1.. SPDX-License-Identifier: GPL-2.0
2
3=========================================
4KUnit - Unit Testing for the Linux Kernel
5=========================================
6
7.. toctree::
8	:maxdepth: 2
9
10	start
11	usage
12	kunit-tool
13	api/index
14	style
15	faq
16	tips
17	running_tips
18
19What is KUnit?
20==============
21
22KUnit is a lightweight unit testing and mocking framework for the Linux kernel.
23
24KUnit is heavily inspired by JUnit, Python's unittest.mock, and
25Googletest/Googlemock for C++. KUnit provides facilities for defining unit test
26cases, grouping related test cases into test suites, providing common
27infrastructure for running tests, and much more.
28
29KUnit consists of a kernel component, which provides a set of macros for easily
30writing unit tests. Tests written against KUnit will run on kernel boot if
31built-in, or when loaded if built as a module. These tests write out results to
32the kernel log in `TAP <https://testanything.org/>`_ format.
33
34To make running these tests (and reading the results) easier, KUnit offers
35:doc:`kunit_tool <kunit-tool>`, which builds a `User Mode Linux
36<http://user-mode-linux.sourceforge.net>`_ kernel, runs it, and parses the test
37results. This provides a quick way of running KUnit tests during development,
38without requiring a virtual machine or separate hardware.
39
40Get started now: Documentation/dev-tools/kunit/start.rst
41
42Why KUnit?
43==========
44
45A unit test is supposed to test a single unit of code in isolation, hence the
46name. A unit test should be the finest granularity of testing and as such should
47allow all possible code paths to be tested in the code under test; this is only
48possible if the code under test is very small and does not have any external
49dependencies outside of the test's control like hardware.
50
51KUnit provides a common framework for unit tests within the kernel.
52
53KUnit tests can be run on most architectures, and most tests are architecture
54independent. All built-in KUnit tests run on kernel startup.  Alternatively,
55KUnit and KUnit tests can be built as modules and tests will run when the test
56module is loaded.
57
58.. note::
59
60        KUnit can also run tests without needing a virtual machine or actual
61        hardware under User Mode Linux. User Mode Linux is a Linux architecture,
62        like ARM or x86, which compiles the kernel as a Linux executable. KUnit
63        can be used with UML either by building with ``ARCH=um`` (like any other
64        architecture), or by using :doc:`kunit_tool <kunit-tool>`.
65
66KUnit is fast. Excluding build time, from invocation to completion KUnit can run
67several dozen tests in only 10 to 20 seconds; this might not sound like a big
68deal to some people, but having such fast and easy to run tests fundamentally
69changes the way you go about testing and even writing code in the first place.
70Linus himself said in his `git talk at Google
71<https://gist.github.com/lorn/1272686/revisions#diff-53c65572127855f1b003db4064a94573R874>`_:
72
73	"... a lot of people seem to think that performance is about doing the
74	same thing, just doing it faster, and that is not true. That is not what
75	performance is all about. If you can do something really fast, really
76	well, people will start using it differently."
77
78In this context Linus was talking about branching and merging,
79but this point also applies to testing. If your tests are slow, unreliable, are
80difficult to write, and require a special setup or special hardware to run,
81then you wait a lot longer to write tests, and you wait a lot longer to run
82tests; this means that tests are likely to break, unlikely to test a lot of
83things, and are unlikely to be rerun once they pass. If your tests are really
84fast, you run them all the time, every time you make a change, and every time
85someone sends you some code. Why trust that someone ran all their tests
86correctly on every change when you can just run them yourself in less time than
87it takes to read their test log?
88
89How do I use it?
90================
91
92*   Documentation/dev-tools/kunit/start.rst - for new users of KUnit
93*   Documentation/dev-tools/kunit/tips.rst - for short examples of best practices
94*   Documentation/dev-tools/kunit/usage.rst - for a more detailed explanation of KUnit features
95*   Documentation/dev-tools/kunit/api/index.rst - for the list of KUnit APIs used for testing
96*   Documentation/dev-tools/kunit/kunit-tool.rst - for more information on the kunit_tool helper script
97*   Documentation/dev-tools/kunit/faq.rst - for answers to some common questions about KUnit
98