Xen and the Art of Virtualization

Class note

Overview

• High-performance VMM that support strict resource control among guest OSes

• A vitual machine monitor?

  • exports virtual machine abstraction

• domain: a running virtual machine within which a guest OS executes

Hardware

• Run 多个os on a single physical machine
• Example: VMWare, Microsoft’s VitrualPC / VirtualServer, Parallels (Macintosh), Xen

A Java virtual machine (JVM) is an abstract computing machine that enables a computer to run a Java program. There are three notions of the JVM: specification, implementation, and instance. The specification is a document that formally describes what is required of a JVM implementation.

hypervisor

A computer on which a hypervisor is running one or more virtual machines is defined as a host machine. Each virtual machine is called a guest machine.

Type-1, native or bare-metal hypervisors These hypervisors run directly on the host's hardware to control the hardware and to manage guest operating systems.

Type-2 or hosted hypervisors These hypervisors run on a conventional operating system just as other computer programs do.

VMM Implementation Overview

VMM is an operating system that exposes a (virtual) hardware interface

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Emulator and VM

Emulator is an interpreter, VM is a real hardware interface

Benefits of VM in Company

• Server consolidation
• Isolation

Internet Services

• Many applications now run inside the Internet
• Goal: allow anyone to upload to a new service into the Internet
• Approach: use VMMs to provide a rent-a-server economy

EC2

• Scalability
  • New machines can be created in minutes
• Security
  • Isolation
• Developer control
• Low cost

Xen Requirement

• isolation among virtual machines

Paravirtualization

Paravirtualization is an enhancement of virtualization technology in which a guest OS is recompiled prior to installation inside a virtual machine. Paravirtualization allows for an interface to the virtual machine that can differ somewhat from that of the underlying hardware.

• Different from full virtualization? Full: Directly run on the top

Os can’t directly control the hardware Hyper call: call Hyperviosr

Xen provides Machine interface

Comparing the Unix and VMM APIs

Boundary

• Both the guest OS and applications run in (physical) user-mode
• For each virtual machine, the VMM keeps a software mode bit:
  • During a system call, switch to “kernel” mode
  • On system call return, switch to “user” mode

CPU Virtualization

• How Xen virtualize the CPU
  • Guest OS no longer run on highest privilege
    • using ring 1 instread of ring 0
  • Explicitly schedule CPU among guest OSes

Memory

• Virtual Memory (application) -> Physical Memory (Guest OS)-> Machine Memory (VMM)
• (increasing privilege)
• assume a software-loaded TLB