OS Virtualization

OS Virtualization

Tanenbaum 8.3

See references

Outline

• What is Virtualization?

• Why would we want it?

• Why is it hard?

• How do we do it?

• Choices cs431-cotter 2

What is Virtualization?

• OS virtualization

– Create a platform that emulates a hardware platform and allow multiple instances of an

OS to use that platform, as though they have full and exclusive access to the underlying hardware cs431-cotter 3

What is Virtualization?

Applications Applications Applications Applications

OS 1 OS 4 OS 2 OS 3

Virtualization Platform

Hardware cs431-cotter 4

Virtualization – Why?

• Server Consolidation

– Often many servers support 1 major application

– Strong isolation between VMs

– Virtualization saves on hardware & energy

• Disaster Recovery

• High Availability

• Testing and Deployment cs431-cotter 5

Virtualization – Why?

• Desktop Consolidation

– Support for legacy applications

– Software Development

– Training cs431-cotter 6

The Problem

• OS uses kernel mode / user mode to protect the OS.

– System calls ( privileged instructions ) generate a trap (software interrupt) that forces a switch to kernel mode

– These calls trigger sensitive instructions (I/O,

MMU control, etc.) that must only be executed by the kernel cs431-cotter 7

The Problem

• If our VM now runs in user space, we cannot run sensitive instructions in it, since those must trap to kernel space.

• Solved in 2005 with new CPUs

– Intel Core 2 – VT (Virtualization Technology)

– AMD Pacific – SVM (Secure Virtual Machine)

– Provides new instructions that allow VM to capture traps cs431-cotter 8

Implementation

• Type 1 Hypervisor

• Type 2 Hypervisor

• Paravirtualization cs431-cotter 9

Type 1 Hypervisor

• Runs on “bare metal”

• Virtual machines run in user mode

– VM runs the guest OS (which thinks it is running in kernel mode) – Virtual kernel Mode

– If guest OS calls sensitive instructions, hypervisor will trap and execute the instructions.

– If application on guest OS calls sensitive instructions (system calls), hypervisor traps to guest OS.

cs431-cotter 10

Type 1 Hypervisors

Figure 8-26. When the operating system in a virtual machine executes a kernel-only instruction, it traps to the hypervisor if virtualization technology is present.

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Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-136006639

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Type 2 Hypervisor

• Runs from within a OS.

• Supports guest OSs above it.

– Boot from CD to load new OS

– Read in code, looking for basic blocks

– Then inspect basic block to find sensitive instructions.

If found, replace with VM call (process called binary translation )

– Then, cache block and execute.

– Eventually all basic blocks will be modified and cached, and will run at near native speed.

cs431-cotter 12

Type 2 Hypervisor

Applications Applications Applications

OS 1 OS 2 OS 3

Virtualization Platform

Base Operating System

Applications

Hardware cs431-cotter 13

Paravirtualization

• Modify Guest OS so that all calls to sensitive instructions are changed to hypervisor calls.

• Much easier (and more efficient) to modify source code than to emulate hardware instructions (as in binary translation).

• In effect, turns the hypervisor into a microkernel.

cs431-cotter 14

Paravirtualization (1)

Figure 8-27. A hypervisor supporting both true virtualization and paravirtualization.

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Problems with Paravirtualization

• Paravirtualized systems won’t run on native hardware

• There are many different paravirtualization systems that use different commands, etc.

– VMware, Xen, etc.

• Proposed solution:

– Modify the OS kernel so that it calls a special set of procedures to execute sensitive instructions ( Virtual Machine Interface )

• Bare metal – link to library that implement code

• On VM – link to VM specific library cs431-cotter 16

Paravirtualization (2)

Figure 8-28. VMI Linux running on (a) the bare hardware (b) VMware (c) Xen.

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Products (partial List)

• Microsoft – Virtual PC, Hyper-V

• QEMU – Processor Emulation & VM

• Sun Microsystems – xVM, VirtualBox

• VMware – ESX Server, Workstation,

Fusion, Player, Server

• Xen – Xen

• VirtualIron cs431-cotter 18

Memory Virtualization

• OS tracks mapping of virtual memory pages to physical memory pages.

• Builds page tables, then update paging register (trap).

• Allow hypervisor to manage page mapping, and use shadow page tables for the VMs cs431-cotter 19

Memory Virtualization

• Changes to page tables do NOT trap!

– One solution: Mark shadow page tables as read only. Then when VM tries to write to table, page fault traps to hypervisor.

– Paravirtualized OS: Since OS has been modified to account for hypervisor, page table updates can be followed by call to hypervisor about changes. cs431-cotter 20

I/O Virtualization

• Each guest OS holds its own “partition”.

– Typically implemented as a file or region on disk

– Hypervisor must convert guest OS address

(block #) into physical address in region

– May convert between storage types.

– Must deal with DMA requests cs431-cotter 21

VM on Multi-core CPUs

• Each core can be configured for multiple virtual machines.

– A Quad-core CPU could be configured as a

32 node multi-computer

– Limiting factor is often memory. Each guest

OS has its own requirements (512 MB?) cs431-cotter 22

Installing a Virtual machine

• Will first install VirtualBox as hypervisor

• Base OS is Windows 7

• Guest OS will be Ubuntu 12.04.1

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Installing VirtualBox

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Installing Ubuntu VM

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Installing

Ubuntu

VM cs431-cotter 32

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Summary

• Virtualization provides a way to consolidate OS installations onto fewer hardware platforms

• 3 basic approaches

– type 1 hypervisor

– type 2 hypervisor

– Paravirtualization

• Must also account for virtual access to shared resources (memory, I/O) cs431-cotter 36

References

• Virtual Machine Interface

– http://vmi.ncsa.uiuc.edu/

• VirtualBox

– https://www.virtualbox.org

• Xen Hypervisor (Red Hat Linux)

– http://www.xen.org/

• Virtual PC 2007

– http://www.microsoft.com

cs431-cotter 37

Questions

• In terms of resource allocation does a type 1 hypervisor leave more or less space for guest

OSs than a type 2 hypervisor? Why?

• In terms of a access to a guest OS, what is the difference between a bridged interface and a

NAT interface?

• What changes are needed to convert a guest

OS into a paravirtualized OS?

• Why has virtualization not been available on

PCs until recently (2005)?

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OS Virtualization

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