Beautiful Architecture

Changing Hardware, Changing Xen

Up to this point, our discussion has concentrated on paravirtualization. However, between Xen versions 2.0 and 3.0, Intel and AMD introduced distinct but similar support in their processors for hardware virtual machines. It became possible to run unmodified operating systems, including Microsoft Windows or native Linux, in virtual machines. So did this spell the end for paravirtualization?

First of all, let’s look at how hardware virtual machines are implemented. Both Intel and AMD introduced a new mode (nonroot mode on Intel and guest mode on AMD) in which attempting to execute a privileged operation, even at the highest (virtual) privilege level, generates an exception that notifies the hypervisor. Therefore it is no longer necessary to scan the code and replace these instructions (either at runtime or in advance through paravirtualization). The hypervisor can use shadow page tables to provide the virtual machine with an illusion of contiguous memory, and it can trap I/O operations in order to emulate physical devices.

Xen added support for hardware virtual machines in version 3.0. The transition was aided greatly by open source development. Since Xen is an open source project, it was possible for developers from Intel and AMD to contribute low-level code that supports the new processors. Furthermore, thanks to its GPL status, Xen could incorporate code from other open source projects. For example, the new hardware virtual machines required an emulated ...