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