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

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

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

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

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

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

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

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

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

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

Two Types of VMs Process VMs System VMs Introduction

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

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

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

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

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

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

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

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

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

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

Taxonomy Introduction

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