1. Introduction to Virtual Machines
From
“Virtual Machines”
Smith and Nair
Chapter 1
Introduction

2. Two fundamental notions in computer system design
Levels of Abstraction …
….separated by well-defined Interfaces
Keys to managing complexity in computer systems.
Introduction

3. Abstraction
Abstraction allows lower levels of design to be ignored/simplified while designing higher levels.
E.g. Details of hard disk abstracted by operating system into multiple variable sized partitions and their file systems.
Disadvantage: Sometimes low-level details are necessary to optimize for performance. E.g. File systems might use better layout if they knew the disk geometry.
Introduction

4. Interfaces
Allow computer design tasks to be decoupled so that different development teams can work independently at different levels of abstraction.
E.g. Instruction set: Intel and AMD implement the same IA-32 (x86) instruction set interface.
Software designers don’t need to worry about their different implementations.
Disadvantage: Components designed for one interface cannot work on another
E.g. x86 vs IBM PowerPC
Diversity of interfaces can be restrictive for applications.
Introduction

5. Virtualization Provides a way to increase flexibility.
Real system (and its interfaces) appear to be a set of virtual systems (and virtual interfaces).
Virtualization vs. abstraction
Virtualization does not necessarily hide the level of details of the real system
Introduction

6. Example: Disk Virtualization
File 1
File 2
Interface
Real
Disk
Introduction

7. Virtual Machines Same concept as disk virtualization in last slide
Implemented by adding layers of software to the real machine to support the desired VM architecture.
E.g. Virtual PC on Apple MAC/PowerPC emulates Windows/x86.
Uses:
Multiple OSes on one machine
Isolation,
Enhanced security
Platform emulation
On-the-fly optimization
Realizing ISAs not found in physical machines
Introduction

8. Virtualization – Isomorphism
Maps a virtual guest system to a real host system.
e(Si) Si
Si’
Guest
V(Si)
V(Sj)
e’(Si’)
Si’
Sj’
Host
Introduction

9. Computer Architecture
User ISA : 7
System ISA : 8
Syscalls : 3
ABI : 3, 7
API : 2,7
Introduction

10. Machine Interfaces Application Binary Interface ISA Interface
(Process View)
(OS View)
Introduction

11. Two Types of VMs Process VMs System VMs
Introduction

12. Process Virtual Machine
Virtualizing software translates instructions from one platform to another.
Helps execute programs developed for a different OS or different ISA. (Think of java)
VM terminates when guest process terminates.
Introduction

13. System Virtual Machine
Provides a complete system environment
OS+user processes+networking+I/O+display+GUI
Lasts as long as host is alive
Usually requires guest to use same ISA as host
Introduction

14. Virtual Machine Applications
Emulation &
Optimization
Replication
Composition
Emulation: Mix-and-match cross-platform portability
Optimization: Usually done with emulation for platform-specific performance improvement
Replication: Multiple VMs on single platform
Composition: form more complex flexible systems
Introduction

15. Types of Process Virtual Machines
Multiprogramming
Standard OS syscall interface + instruction set
Can support multiple processes with its own address space and virtual machine view.
Emulators
Support one instruction set on hardware designed for another
Interpreter:
Fetches, decodes and emulates the execution of individual source instructions. Can be slow.
Dynamic Binary Translator:
Blocks of source instructions converted to target instructions.
Translated blocks cached to exploit locality.
IA-32 Windows APP
Digital FX!32
System
Windows NT
Runtime
Alpha ISA
Introduction

16. Types of Process Virtual Machines (contd)
Same ISA Binary Optimizers
Optimize code on the fly
Same as emulators except source and target ISAs are the same.
High-Level Language VMs
Virtual ISA (bytecode) designed for platform independence
Platform-dependent VM executes virtual ISA
E.g. Sun’s JVM and Microsoft’s CLI (part of .NET)
Both are stack-based VMs that run on register-based m/c.
Introduction

17. Types of System VMs Originally developed for large mainframes Today:
Secure way of partitioning major software systems on a common platform
Ability to run multiple OSes on one platform
Platform replication provided by VMM
VMM controls access to hardware resources
When guest OS performs a privileged operation, VMM intercepts it, checks for correctness and performs the operation.
Transparent to guest OS.
Introduction

18. Classic System VMs Try to execute natively on the host ISA
VMM directly controls hardware
Provides all device drivers
Traditional mainframe model
Introduction

19. Hosted VMs Similar to classic system VM Operates in process space
Relies on host OS to provide drivers
E.g. VMWare
Introduction

20. Whole System VMs: Emulation
Host and Guest ISA are different
Hosted VM + emulation
So emulation is required
E.g. Virtual PC (Windows on MAC)
Introduction

21. Co-designed VMs Performance improvement of existing ISA
Customized microarchitecture and ISA at hardware level
Native ISA not exposed to applications
VMM
co-designed with native ISA
Part of native hardware implementation
Emulation/translation
E.g. Transmeta Crusoe
Native ISA based on VLIW
Guest ISA = x86
Goal power savings
Introduction

22. Taxonomy
Introduction

23. Versatility Java App JVM Linux IA-32 Windows IA-32 Crusoe VLIW VMWare
Code Morphing
Introduction