Devices are computer components that can be attached to machines. They can be classified in two ways : physical or emulated.
| Physical hardware | Emulated: model-based | Emulated: paravirtual | |
|---|---|---|---|
| Design | Specific | Specific | Generic |
| Type | Silicon-based | Software-based | Software-based |
Physical
Physical components refer to devices that can be attached to a system. For instance, a dedicated physical graphics card attached to a physical system can be directly attached to a virtual machine, which then becomes responsible for managing it, a technique called passthrough. The PCI-SIG standards provide IOMMU-related specifications to allow a host operating system to not have a driver for a particular device[1] and passthrough the device to the guest. The guest will have a device with nearly native performance, and use the standard vendor's drivers for the device.
The PCI-SIG standards also provide a way to partition compatible devices using so-called Virtual Functions (VFs). In this case, the host manages the way a physical device is used by the guest. Both host and guest must have specific device drivers. It offers nearly native performance.
Emulated
Paravirtual
Emulators or virtualizers are software that provide material components similar to physical hardware, but that are made of computer code instead of silicon, -- virtual hardware --, such as virtual floppy disks.
QEMU[3] is a popular emulator that can act as a simulator or virtual machine monitor. In the latter case, it can leverage hardware acceleration,
Firmware are software that are tightly integrated with the hardware. Almost all devices are shipping with some kind of firmware associated to it. For instance, motherboards include such programs, which in their case is tasked to properly boot hardware devices such as RAM modules, check their state and make them ready-to-use by an operating system.
Hardware-assisted virtualization is a feature of certain computer hardware made to take advantage of virtualization.
Such hardware reduces the overheads associated with virtualization and is thus key to unlocking near-native performance for virtual machines.
In other words, hardware-assisted virtualization translates into better performance for virtualized workloads, significantly reducing the gap in performance between a virtual machine and a physical one.
A hypervisor is an operating system or firmware that is designed to run guest systems: it handles scheduling, execution of hyper privileged instructions, memory management and over-commitment, and provides drivers for physical devices.
The Virtual Machine Monitor (VMM) is a software that runs on top of the hypervisor and manages the life cycle of a virtual machine. It provides device models for emulated devices and implements tasks such as start, suspend, migrate, and stop virtual machines.
The hypervisor and VMM work in tandem with emulators, which provide them virtual hardware.
As of 2021, there are two major open-source hypervisor that are both are able to leverage hardware-assisted virtualization:
Nested-virtualization refers to the ability to run a virtual machine inside another virtual machine.
Paravirtualization refers to the emulation practice of letting an operating system running in a virtualized environment know that it is running in such an environment.
Under this configuration, more efficient communication methods are available between the host and the guest, including VirtIO devices (e.g.: virtio-net for network devices, virtio-blk for block storage devices). Such devices can communicate directly with the host, instead of emulating every single command of an IDE, SATA, SCSI or NVMe device, as it is the case for model-based emulation.
A virtual machine is a recreation of a real, physical, silicon-based computer using software. It performs almost exactly as a physical computer, and can thus host an operating system.
The expression "virtual machine" is often abbreviated VM. VMs are also often referred to as guests, in contrast to the hosts that host them.
Most computers are made of hardware and software. By analogy, the brain that animates the cells to control a body can be thought as the operating system that controls components of its body.
Virtualization can be defined as the ability to run a software-based computer inside a physical computer. It is a set of computer-related techniques that make it possible to create replicas of computer hardware out of computer code. Those replicas are often referred to as virtual
machines.
There are roughly three types of virtualization:
Simulation or emulation: when a computer is fully emulated and can be made to look like any device to an operating system
Partition: when computer resources are split such that each operating system can only see a subset of available hardware resources
Paravirtualization: when both hardware and software-assisted virtualization is being used. In this case, the guest is aware that it is running in the virtualized environment, and acts accordingly.
Virtualization is used to better isolate resources on a physical computer and to distribute them across various workloads, enabling better use of resources through consolidation. For instance, with virtualization, multiple operating systems can run concurrently on a physical machine.
Thanks to Pasha Tatashin for pointing this out. ↩︎
The Q35 chipset, contrary to i440fx, provides a PCI Express (PCIe) bus. Alas, the i440fx, despite its limitations to PCI-only, does support passthrough of PCIe devices, as long as the driver follows suit. Thanks to Stefan Reiter for pointing that out. ↩︎
In its online manual, QEMU is described as follows: “QEMU is a generic and open source machine emulator and virtualizer. QEMU can be used in several different ways. The most common is for “system emulation”, where it provides a virtual model of an entire machine (CPU, memory and emulated devices) to run a guest OS. In this mode the CPU may be fully emulated, or it may work with a hypervisor such as KVM, Xen, Hax or Hypervisor. Framework to allow the guest to run directly on the host CPU.” ↩︎
Originally developed at Cambridge, Xen was first released in 2003. It is primarily used alongside Linux, although Xen is compatible with other operating systems as well. Interestingly, it can function on hardware that lacks hardware-assisted virtualization. AWS is by far the largest Xen shop, but is slowly shifting towards KVM. ↩︎
The Kernel-based Virtual Machine (KVM) module for Linux was first developed by Qumranet, Inc, a former software company focusing on Desktop virtualization which was later acquired by Red Hat. Qumranet, Inc has also developed the SPICE protocol, which among other things allows for interaction with a guest display machine over a network. ↩︎