VMM Definition
System virtual machines are capable of virtualizing a full set of hardware resources, including a processor (or processors), memory and storage resources and peripheral devices. A virtual machine monitor (VMM, also called hypervisor) is the piece of software that provides the abstraction of a virtual machine. There are three properties of interest when analyzing the environment created by a VMM:
- Equivalence / Fidelity
- A program running under the VMM should exhibit a behavior essentially identical to that demonstrated when running on an equivalent machine directly.
- Resource control / Safety
- The VMM must be in complete control of the virtualized resources.
- Efficiency / Performance
- A statistically dominant fraction of machine instructions must be executed without VMM intervention.
In the terminology of Popek and Goldberg, a VMM must present all three properties. In the terminology used in the reference book of Smith and Nair (2005), VMMs are typically assumed to satisfy the equivalence and resource control properties, and those additionally meeting the performance property are called efficient VMMs.
Popek and Goldberg describe the characteristics that the Instruction Set Architecture (ISA) of the physical machine must possess in order to run VMMs which possess the above properties. Their analysis derives such characteristics using a model of "third generation architectures" (e.g., IBM 360, Honeywell 6000, DEC PDP-10) that is nevertheless general enough to be extended to modern machines. This model includes a processor that operates in either system or user mode, and has access to linear, uniformly addressable memory. It is assumed that a subset of the instruction set is available only when in system mode and that memory is addressed relative to a relocation register. I/O and interrupts are not modelled.
Read more about this topic: Popek And Goldberg Virtualization Requirements
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