Short introduction to Virtual Machine Extensions

Things have been changed greatly in the virtualization area. A few years ago, we had to emulate hardware by either binary patch at runtime or modify the guest OS. So not all the instruction was run natively and the goal was to run as much instruction as possible natively on the actual processors without the VMM intervention.

Nowadays, there is a better way to do virtualization. Namely, hardware support! Hardware vendors came up with many new features in CPU so that we do not have to do all the work in the software any more. At the heart of this technology is VMX in Intel and SVM in AMD. Here is high level overview of these technology. The goal is that from time to time VMM needs a way to gain control to feed different information to guest.
First, system software will have to register with processors that it is interested in gaining control when the guest OS executes such and such instruction. We do this by configuring virtual-machine control structure (VMCS).

Once this is set up and the guest OS actually executes instructions we registered, it will cause the transition from VMX guest operation to VMX root operation so that it can give processed information to the guest. This is called VM exits.

Following is a diagram from Intel Manual with regard to the interactions between VMM and VM guests.

There are some other ways to enter VMM mode but that's essentially how the virtualization with hardware support works in the core. Here I have skipped how the system software discovers VMX support in the processor and how it enables the features. For more information, please refer to Intel/AMD documentation. Hopefully, I will have more to say about those in the next posts.