Michael L. Collard, Ph.D.

Department of Computer Science, The University of Akron

Running non-native Instruction Sets


  simulator emulator
speed fast slow
behavior approximation accurate (except for response)

Platform Virtualization

  • Hide the physical characteristics of computer resources from the applications
  • Not a new idea: IBM’s CP-40 1967, CP/CMS, VM
  • Full Virtualization
  • Simulate enough hardware so that an unmodified guest operating system can run
  • Provides a full “virtual machine”
  • Scenarios:
  • Run Linux in a virtual machine on Windows
  • Run multiple logical servers (each with a separate virtual machine) on a single physical server

Virtual Machine Monitors (VMMs)

Challenges of Running a VMM

Popek & Goldberg

  • Formal Requirements for Virtualizable Third Generation Architectures - Popek & Goldberg, Communications of the ACM, July 1974
  • Virtual Machine
  • capable of virtualizing all hardware resources, processors, memory, storage, and peripherals
  • Virtual Machine Monitor (VMM)
  • provides virtual machine abstraction
  • Also referred to as a hypervisor

Popek & Goldberg VMM Properties

  • Equivalence
  • Program running under a VMM should exhibit a behavior identical to that of running on the equivalent machine
  • Resource Control
  • VMM is in full control of virtualized resources
  • Efficiency
  • A statistically dominant fraction of machine instructions may be executed without VMM intervention

Popek & Goldberg: Instruction Characteristics

  • Privileged instructions
  • those that trap in user mode and only run in system mode
  • Control-sensitive instructions
  • those that change the resource configuration of the system
  • Behavior-sensitive instructions
  • behavior or results depend on the configuration of resources

Theorem 1: Virtualization

Can construct a VMM if the set of control-sensitive instructions is a subset of the privileged instructions

  • All control-sensitive instructions always trap and pass control to the VMM
  • Guarantees resource control property
  • Non-privileged instructions must be executed natively

Theorem 2: Recursive Virtualization

A VMM can run on a copy of itself

  • For an architecture, this is possible if:
  • The architecture is virtualizable
  • A VMM without timing dependencies can be built

Non-Virtualizable Machines

  • VMMs (under Popek & Goldberg's definition) are not possible on non-virtualizable machines
  • Workarounds:
  • patching – critical instructions removed and replaced with a trap to VMM
  • paravirtualization – guest o.s. is modified (e.g., IBM VM)

x86 Virtualization

  • x86
  • Pre 2005:
  • x86 processor architecture did not meet Popek & Goldberg virtualization requirements
  • Instructions that can access resources with a trap
  • Intel VT (Virtual Technology) (IVT)
  • IA-32, IA-64, Directed I/O, DMA & interrupt
  • Solution: Add instructions that allow the host to trap resource instructions remapping, Connectivity
  • AMD-V (Pacifica)
  • AMD Athlon 64, Turion, Opteron

Virtualization: Isolation

Virtualization: Consolidation

Virtualization: Migration


  • Legacy software support – Consolidation
  • Training/QA – Consolidation
  • Activity Partitioning – Isolation
  • Administration – Consolidation, Isolation, Migration
  • Failover Infrastructure - Migration

Questions about a VMM

  • What host OS can it run? Does it need a host OS?
  • What OS does it allow as a guest?
  • Can it support a VM even if the instructions are not on the physical CPU, e.g., IA-64 VM on an IA-32 machine?
  • How much are the resources shared between guest OSs?
  • What tools does it provide for managing VMs?


  • Desktop – runs in a host OS
  • VMWare Workstation (1999) – runs on PC
  • VMWare Fusion – runs on Mac OS X
  • VMWare Player – run, but not create images
  • Server
  • VMWare Server (GSX Server) –hosted on Linux or Windows
  • VMWare ESX (ESX Server) – no host OS
  • VMWare ESXi (ESX 3i) – freeware (July 2008)


  • Part of VMWare Infrastructure
  • Runs “bare-metal”, i.e., has its own vmkernel, with near-native performance
  • Guest/Host CPU: x86 (ESXi only), x86-64, AMD
  • Guest OS: Windows, Linux
  • Management capabilities:
  • Move VM between servers on the fly
  • Move VM based on host/VM load requirements/demands

Microsoft Hyper-V

  • Viridian, Windows Server Virtualization
  • Host CPU: x64 + IVT or AMD-V
  • VMM runs directly on the hardware; however, it requires one parent partition of the Windows Server
  • Parent manages child VMs
  • Guest OS: Windows, Linux
  • Enlightened I/O: direct access for enabled guests


  • Free and open-source hypervisor
  • Host: Windows, macOS, Linux, Solaris, and OpenSolaris
  • Guest: Windows, Linux, BSD, OS/2, Solaris, Haiku, and macOS (on macOS host)


  • Xen
  • free software
  • runs native but with the required dom0 OS (Linux, NetBSD, Solaris), which starts other VMs
  • UML (User Mode Linux)
  • free software
  • Run Linux in Linux
  • Support integrated into Linux kernel

Virtualization-Specific Challenges

  • Performance Overheads
  • Resource Contention
  • CPU, memory, I/O
  • I/O Performance
  • Management Complexity
  • Scaling
  • Automation
  • Security


  • Virtualization is an old approach, but updated software techniques and hardware support made it more visible
  • The mainstream of system administration
  • More movement into containers and away from dealing directly with the VM