1. E6998 - Virtual Machines Lecture 4 Device Virtualization
Scott Devine
VMware, Inc.

2. Outline Types of Device Virtualization Storage Virtualization
Direct Access
Emulated
Para-virtualized
Storage Virtualization
Copy-on-Write Disks

3. Computer System Organization
CPU
Memory
MMU
Controller
Local Bus
Interface
High-Speed
I/O Bus
Frame Buffer
NIC
Controller
Bridge
LAN
Low-Speed
I/O Bus
CD-ROM
USB

4. Device Virtualization
Goals
Isolation
Multiplexing
Speed
Mobility
Interposition
Device Virtualization Strategies
Direct Access
Emulation
Para-virtualization

5. Direct Access Device CPU VM Local Bus High-Speed I/O Bus Low-Speed
Guest OS
CPU
Memory
MMU
Controller
Local Bus
Interface
High-Speed
I/O Bus
Frame Buffer
NIC
Controller
Bridge
LAN
Low-Speed
I/O Bus
CD-ROM
USB

6. Memory Isolation w/ Direct Access DeviceVM
Guest OS
CPU
Memory
MMU
Controller
Local Bus
Interface IOMMU
High-Speed
I/O Bus
Frame Buffer
NIC
Controller
Bridge
LAN
Low-Speed
I/O Bus
CD-ROM
USB

7. Virtualization Enabled Device
VM1
Guest OS
VM2
Guest OS
CPU
Memory
MMU
Controller
Local Bus
Interface IOMMU
High-Speed
I/O Bus
vNIC 1
vNIC 2
Frame Buffer
NIC
Controller
Bridge
LAN
Low-Speed
I/O Bus
CD-ROM
USB

8. Direct Access Device Virtualization
Allow Guest OS direct access to underlying device
Positives
Fast
Simplify monitor
Limited device drivers needed
Negatives
Need hardware support for safety (IOMMU)
Need hardware support for multiplexing
Hardware interface visible to guest
Limits mobility of VM
Interposition hard by definition

9. Emulated Devices Emulate a device in class Convert
Emulated registers
Memory mapped I/O or programmed I/O
Convert
Intermediate representation
Back-ends per real device

10. Serial Port Example User App Guest Host OS Monitor Serial Chip ABC
Emulation
Generic Serial Layer
Serial Chip
ABC Driver
Host OS
Monitor
Serial Chip XYZ
LAN

11. Emulated Devices Positives Negatives Platform stability
Allows interposition
No special hardware support needed
Isolation, multiplexing implemented by monitor
Negatives
Can be slow
Drivers needed in monitor or host

12. Para-Virtualized Devices
Guest passes requests to Monitor at a higher abstraction level
Monitor calls made to initiate requests
Buffers shared between guest / monitor
Positives
Simplify monitor
Fast
Negatives
Monitor needs to supply guest-specific drivers
Bootstrapping issues

13. Traditional Architecture
Linux
Linux (devel) XP
Vista
MacOS
Virtual Machine Monitor
Here the monitor needs to be self-sufficient. There is no help from an Operating System. What does this mean the monitor needs to have:
CPU scheduler
Memory allocator
Device Drivers
File system
Network stack for administration
Hardware

14. Hosted Monitor Architecture
User App
Guest OS (Linux)
Host OS (Window XP)
Virtual Machine Monitor
Kernel Module
The virtual machine monitor runs in its own address space at kernel level. The VMM time shares the hardware with the host. When the UserApp runs in the host, it switches to the VMM by way of a World Switch. The world switch save all the registers, page tables, etc of the host and then loads the state of the VMM. Initially the state of the VMM is setup up by the UserApp. This includes what the registers should be and the structure and contents of the page tables. When the VMM voluntarily gives up the CPU, it World Switches back to the host.
Hardware

15. Source: http://www.vmware.com/pdf/esx2_performance_implications.pdf
VMware ESX 2.0
Traditional arch for cpu scheduling, memory allocation, virtual disk, virtual network. Console OS is there for legacy devices, USB, and as a management platform, i.e. it has the network stack.
Source:

16. Hybrid Ex 2 - Xen 3.0 Para –virtualization
Linux Guest
Hardware-supported virtualization
Unmodified Windows
Isolated Device Drivers
Source: Ottawa Linux Symposium 2006 presentation.

17. Hypervisor Hardware-supported single-use monitor Characteristics Uses
Small size
Runs in a special hardware mode
Guest OS runs in normal priviledge level
Uses
Security
System management
Fault tolerance
User
Mode
Operating System
Kernel
Mode
Monitor
Mode
Hypervisor
Hardware

18. Virtual Disks File on file system Meta-data in file Host file system
Hypervisor file system
Meta-data in file
Maps disk blocks to file offsets
Flat file
Indexed file
Allows disk to grow on demand

19. Copy-on-Write Disks
Win XP
+ SP1
+ SP2
+ SP2
IE 7

20. Copy-on-Write Disks
Win XP
Reads
+ SP1
+ SP2
+ SP2
IE 7
Writes

21. Non-persistent Execution
Win XP
+ SP1
+ SP2
+ SP2
IE 7
Reads
Temp
Writes