An Overview of Virtual Machine Architectures by J

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 2 nd year Ph. D 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 1. Interface at the top 1. ABI (application binary interface) 2. ISA (instruction set architecture) 2. 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 – 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: – OS is different, ISA is same – for example, run Windows x 86 applications in a Linux x 86 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?

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