Transcript Lecture 18
CSE 451: Operating Systems Winter 2011 Module 18 Virtual Machines Mark Zbikowski and Gary Kimura What is “Virtualization”? • Beyond the buzz-word du jour: – Abstraction of computing resources – Hide the physical characteristics of a resource – Allow multiple “instances” of the resource • Why? – Sharing the resource among multiple clients • Disks (I/O system) • Cpu (scheduler and context switches) • Memory (processes and virtual memory) – Fault isolation • Application • Operating System • Hardware(!) 4/8/2016 2 Classes of Virtualization • Resource – aggregation, spanning, or concatenation combines individual components into larger resources or resource pools (RAID) – VPN or NAT – Multiprocessor/multicore – Encapsulation: hide complexity by the creation with simplified interface. 4/8/2016 3 Classes of Virtualization • Platform – 1960’s term: virtual machines used hardware and software – Host running sofware using hardware assistance to execute a guest – Guest can be application or entire OS – Guest runs as if it were installed on bare metal – Typically many guests run on a single host 4/8/2016 4 Platform Virtualization – What? • Emulation/simulation – Software/hardware decodes each instruction and emulates the guest hardware – Emulate completely different CPU (VirtualPC on PowerPC Mac) – Typically slow (but some clever tricks can be used) • Native/Full Virtualization – Host emulates only enough hardware to allow guest to run unmodified • OS-Level Virtualization – “personalities” – Underlying OS emulates programming environment: Win32, POSIX, OS/2 4/8/2016 5 Platform Virtualization – How? • Emulation/Simulation – Naively, very slow: decode and execute instruction – Why not play compiler? • • • • “dynamic recompilation” Decode sequence of guest instructions (basic block) Generate native host instructions Hash table of guest address -> codelet block – Memory management emulation difficult • Simulate page directory/table and TLB • Java VM and VirtualPC 4/8/2016 6 Platform Virtualization – Difficulties • Requirements – Fidelity: guest executes identically to execution on native hardware, barring timing effects – Performance: an overwhelming majority of guest instructions are executed by the hardware without intervention by host – Safety: host manages all hardware resources • X86 is not classically virtualizable – Run guest in unprivileged mode (“deprivileging”) – Trap to host on illegal instructions, emulate effects in guest – Illegal instructions: • CR (chip control registers) • I/O port • Memory-mapped devices 4/8/2016 7 More x86 woes • Primary and shadow structures – Host maintains shadows of guest’s primary structures – On-CPU state: • Page directory pointer • Processor status register (interrupt state, flags, debug state) • Trivial to maintain – Off-CPU state: • Page tables • Encodes mappings and permissions • Modified by guest AND guest H/W 4/8/2016 8 Improving host performance • Allow slightly modified guests – Augment the “hardware” to add host-calls – VMWare Tools/VirtualPC Machine Additions • Device drivers that export the I/O at a high level for emulation • Dynamic recompliation • Hardware extensions – – – – 4/8/2016 Intel VMRUN VMEXIT instructions Hardware maintains shadow state Traps to host when needed Too limited to be of value 9 Platform Virtualization • Emulating an “environment” – – – – “Win32 API” implemented on Windows 95 Native API to DOS/Win-based kernel Windows NT built to allow emulation DLLs in application address space emulate Windows 95 API but run on an entirely different operating system – Vast majority of Windows 95 applications ran on Windows NT 4 without modification • Java – Byte-code defines a virtual machine – Emulator written in portable C, can be compiled and run on almost all hardware architectures – Pcode can be run anywhere – Dynamic recompilation (aka incremental compilation) 4/8/2016 10