Performance in Practice
The efficiency requirement in Popek and Goldberg's definition of a VMM concerns only the execution of non-privileged instructions, which must execute natively. This is what distinguishes a VMM from the more general class of hardware emulation software. Unfortunately, even on an architecture that meets Popek and Goldberg's requirements, the performance of a virtual machine can differ significantly from the actual hardware. Early experiments performed on the System/370 (which meets the formal requirements of Theorem 1) showed that performance of a virtual machine could be as low as 21% of the native machine in some benchmarks. The cost of trapping and emulating privileged instructions in the VMM can be significant. This led the IBM engineers to introduce a number of hardware assists, which roughly doubled the performance of the System/370 virtual machines. Assists were added in several stages. In the end, there were over 100 assists on the late models System/370.
One of the main driving factors for the development of hardware assists for the System/370 was virtual memory itself. When the guest was an operating system that itself implemented virtual memory, even non-privileged instructions could experience longer execution times - a penalty imposed by the requirement to access translation tables not used in native execution (see shadow page tables).
Read more about this topic: Popek And Goldberg Virtualization Requirements
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