In order to virtualize the CPU, the operating system needs to somehow share the physical
CPU among many jobs running seemingly at the same time. The basic idea is simple: run
one process for a little while, then run another, and so forth. By time sharing the CPU in this
manner, virtualization is achieved. There are a few challenges, however, in building such
virtualization machinery. The first is performance: how can we implement virtualization without
adding excessive overhead to the system? The second is control: how can we processes efficiently
while retaining control over the CPU? Control is particularly important to the OS, as it is in
charge of resources; without control, a process could simpley run forever and take over the
machine, or access information that it should not be allowed to access. Attaining performance
while maintaining control is thus one of the central challenges in building an Operating System.
Operating System: Three Easy Pieces --- Mechanism: Limited Direct Execution (Note)
原文：http://www.cnblogs.com/miaoyong/p/4855584.html