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Reviewed-by: Richard Henderson <richard.henderson@linaro.org> Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org> Message-Id: <20241209183104.365796-7-pierrick.bouvier@linaro.org> [AJB: update MAINTAINERS] Signed-off-by: Alex Bennée <alex.bennee@linaro.org> Message-Id: <20250116160306.1709518-37-alex.bennee@linaro.org>pull/281/head
Pierrick Bouvier
1 year ago
committed by
Alex Bennée
Alex Bennée
8 changed files with 292 additions and 0 deletions
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.. _Glossary: |
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|
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-------- |
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Glossary |
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-------- |
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|
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This section of the manual presents brief definitions of acronyms and terms used |
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by QEMU developers. |
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|
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Accelerator |
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----------- |
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|
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A specific API used to accelerate execution of guest instructions. It can be |
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hardware-based, through a virtualization API provided by the host OS (kvm, hvf, |
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whpx, ...), or software-based (tcg). See this description of `supported |
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accelerators<Accelerators>`. |
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Board |
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----- |
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Another name for :ref:`machine`. |
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Block |
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----- |
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|
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Block drivers are the available `disk formats and front-ends |
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<block-drivers>` available, and block devices `(see Block device section on |
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options page)<sec_005finvocation>` are using them to implement disks for a |
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virtual machine. |
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CFI |
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--- |
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|
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Control Flow Integrity is a hardening technique used to prevent exploits |
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targeting QEMU by detecting unexpected branches during execution. QEMU `actively |
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supports<cfi>` being compiled with CFI enabled. |
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|
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Device |
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------ |
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|
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In QEMU, a device is a piece of hardware visible to the guest. Examples include |
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UARTs, PCI controllers, PCI cards, VGA controllers, and many more. |
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|
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QEMU is able to emulate a CPU, and all the hardware interacting with it, |
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including `many devices<device-emulation>`. When QEMU runs a virtual machine |
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using a hardware-based accelerator, it is responsible for emulating, using |
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software, all devices. |
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|
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EDK2 |
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---- |
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|
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EDK2, as known as `TianoCore <https://www.tianocore.org/>`_, is an open source |
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implementation of UEFI standard. QEMU virtual machines that boot a UEFI firmware |
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usually use EDK2. |
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|
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gdbstub |
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------- |
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|
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QEMU implements a `gdb server <GDB usage>`, allowing gdb to attach to it and |
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debug a running virtual machine, or a program in user-mode. This allows |
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debugging the guest code that is running inside QEMU. |
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|
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glib2 |
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----- |
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|
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`GLib2 <https://docs.gtk.org/glib/>`_ is one of the most important libraries we |
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are using through the codebase. It provides many data structures, macros, string |
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and thread utilities and portable functions across different OS. It's required |
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to build QEMU. |
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|
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Guest agent |
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----------- |
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|
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The `QEMU Guest Agent <qemu-ga>` is a daemon intended to be run within virtual |
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machines. It provides various services to help QEMU to interact with it. |
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.. _guest: |
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Guest |
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----- |
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|
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Guest is the architecture of the virtual machine, which is emulated. |
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See also :ref:`host`. |
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Sometimes this is called the :ref:`target` architecture, but that term |
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can be ambiguous. |
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.. _host: |
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Host |
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---- |
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|
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Host is the architecture on which QEMU is running on, which is native. |
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See also :ref:`guest`. |
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Hypervisor |
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---------- |
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|
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The formal definition of an hypervisor is a program or API than can be used to |
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manage a virtual machine. QEMU is a virtualizer, that interacts with various |
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hypervisors. |
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|
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In the context of QEMU, an hypervisor is an API, provided by the Host OS, |
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allowing to execute virtual machines. Linux implementation is KVM (and supports |
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Xen as well). For MacOS, it's HVF. Windows defines WHPX. And NetBSD provides |
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NVMM. |
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.. _machine: |
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Machine |
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------- |
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|
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QEMU's system emulation models many different types of hardware. A machine model |
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(sometimes called a board model) is the model of a complete virtual system with |
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RAM, one or more CPUs, and various devices. It can be selected with the option |
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``-machine`` of qemu-system. Our machine models can be found on this `page |
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<system-targets-ref>`. |
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Migration |
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--------- |
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|
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QEMU can save and restore the execution of a virtual machine between different |
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host systems. This is provided by the `Migration framework<migration>`. |
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NBD |
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--- |
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|
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The `QEMU Network Block Device server <qemu-nbd>` is a tool that can be used to |
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mount and access QEMU images, providing functionality similar to a loop device. |
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|
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Mailing List |
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------------ |
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|
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This is `where <https://wiki.qemu.org/Contribute/MailingLists>`_ all the |
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development happens! Changes are posted as series, that all developers can |
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review and share feedback for. |
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|
