 Virtual machine systems: simulators for multiple copies of a machine on itself.  Virtual machine (VM): the simulated machine.  Virtual machine monitor (VMM): the simulator software.  Pure software simulator is too slow; Direct hardware execution is necessary  Complicated when OS involved; especially with multiple OS; build separate VMM layer Virtual Machine - Introduction CDA6159fa07 peir

 Principal characteristics of the original (IBM 360) architectures for which virtual machine systems were developed was the dual-state hardware organization with a privileged and non-privileged mode.  Privileged software nucleus (kernel)  Extended machine: set of non-privileged instructions together with the supervisory calls.  VMM runs on privileged mode, while guest OS runs on non-privileged mode Virtual Machine - Introduction CDA6159fa07 peir

Survey of Virtual Machine Research IEEE Computer, 1974

Conventional and Virtual Machine Organizations

Flexible Virtual Machine Model

The Renaissance of Virtualization - X86 Prospective from VMware CDA6159fa07 peir  1970s: Virtual machines first used  1990s: x86 becomes prominent server platform, not design for virtualization No vertical integration in x86 Lack of enterprise features in commodity OSs  1999: VMWare first product to virtualize x86  2006: AMD and Intel offer hardware support

 Extra resources needed: main memory, processor cycles.  Sources of overhead: Maintaining the status of the virtual processor. Maintaining the status of the virtual processor. Support of privileged instructions. Support of privileged instructions. Support of paging within virtual machines. Support of paging within virtual machines. Console functions. Console functions.  A problem: resource allocation algorithms in operating system (these are meant for managing real resources, but the virtual resources may not correspond precisely to real resources). Virtual Machine - Performance CDA6159fa07 peir

 Privileged software development, test and diagnostic functions, multiple operating system execution concurrently with production uses of the system.  Retrofit old operating systems with new features.  Improved testing of privileged software.  Testing of computer network software on a single physical machine.  Debugging and performance monitoring tools.  Software reliability through isolation.  Data security. Virtual Machine - Applications CDA6159fa07 peir

Basic Computer Systems

Hardware / Software Interface 3: “system” ISA: I/O, memory mgmt, CPU intercept & emulate 4: “user” ISA: ALU, branch, load/store direct execution

Basic VM Architecture CDA6159fa07 peir  Classic VMs are enabled by “Virtual Machine Monitor” (VMM) software – VMM may be in total control of the underline physical machine No O/S below the VMM – Or, VMM may share the physical machine with a “host” operating system VMM runs on top of host O/S (with user-level processes, as well as with kernel code, e.g. modules)

Classic VMM

VMM Integrated with Host OS  Advantages: Simple to install VMM, hosted OS accommodate rich I/O devices, take advantage hosted OS’ resource management  Disadvantages: Performance overhead, no performance isolation

 Present a “replica” of the interfaces to Hardware  provided by physical machine to each VM – Interface to processor (i.e. instruction set) – Interface to memory (i.e. use physical memory addresses) – Interface to I/O devices (e.g. memory mapped I/O, interruptions) * Different views of physical I/O device possible; e.g. a SCSI physical disk may be viewed as many virtual IDE disks Fundamental task of VM monitor CDA6159fa07 peir

 Analogy with operating systems: – Most application instructions execute directly on processor (O/S does not see them) – Those that require access to shared resources are handled indirectly by calling the O/S via system calls  VMM: – Most application instructions execute directly on processor (VMM does not see them) – Those that require privileged processor access are handled indirectly – they are trapped and emulated by the VMM VMM – Processor Virtualization CDA6159fa07 peir

Challenges and Techniques  X86 architecture is not designed for virtualization POPF (pop CPU flags from stack) set / clear interrupt- disable flag; does not trap in user mode in guest OS  Paravirtualization Replace nonvirtualizable instructions with virtualized equivalent OS must be ported in order to run in a virtual machine Disco (for MIPS) changes MIPS interrupt flag from a privileged register to a special memory location

 Aided by processor’s ability to support virtual memory  One more level of indirection is implemented – O/S: application loads/stores virtual address; map to physical addresses of physical machine – Classic VM: application loads/stores virtual address; map to physical address of the VM; then map to physical address of physical machine VMM - Memory Virtualization CDA6159fa07 peir

Page Replication and Page Sharing in Disco

VMM - I/O Virtualization  Hosted versus non-hosted – Non-hosted: VMM must implement drivers for each device Since it interfaces directly with hardware; must go through VMM, incur overhead – Hosted: VMM may build on top of existing device drivers of host O/S E.g. VMware: low-level I/O accesses to virtual disk are mapped into accesses to a regular file of the host via system calls Host O/S drivers, in turn, maps to I/O accesses to physical disk