Clemson University -- CPSC 231 -- Spring 2010


Virtual machines


benefits

  mutliple flavors - run a different OS in each VM
    - provide for user preference
    - legacy OS/application (run one VM with old OS/app to avoid porting)

  fault isolation / security
    - OS in one VM crashes without taking the whole system down

  development/testing
    - develop/test in one VM without affecting others

  migration - move a VM from one physical server to another
    - load balancing
    - fault recovery


approaches

  bare metal approach

    | user apps | user apps | user apps | user apps |
    | client OS | client OS | client OS | client OS |
    +-----------+-----------+-----------+-----------+
    |    VM     |    VM     |    VM     |    VM     |
    +-----------+-----------+-----------+-----------+
    |     virtual machine monitor (hypervisor)      |
    +-----------------------------------------------+
    |                    hardware                   |
    +-----------------------------------------------+

  hosted approach

    | user apps | user apps |
    | client OS | client OS |
    +-----------+-----------+
    |    VM     |    VM     |
    +-----------+-----------+
    |       VM monitor      | user app ... user app |
    +-----------------------------------------------+
    |                    host OS                    |
    +-----------------------------------------------+
    |                    hardware                   |
    +-----------------------------------------------+

  also:
    dynamic recompilation of user and target OS code
    modify client OS to work with VMM (paravirtualization)


VM should mimic I/O, interrupts, etc. - but should run w/ limited privileges

  1) software simulate the actual machine => slow
  2) run directly on hardware but each privileged operation is trapped
       and simulated
  3) code VM with special assists


history

  1964 - work starts on IBM CP-40 (control program for S/360 Model 40)
  1966 - IBM CP-67 (control program for S/360 Model 67, virtual memory)

  1972 - IBM VM/370 for S/370 computers

  1988 - SoftPC for Apple and Sun systems

  1999 - VMware

  today - Microsoft Virtual PC, VMware, and Xen are but three among many

AMD provides AMD-V and Intel provides Intel-VT
  (will allow Xen to host Windows)