slow
Fast interception of signals and exceptions
Expose power of underlying hardware
Type II VMM CAN be as fast as type I
Questions?
Sam King
Operating System Support for Virtual Machines

19. Operating System Support for Virtual Machines
Related Work
User-Mode-Linux
SUNY Palladium
Mach, Exokernel, L4
Wisconsin Wind Tunnel
Sam King
Operating System Support for Virtual Machines

20. Further Areas of Improvement
Asynchronous I/O
Exception Handling
Sam King
Operating System Support for Virtual Machines