docs: update virt/lexicon
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title: Lexicon
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description:
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published: true
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date: 2021-11-13T18:47:23.476Z
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date: 2022-01-12T15:43:12.102Z
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tags:
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editor: markdown
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dateCreated: 2021-11-13T11:58:43.776Z
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---
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> Section under construction.
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{.is-warning}
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# Key terms related to virtualization
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## Terms related to virtualization
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### Emulator
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## Emulator
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Emulators or virtualizers are software that provide material components
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similar to physical hardware, but that are made of computer code instead
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of silicon, -- virtual hardware --, such as virtual floppy disks.
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QEMU[^6] is a popular emulator that can act as a simulator or virtual
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QEMU is a popular emulator that can act as a simulator or virtual
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machine monitor. In the latter case, it can leverage hardware
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acceleration,
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### Hardware-assisted virtualization
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## Hardware-assisted virtualization
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Hardware-assisted virtualization is a feature of certain computer
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hardware made to take advantage of virtualization.
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@@ -34,12 +31,12 @@ In other words, hardware-assisted virtualization translates into better
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performance for virtualized workloads, significantly reducing the gap in
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performance between a virtual machine and a physical one.
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### Nested-virtualization
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## Nested-virtualization
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Nested-virtualization refers to the ability to run a virtual machine
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inside another virtual machine.
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### Virtualization
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## Virtualization
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Most computers are made of hardware and software. By analogy, the brain
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that animates the cells to control a body can be thought as the
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@@ -53,12 +50,12 @@ machines.
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There are roughly three types of virtualization:
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- **Simulation or emulation**: when a computer is fully emulated and
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* **Simulation or emulation**: when a computer is fully emulated and
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can be made to look like any device to an operating system
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- **Partition**: when computer resources are split such that each
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* **Partition**: when computer resources are split such that each
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operating system can only see a subset of available hardware
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resources
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- **Paravirtualization**: when both hardware and software-assisted
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* **Paravirtualization**: when both hardware and software-assisted
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virtualization is being used. In this case, the guest is aware that
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it is running in the virtualized environment, and acts accordingly.
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@@ -68,7 +65,7 @@ better use of resources through consolidation. For instance, with
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virtualization, multiple operating systems can run concurrently on a
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physical machine.
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### Hypervisor
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## Hypervisor
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A hypervisor is an operating system or firmware that is designed to run
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guest systems: it handles scheduling, execution of hyper privileged
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@@ -86,56 +83,51 @@ virtual hardware.
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As of 2021, there are two major open-source hypervisor that are both are
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able to leverage hardware-assisted virtualization:
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- Xen (2003)[^7].
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- Kernel-based Virtual Machine (KVM) module for Linux (2007)[^8].
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- Xen (2003)
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- Kernel-based Virtual Machine (KVM) module for Linux (2007)
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### Device
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## Device
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Devices are computer components that can be attached to machines. They
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can be classified in two ways : physical or emulated.
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+--------+-------------------+----------------+------------------------+
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| | Physical hardware | Emulated : | Emulated : paravirtual |
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| | | | |
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| | | model-based | |
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+--------+-------------------+----------------+------------------------+
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| Design | Specific | Specific | Generic |
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+--------+-------------------+----------------+------------------------+
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| Type | Silicon-based | Software-based | Software-based |
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+--------+-------------------+----------------+------------------------+
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| | Physical hardware | Emulated: model-based | Emulated: paravirtual |
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| :- | :-: | :-: | :-: |
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| *Design* | Specific | Specific | Generic |
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| *Type* | Silicon-based | Software-based | Software-based |
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- Physical
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* Physical
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- Physical components refer to devices that can be attached to a
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* Physical components refer to devices that can be attached to a
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system. For instance, a dedicated physical graphics card
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attached to a physical system can be directly attached to a
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virtual machine, which then becomes responsible for managing it,
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a technique called *passthrough*. The PCI-SIG standards provide
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IOMMU-related specifications to allow a host operating system to
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not have a driver for a particular device[^9] and passthrough
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not have a driver for a particular device and passthrough
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the device to the guest. The guest will have a device with
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nearly native performance, and use the standard vendor's drivers
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for the device.
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- The PCI-SIG standards also provide a way to partition compatible
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* The PCI-SIG standards also provide a way to partition compatible
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devices using so-called Virtual Functions (VFs). In this case,
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the host manages the way a physical device is used by the guest.
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Both host and guest must have specific device drivers. It offers
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nearly native performance.
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- Emulated
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* Emulated
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- Model-based
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* Model-based
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- Model-based emulated hardware are designed after real
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* Model-based emulated hardware are designed after real
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devices, but are made out of computer code, not silicon. The
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i440fx and Q35[^10] chipsets are both instances of emulated
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i440fx and Q35 chipsets are both instances of emulated
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hardware. This is the slowest (but most compatible) way to
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provide a device to a guest. An emulated GPU is not going to
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be fast enough in an emulated mode to do 3D rendering.
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- Paravirtual
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* Paravirtual
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- Paravirtual hardware, also known as paravirtualized Virtual
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* Paravirtual hardware, also known as paravirtualized Virtual
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I/O devices or simply virtio, are also made out of computer
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code. Contrary to emulated hardware, they function as a
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generic piece of software-based hardware which doesn't
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@@ -145,7 +137,7 @@ can be classified in two ways : physical or emulated.
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Paravirtualization refers to the emulation practice of letting an
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operating system running in a virtualized environment know that it is
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running in such an environment[^11].
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running in such an environment.
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Under this configuration, more efficient communication methods are
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available between the host and the guest, including VirtIO devices
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