Virtual Machine Monitor

Overview

All computer problems can be solved with another layer of redirection.

VMM Definition

  • VMM transform the single machine interface into the illusion of many. Each interfaces is a replica of original computer system, complete with all of the processor instructions.

    VMM 能操纵 processors

  • VMM implemented by adding software to an execution for all Platform HW

    Platform HW: Mem, Processors, I/O devices

Why use VMM

许多商用 PC 都为大型机，闲置的时候有很多空闲算力，可以以 VMM 的形式抽象多个虚拟 PC，作为 Server 向其他用户提供计算服务。这样能提高物理机的利用率。

VMM Essential Features

  • Equivalence: VMMs are identical virtual platform, excepts some system resources differences, e.g. memory size.

    VMM 几乎和物理机一样，但是物理机分配给 VMM 的资源可以有差异

  • Isolation: resource control, VMM is a complete control of system resources

    VMMs couldn’t disturbe others

  • Efficiency: speed >> emulator/simulators(模拟器)

    Emulator(仿真器): QEMU, VMM 不需要多一步解释，肯定是要比 QEMU 快滴

How VMM Work

With or Without VMM?

  1. Without VMM, user mode communicate with kernel mode by syscall.
  1. 当前 OS 架构: 由 VMM 分割了 user-mode syscall, virtual HW/ `SW interface, physical HW/SW interface.

Run OS as user-level program

运作方式：依靠 CPU 来捕获 (trap) sensitive instructions，并将其移交给 VMM

  • VMM emulates guest OS’s instructions effects on HW.
  • VMM provides virtual HW/SW interface to guest OSs by trapping and emulating sensitive instructions.

Processes 由 CPU 控制，OS 仅提供服务

Two classes of instructions:

  1. priviledged instructions: those that trap when CPU is in user-mode
  2. sensitive instructions: which modify HW configuration or resources, 和一些需要依赖 HW configuration 的 behaviors

Memory with/without virtualization

  1. Without virtualization

VPN: Virtual Page Number

VA: Virtual Address

TLB: Translation Lookaside Buffer - 页表缓存

2. With Virtualization

VMM will check TLB miss handler and then the sensitive instructions reduced to priviledge instructions.