Running non-native Instruction Sets

• emulator
• Reproduction of the function or action of a different computer, software system, etc.
• Replicates the original guest instruction set

• E.g., Android emulator

• simulator
• Models the original guest instruction set
• realistic imitation
• E.g., flight simulator
• E.g., iPhone simulator

Tradeoffs

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 https://doi.org/10.1145/361011.361073
• 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

Usages

• 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?

VMWare

• 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)

VMWare ESX

• 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

VirtualBox

• 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)

Other

• 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

Summary

• 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