Summary

  • 5 minutes
  • Virtualization involves the construction of an isomorphism that maps a virtual guest system to a real (or physical) host system.
  • An underlying physical machine (PM) usually is called a host, and an OS running on a VM is called a guest OS.
  • As compared to a host PM, a VM can have resources different in quantity and in type. (For example, a host can contain one Intel IA-32 physical CPU, while a VM can include eight PowerPC virtual CPUs that all map to the single physical CPU.)
  • A VM can run only at a single host at a certain point in time, yet can be migrated to a different host (and run at that host) at a different point in time.
  • There are two types of VMs, process VMs and system VMs.
  • A process VM (for example, JVM) consists of a virtual memory address space, user-level registers, and instructions assigned to an OS process to execute a user program (such as, no OS can run within a process VM).
  • Process VMs can support ISAs that differ from host ISAs.
  • The abstraction of a process VM is provided by a piece of virtualizing software denoted as the runtime.
  • The runtime of a process VM is placed at the ABI interface, on top of a host OS.
  • As opposed to process VMs, a system VM provides a complete system environment (such as, an OS image can be run in a system VM).
  • System VMs can support ISAs that differ from host ISAs.
  • The abstraction of a system VM is provided by a piece of a virtualizing software called the hypervisor (or the virtual machine monitor [VMM]).
  • There are three main classes of system VMs, which are defined according to where in the system the hypervisor is placed.
  • A system VM is called a native system VM when its hypervisor is placed on bare metal (such as the raw hardware).
  • In native system VMs, the hypervisor is run in system mode, and the VMs (alongside their associated OSs) are run in user mode.
  • Hypervisors in native system VMs should specify and implement every function required for managing hardware resources.
  • With native system VMs, every privileged instruction issued by a user program at any guest OS has to trap to the hypervisor.
  • A system VM is called a user-mode hosted VM when its hypervisor is placed on top of a host OS.
  • In user-mode hosted VMs, the hypervisor and all its managed VMs run in user mode, while the underlying host OS runs in system mode.
  • With user-mode hosted VMs, privileged instructions from guest OS(s) need to trap to the hypervisor, but the hypervisor needs not implement every function required for managing hardware resources.
  • A system VM is called a dual-mode hosted VM when its hypervisor is placed partly on bare metal and partly on a host OS.
  • In dual-mode hosted VMs, the hypervisor can operate partly in system mode and partly in user mode. Hence, it can use the best of native system VMs and user-mode hosted VMs.