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Tanenbaum 8.3 See references

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1 Tanenbaum 8.3 See references
OS Virtualization Tanenbaum 8.3 See references

2 Outline What is Virtualization? Why would we want it? Why is it hard?
How do we do it? Choices cs431-cotter

3 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

4 What is Virtualization?
Applications Applications Applications Applications OS 1 OS 2 OS 3 OS 4 Virtualization Platform Hardware cs431-cotter

5 Virtualization – Why? Server Consolidation Disaster Recovery
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

6 Virtualization – Why? Desktop Consolidation
Support for legacy applications Software Development Training cs431-cotter

7 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

8 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

9 Implementation Type 1 Hypervisor Type 2 Hypervisor Paravirtualization

10 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

11 4/20/2017 Type 1 Hypervisors Figure When the operating system in a virtual machine executes a kernel-only instruction, it traps to the hypervisor if virtualization technology is present. cs431-cotter Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved cs431-cotter

12 Type 2 Hypervisor Runs from within a OS. Supports guest OSs above it.
4/20/2017 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 cs431-cotter

13 Virtualization Platform
Type 2 Hypervisor Applications Applications Applications OS 1 OS 2 OS 3 Virtualization Platform Applications Base Operating System Hardware cs431-cotter

14 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

15 Paravirtualization (1)
4/20/2017 Paravirtualization (1) Figure A hypervisor supporting both true virtualization and paravirtualization. cs431-cotter Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved cs431-cotter

16 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

17 Paravirtualization (2)
4/20/2017 Paravirtualization (2) Figure VMI Linux running on (a) the bare hardware (b) VMware (c) Xen. cs431-cotter Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved cs431-cotter

18 Products (partial List)
4/20/2017 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 cs431-cotter

19 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

20 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

21 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

22 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

23 Installing a Virtual machine
4/20/2017 Installing a Virtual machine Will first install VirtualBox as hypervisor Base OS is Windows 7 Guest OS will be Ubuntu cs431-cotter cs431-cotter

24 Installing VirtualBox

25 Installing VirtualBox

26 Installing VirtualBox

27 Installing VirtualBox

28 Installing Ubuntu VM cs431-cotter

29 Installing Ubuntu VM cs431-cotter

30 Installing Ubuntu VM cs431-cotter

31 Installing Ubuntu VM cs431-cotter

32 Installing Ubuntu VM cs431-cotter

33 Installing Ubuntu VM cs431-cotter

34 Installing Ubuntu VM cs431-cotter

35 Installing Ubuntu VM cs431-cotter

36 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

37 References Virtual Machine Interface VirtualBox
VirtualBox Xen Hypervisor (Red Hat Linux) Virtual PC 2007 cs431-cotter

38 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)? cs431-cotter

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