Introduction to Virtual Machines

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Introduction to Virtual Machines

• From
• Virtual Machines
• Smith and Nair
• Chapter 1

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Two fundamental notions incomputer system design

• Levels of Abstraction
• .separated by well-defined Interfaces
• Keys to managing complexity in computer systems.

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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.

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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 dont 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.

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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

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Example Disk Virtualization
Virtual Disk 1
Virtual Disk 2
Virtualization
File 1
File 2
Interface
Real Disk
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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

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Virtualizatilon Isomorphism

• Maps a virtual guest system to a real host
  system.

e(Si)
Si
Si
Guest
V(Si)
V(Sj)
e(Si)
Sj
Si
Host
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Computer Architecture
User ISA 7 System ISA 8 Syscalls 3 ABI 3,
7 API 2,7
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Machine Interfaces
Application Binary Interface
ISA Interface
(Process View)
(OS View)
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Two Types of VMs Process VMs System VMs
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Process Virtual Machine

• Virtualizing software translates instructions
  from one platform to another.
• Helps execute programs developed for a different
  OS or different ISA.
• VM terminates when guest process terminates.

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System Virtual Machine

• Provides a complete system environment
• OSuser processesnetworkingI/OdisplayGUI
• Lasts as long as host is alive

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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

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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
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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. Suns JVM and Microsofts CLI (part of .NET)
• Both are stack-based VMs that run on
  register-based m/c.

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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.

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Classic System VMs

• Try to execute natively on the host ISA
• VMM directly controls hardware
• Provides all device drivers
• Traditional mainframe model

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Hosted VMs

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

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Whole System VMs Emulation

• Host and Guest ISA are different
• Hosted VM emulation
• So emulation is required
• E.g. Virtual PC (Windows on MAC)

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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

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Taxonomy
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Versatility

• Java App
• Linux IA-32
• Windows IA-32
• Crusoe VLIW

JVM
VMWare
Code Morphing