Today most of us enjoy the benefits of virtualization. With hardware support that finally came to x86 a decade ago and powerful open source hypervisors (also called virtual machine managers), such as VirtualBox or KVM, it's pretty straightforward to run Windows alongside Linux or share a single physical server between a dozen of tenants. In early 2000, to give a new Linux distro a try, you'd install it on a separate hard disk or perhaps run it directly from the CD. Now, you can just boot a virtual machine (VM) from the downloaded ISO image and have fun while reading your friends' tweets.

Yet virtualization internals is still a hairy topic that you may not have a complete picture of. In Linux, Qemu/KVM is the de facto standard tool – or tools (Figure  1). These two have quite a different history but are now seen together more often than not. What's the reason? How does Qemu interact with KVM (and vice versa) to keep your VMs running? In this Core Tech, we'll build a big picture of how an x86 hypervisor operates internally. Qemu, however, is a large and complex project, so we'll dissect a lighter alternative instead: kvmtool. It is simpler yet still real-world and functional, so you may find it useful in your virtualization scenarios.

A Need for Qemu (or Something)

KVM stands for Kernel-based Virtual Machine, so why is there a userspace part in the first place? To answer this question, we need to learn a little bit about how virtualization works in x86.

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