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