1. An Overview of Virtual Machine Architectures
by J.E. Smith and Ravi Nair
presented by Sebastian Burckhardt
University of Pennsylvania
CIS 700 – Virtualization Seminar
Friday, September 17, 2004

2. About me I am a 2nd year PhD student
I was born and raised in Switzerland
My academic advisor is Rajeev Alur
Research Interests:
Hardware Verification
Model checking
Refinement and Abstraction
Compositional Verification
Architecture
Formal methods

3. Contributions of the paper
Systematic taxonomy of VMs
demarcate what to include
classify into types, name each type
Introduce a diagram language
comprehensive: can draw all VM types
precise: can distinguish all VM types
Discuss each type
purpose
implementation issues
examples

4. Organization of the paper
1.1 Motivations for using VMs
1.2 System Layers, Interfaces, and VM classes
1.3 Process VMs
1.4 System VMs
1.5 Virtualization
1.6 Taxonomy summary

5. Standard System Layers
Benefits
decoupled design tasks
software reuse across varying hardware configurations and generations
Starting point for all virtual machines in this paper

6. Disadvantages and Limitations ?
Suboptimal performance across interfaces
SW still depends on ISA and OS interfaces
OS still depends on ISA
Get locked into interface

7. VMs for Flexibility
Software perspective: want ability to run on all machines
reach more customers
move code between networked computers
Hardware perspective: want ability to run all programs
server consolidation
legacy applications
cross-platform software development

8. VMs for Performance Optimize just in time
The more you know, the more you can optimize
Isolate faults and security risks
Many server images more stable than single server
Increase utilization of resources
adjust allocations dynamically
reduce fragmentation

9. Examples

10. VM Puzzles
show position of VM with respect to the standard system layers
show kind of interface used between components
show replication

11. Taxonomy Overview Interface at the top
ABI (application binary interface)
ISA (instruction set architecture)
same/different ISA at top and bottom

12. ISA and ABI Instruction Set Architecture separates hardware from rest
Application Binary Interface separates processes from rest

13. Process vs. System VMs Process level VMs provide ABI to application
System VMs provide ISA to OS and applications

14. Process VMs (1) Multiprogramming Note: replication can go both ways
same ISA, same OS
replicates ABI so that each process thinks it has its own machine
standard in “modern” OS
can argue whether to call this a VM
Note: replication can go both ways
multiprocessor appears as single environment
clusters, grid computing

15. Process VMs (2) “Emulation” and “Dynamic Binary Translation”
OS is same, ISA is different
better known as “interpretation” and “compilation”
Dynamic Optimizers
same OS and ISA
performance is purpose

16. Process VMs (3) High-level VMs Use synthetic ISA
P-code (somewhat anachronistic)
Java bytecode
Maximal platform independence
what about OS calls?
High performance penalty?
what about just-in-time compilation?

17. Process VMs (4) This combination is left out by taxonomy:??
This combination is left out by taxonomy:
OS is different, ISA is same
for example, run Windows x86 applications in a Linux x86 environment (e.g. WINE)
Don’t know specific term for this VM type

18. System VMs (1) Classic System VMs
VMM (Virtual Machine Monitor) provides replication and resource management
possible benefits: flexibility, utilization, isolation
similar to what an OS does for processes
sits on hardware
(super)privileged mode

19. System VMs (2) Hosted System VMs
Like classic system VM, but operates within process space
Can play tricks to overcome limitations
Common to Classic/Hosted System VM:
try to do as much as you can natively
more difficult for hosted VMs than for classic VMs
underlying ISA can make big difference

20. System VMs (3) Whole System VMs ISA is different
no ‘native’ execution possible. Complete emulation/translation required.
Usually done as a hosted VM

21. System VMs (4) Co-Designed VMs use synthetic custom ISA at bottom
goal: reconcile diverging requirements between ISA and microarchitecture
no ‘native’ execution possible
Emulation/translation
can be joint effort by hardware and software
can be made completely transparent

22. General implementation issues for virtual machines
How to design/verify implementation
need to
map features of guest to features of host
know how to materialize state (e.g. on interrupts)
compare this to architecture/microarchitecture
describe typical issues for each VM type

23. Analogy: Architecture vs. Microarchitecture (1)
Map architected state to implementation state
No one-to-one relation: many implementation states per architecture state
map direction is wrong?

24. Analogy: Architecture vs. Microarchitecture (2)
When designing the microarchitecture, we map features of the ISA into features of the microarchitecture
When verifying the implementation, we
map implementation states to architecture state
map implementation steps to (zero or more) architecture steps
diagram must commute

25. How to implement a VM
Map guest features to host features

26. Implementation issues: Process VMs

27. Implementation issues: System VMs

28. Finally: VMs on top of VMs

29. Discussion questions Is this taxonomy good?
too broad or too narrow?
too simple or too complicated?
What VM types are most relevant?
Would a proliferation of VMs change
ABI’s?
ISA’s?
OS’s?

30. - THE END -