Open source virtualization projects
The following table is a list of open source virtualization projects in Linux:
Figure 1.3 – Open source virtualization projects in Linux
In the upcoming sections, we will discuss Xen and KVM, which are the leading open source virtualization solutions in Linux.
Xen
Xen originated at the University of Cambridge as a research project. The first public release of Xen was in 2003. Later, the leader of this project at the University of Cambridge, Ian Pratt, co-founded a company called XenSource with Simon Crosby (also from the University of Cambridge). This company started to develop the project in an open source fashion. On 15 April 2013, the Xen project was moved to the Linux Foundation as a collaborative project. The Linux Foundation launched a new trademark for the Xen Project to differentiate the project from any commercial use of the older Xen trademark. More details about this can be found at https://xenproject.org/.
The Xen hypervisor has been ported to a number of processor families, such as Intel IA-32/64, x86_64, PowerPC, ARM, MIPS, and so on.
The core concept of Xen has four main building blocks:
- Xen hypervisor: The integral part of Xen that handles intercommunication between the physical hardware and virtual machine(s). It handles all interrupts, times, CPU and memory requests, and hardware interaction.
- Dom0: Xen's control domain, which controls a virtual machine's environment. The main part of it is called QEMU, a piece of software that emulates a regular computer system by doing binary translation to emulate a CPU.
- Management utilities: Command-line utilities and GUI utilities that we use to manage the overall Xen environment.
- Virtual machines (unprivileged domains, DomU): Guests that we're running on Xen.
As shown in the following diagram, Dom0 is a completely separate entity that controls the other virtual machines, while all the other are happily stacked next to each other using system resources provided by the hypervisor:
Figure 1.4 – Xen
Some management tools that we're going to mention a bit later in this book are actually capable of working with Xen virtual machines as well. For example, the virsh command can be easily used to connect to and manage Xen hosts. On the other hand, oVirt was designed around KVM virtualization and that would definitely not be the preferred solution to manage your Xen-based environment.
KVM
KVM represents the latest generation of open source virtualization. The goal of the project was to create a modern hypervisor that builds on the experience of previous generations of technologies and leverages the modern hardware available today (VT-x, AMD-V, and so on).
KVM simply turns the Linux kernel into a hypervisor when you install the KVM kernel module. However, as the standard Linux kernel is the hypervisor, it benefits from the changes that were made to the standard kernel (memory support, scheduler, and so on). Optimizations for these Linux components, such as the scheduler in the 3.1 kernel, improvement to nested virtualization in 4.20+ kernels, new features for mitigation of Spectre attacks, support for AMD Secure Encrypted Virtualization, Intel iGPU passthrough in 4/5.x kernels, and so on benefit both the hypervisor (the host OS) and the Linux guest OSes. For I/O emulations, KVM uses a userland software, QEMU; this is a userland program that does hardware emulation.
QEMU emulates the processor and a long list of peripheral devices such as the disk, network, VGA, PCI, USB, serial/parallel ports, and so on to build a complete piece of virtual hardware that the guest OS can be installed on. This emulation is powered by KVM.