…is a concept whereby an operating system is executed from a simulated environment rather than directly on any physical hardware. These virtualised instances are kept in containers such as files, which contain all the needed configuration and disk information needed to be instantiated.
A normal PC exists in several layers as shown below. To virtualise it the hardware layer through to the application layer are containerised. With the hardware layer being replaced with configuration data, and the data of the system(OS and Applications) existing as a virtual disk(s) within this container.
These virtual machine(VM) instances are managed from a hypervisor. This is implemented in two main types:
Type 1 – Bare Metal Hypervisor
Examples include: ESXi, Xen, Hyper-v and KVM
The hypervisor runs on directly on the host’s hardware acting as a “thin” operating system for the host machine. Guest OS’s run on the hypervisor through virtual machine instances.
This approach is often preferred, as running directly on hardware allows for higher virtualisation efficiency.
Type 2 – Host Based Hypervisor
Examples include: VMware workstation and VirtualBox.
The hypervisor runs through the host operating system. The hypervisor manages resources through the host operating system rather than being able to directly manage these resources.
This type of virtualisation is very useful for temporary instances of machines, which would otherwise need to be placed on another physical machine. This approach in particular can be used to do live forensics on a revert-able disk image.
This approach is less efficient than type-1, and relies upon the interoperability that the host OS provides. For example a host OS, in some circumstances, may not passthrough the CPU’s virtualisation suites. Causing the hypervisor to rely on software emulation.
Virtualisation Implementation
So if a VM is supposed to work as if it were it’s own dedicated machine, how does the host hypervisor support this?
Virtual Cores
Each VM is assigned virtual cores (vCPU) upon creation. Each of these is (usually) a virtualised thread of host CPU execution managed by the hypervisor. In a hypervisor such as ESXi, the inbuilt resource scheduler spreads workload over the physical CPU by taking into account vCPU workload, and allocating physical CPU time to these vCPU’s as needed.
As such an underutilised internal http website VM would be given less physical CPU time, than an intensive video encoding VM running on the same host. These VM’s should still be given the physical CPU time needed to complete their executions, but would be able to share the resources of a capable host system.
Types of Virtualisation
No Virtualisation
An example diagram of requests to the hardware in the normal scenario without virtualisation.
Full Virtualisation
Full virtualisation emulates all instructions sent to the physical CPU by the VM. This is very performance intensive as binary translation is needed for VM OS requests.
Full virtualisation is mainly used in host based virtualisation.
Paravirtualisation
Paravirtualisation lets most instructions run directly on the hardware of the host machine without emulation. The only instructions that are changed are non-virtualisable instructions, these are replaced with hypercalls that communicate directly with the hypervisor. The hypervisor will also provide other hypercall interfaces to the VM such as memory management, interrupt handling and timekeeping.
This involves the modification of the VM OS to handle all, non-privileged but still sensitive instructions. As the OS needs to be modified, any OS’s which are unmodifiable are not compatible with Paravirtualisation.
Hardware Assisted Virtualisation
Hardware enhancements from AMD and Intel assist in the virtualisation process. Technologies such as VT-x (Intel) and AMD-v (AMD), allow the hypervisor to run in a root mode. Privileged and sensitive calls are automatically trapped by these technologies, removing the necessity of either binary translation or Paravirtualisation.
In 2008 under some workloads, hardware assisted virtualisation performed worse than binary translation. As of writing, intel boasts that the VT-x technology is as fast as native CPU utilisation.
Available since 2006, hardware assisted virtualisation enhancements are used by VMware, Microsoft, Parallels and Xen, to name a few.