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For reporting issues, our `GitLab |
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<https://gitlab.com/qemu-project/qemu/-/issues>`_ tracker is the best place. |
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.. _softmmu: |
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|
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MMU / softmmu |
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------------- |
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|
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The Memory Management Unit is responsible for translating virtual addresses to |
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physical addresses and managing memory protection. QEMU system mode is named |
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"softmmu" precisely because it implements this in software, including a TLB |
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(Translation lookaside buffer), for the guest virtual machine. |
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|
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QEMU user-mode does not implement a full software MMU, but "simply" translates |
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virtual addresses by adding a specific offset, and relying on host MMU/OS |
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instead. |
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Monitor / QMP / HMP |
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------------------- |
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The `QEMU Monitor <QEMU monitor>` is a text interface which can be used to interact |
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with a running virtual machine. |
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QMP stands for QEMU Monitor Protocol and is a json based interface. |
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HMP stands for Human Monitor Protocol and is a set of text commands available |
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for users who prefer natural language to json. |
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MTTCG |
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----- |
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|
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Multiple CPU support was first implemented using a round-robin algorithm |
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running on a single thread. Later on, `Multi-threaded TCG <mttcg>` was developed |
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to benefit from multiple cores to speed up execution. |
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Plugins |
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------- |
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|
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`TCG Plugins <TCG Plugins>` is an API used to instrument guest code, in system |
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and user mode. The end goal is to have a similar set of functionality compared |
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to `DynamoRIO <https://dynamorio.org/>`_ or `valgrind <https://valgrind.org/>`_. |
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One key advantage of QEMU plugins is that they can be used to perform |
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architecture agnostic instrumentation. |
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Patchew |
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------- |
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|
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`Patchew <https://patchew.org/QEMU/>`_ is a website that tracks patches on the |
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Mailing List. |
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PR |
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-- |
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|
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Once a series is reviewed and accepted by a subsystem maintainer, it will be |
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included in a PR (Pull Request) that the project maintainer will merge into QEMU |
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main branch, after running tests. |
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The QEMU project doesn't currently expect most developers to directly submit |
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pull requests. |
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QCOW2 |
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----- |
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|
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QEMU Copy On Write is a disk format developed by QEMU. It provides transparent |
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compression, automatic extension, and many other advantages over a raw image. |
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qcow2 is the recommended format to use. |
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|
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QEMU |
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---- |
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|
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`QEMU (Quick Emulator) <https://www.qemu.org/>`_ is a generic and open source |
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machine emulator and virtualizer. |
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QOM |
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--- |
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|
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`QEMU Object Model <qom>` is an object oriented API used to define various |
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devices and hardware in the QEMU codebase. |
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|
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Record/replay |
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------------- |
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|
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`Record/replay <replay>` is a feature of QEMU allowing to have a deterministic |
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and reproducible execution of a virtual machine. |
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Rust |
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---- |
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|
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`A new programming language <https://www.rust-lang.org/>`_, memory safe by |
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default. There is a work in progress to integrate it in QEMU codebase for |
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various subsystems. |
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|
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System mode |
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----------- |
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|
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QEMU System mode provides a virtual model of an entire machine (CPU, memory and |
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emulated devices) to run a guest OS. In this mode the CPU may be fully emulated, |
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or it may work with a hypervisor such as KVM, Xen or Hypervisor.Framework to |
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allow the guest to run directly on the host CPU. |
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QEMU System mode is called :ref:`softmmu <softmmu>` as well. |
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.. _target: |
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Target |
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------ |
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|
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The term "target" can be ambiguous. In most places in QEMU it is used as a |
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synonym for :ref:`guest`. For example the code for emulating Arm CPUs is in |
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``target/arm/``. However in the :ref:`TCG subsystem <tcg>` "target" refers to the |
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architecture which QEMU is running on, i.e. the :ref:`host`. |
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TCG |
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--- |
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|
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TCG is the QEMU `Tiny Code Generator <tcg>`. It is the JIT (just-in-time) |
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compiler we use to emulate a guest CPU in software. |
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It is one of the accelerators supported by QEMU, and supports a lot of |
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guest/host architectures. |
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User mode |
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--------- |
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|
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QEMU User mode can launch processes compiled for one CPU on another CPU. In this |
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mode the CPU is always emulated. In this mode, QEMU translate system calls from |
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guest to host kernel. It is available for Linux and BSD. |
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|
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VirtIO |
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------ |
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|
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VirtIO is an open standard used to define and implement virtual devices with a |
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minimal overhead, defining a set of data structures and hypercalls (similar to |
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system calls, but targeting an hypervisor, which happens to be QEMU in our |
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case). It's designed to be more efficient than emulating a real device, by |
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minimizing the amount of interactions between a guest VM and its hypervisor. |
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vhost-user |
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---------- |
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|
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`Vhost-user <vhost_user>` is an interface used to implement VirtIO devices |
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outside of QEMU itself. |
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