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
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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
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4. What is Virtualization?
Applications
Applications
Applications
Applications
OS 1
OS 2
OS 3
OS 4
Virtualization Platform
Hardware
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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
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6. Virtualization – Why? Desktop Consolidation
Support for legacy applications
Software Development
Training
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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
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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
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9. Implementation Type 1 Hypervisor Type 2 Hypervisor Paravirtualization
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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.
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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.
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Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved
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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.
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13. Virtualization Platform
Type 2 Hypervisor
Applications
Applications
Applications
OS 1
OS 2
OS 3
Virtualization Platform
Applications
Base Operating System
Hardware
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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.
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15. Paravirtualization (1)
4/20/2017
Paravirtualization (1)
Figure A hypervisor supporting both true virtualization and paravirtualization.
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Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved
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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
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17. Paravirtualization (2)
4/20/2017
Paravirtualization (2)
Figure VMI Linux running on (a) the bare hardware (b) VMware (c) Xen.
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Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved
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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
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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
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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.
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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
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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?)
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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
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24. Installing VirtualBox
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25. Installing VirtualBox
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26. Installing VirtualBox
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27. Installing VirtualBox
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28. Installing Ubuntu VM
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29. Installing Ubuntu VM
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30. Installing Ubuntu VM
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31. Installing Ubuntu VM
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32. Installing Ubuntu VM
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33. Installing Ubuntu VM
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34. Installing Ubuntu VM
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35. Installing Ubuntu VM
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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)
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37. References Virtual Machine Interface VirtualBox
VirtualBox
Xen Hypervisor (Red Hat Linux)
Virtual PC 2007
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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)?
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