Discusses virtualization under the assumption that the virtualized system is running the same ISA as the underlying hardware. However, one possible use of virtualization is to emulate non-native ISAs. An example of this is QEMU, which emulates a variety of ISAs such as MIPS, SPARC, and PowerPC. What are some of the difFiculties involved in this kind of virtualization? Is it possible for an emulated system to run faster than on its native ISA?

One of the biggest impediments to widespread use of virtual machines is the performance overhead incurred by running a virtual machine. The table below lists various performance parameters and application behavior.

a. b. Base CPI 1.5 1.75 Priviliged 0/S Accesses per 10,000 Instructions 120 90 Performance Impact to Trap to the Guest O/S 15 cycles 20 cycles Performance Impact to Trap to VMM 175 cycles 140 cycles 1/0 Accesses per 10,000 Instructions 30 25 1/0 Access Time (Includes Time to Trap to Guest 0/S) 1100 cycles 1200 cycles