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

Virtual Machines Software abstraction of a machine Software backward compatibility Convenient place for new services Strong isolation

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

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

Presentation Outline Background on UMLinux Discussion of optimizations Conclusions

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 OS Linux Operating System PC Hardware Guest Process UMLinux VMM

Hardware Equivalents

Performance

Original UMLinux Guest System Call Host operating system VMM Process Guest application Guest operating system

Move VMM to Host Kernel Host operating system VMM Process Guest application Guest operating system VMM Process

Performance

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

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

Performance

Original UMLinux Guest Application Context Switching proc_a mem proc_b mem RAM File process_a running 0x8000 Guest Application Memory proc_a mem proc_b mem 0x8000 RAM File process_b page at 0x8000 mapped in, process_b executes Guest Application Memory

Multiple Address Space Definitions Instead, modify hardware page table pointer Page Table Ptr Host operating system Guest OS guest proc a guest proc bswitchguest syscall

Final Performance Results

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?

Related Work User-Mode-Linux SUNY Palladium Mach, Exokernel, L4 Wisconsin Wind Tunnel

Further Areas of Improvement Asynchronous I/O Exception Handling