Paravirtualization

In computing, paravirtualization is a virtualization technique that presents a software interface to virtual machines that is similar, but not identical to that of the underlying hardware.

The intent of the modified interface is to reduce the portion of the guest's execution time spent performing operations which are substantially more difficult to run in a virtual environment compared to a non-virtualized environment. The paravirtualization provides specially defined 'hooks' to allow the guest(s) and host to request and acknowledge these tasks, which would otherwise be executed in the virtual domain (where execution performance is worse). A successful paravirtualized platform may allow the virtual machine monitor (VMM) to be simpler (by relocating execution of critical tasks from the virtual domain to the host domain), and/or reduce the overall performance degradation of machine-execution inside the virtual-guest.

Paravirtualization requires the guest operating system to be explicitly ported for the para-API — a conventional OS distribution that is not paravirtualization-aware cannot be run on top of a paravirtualizing VMM. However, even in cases where the operating system cannot be modified, components may be available that enable many of the significant performance advantages of paravirtualization. For example, the Xen Windows GPLPV project provides a kit of paravirtualization-aware device drivers, licensed under the terms of the GPL, that are intended to be installed into a Microsoft Windows virtual-guest running on the Xen hypervisor.

Paravirtualization is a new term for an old idea. IBM's VM operating system has offered such a facility since 1972 (and earlier as CP-67). In the VM world, this is referred to as a "DIAGNOSE code", because it uses an instruction code used normally only by hardware maintenance software and thus undefined.

The Parallels Workstation operating system calls its equivalent a "hypercall". All are the same thing: a system call to the hypervisor below. Such calls require support in the "guest" operating system, which has to have hypervisor-specific code to make such calls.

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