Survey of Virtual Machine Research

Motivations:

• Want multiple simulated machines on top of the real machine.
• User's view: select your own version of OS and run on your own private machine.

Definitions:

• Virtual machine systems: systems which have simulators for multiple copies of machines on itself. Much of the software for the simulated machine executes directly on the hardware without software interpretation.
• Virtual machine (VM): the simulated machine.
• Virtual machine monitor (VMM): the simulator software.
• Dual-state hardware organization: privileged and non-privileged mode.
• Extended machine: the conventional organization contains a privileged software nucleus (kernel) which can execute in the privileged mode. User programs make calls to this nucleus to have privileged instructions performed on its behalf.
• Only one bare machine interface thus only one privileged software nucleus can be run at a given time. Therefore, cannot run any other software that requires a bare machine interface. E.g. porting user software, testing of OS, T&D programs.

Solutions and some problems:

• The heart of a VM system is the VMM software which transforms the single machine interface into the illusion of many. Each VM (interface) is an efficient replica of the machine (includes processor instructions and system resources). Can run multiple OSs this way.
• Difficult to implement VMMs on machines not designed for them. E.g. privileged instructions can altered the state of the machine and thus can interfere with other VMs. The VMM must maintain control over the state of the real processor.

Virtualizable Architectures:

• New architectures than provide features to directly support virtual machines.
• f-map: a map between the sets of resources of the VM and the real machine, which can be used to transform a virtual resource name into a real resource name.
• Allows for recursion: a VMM can create another VMM which can interact with the OSs.

Performance:

• Main sources of overhead: maintaining integrity of the virtual processor; support of privileged instructions (trap and simulate); support of paging within VM (2 levels of indirection); console functions.
• Improving performance: policies; compromises to interface (pure vs. impure VMs); improved or new mechanisms (virtualizable architectures).

Questions:

• Instead of using VMM: Since machines are much cheaper now, just find another machine to run another OS for testing, porting etc?