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	api/index
13	faq
14
15What is KUnit?
16==============
17
18KUnit is a lightweight unit testing and mocking framework for the Linux kernel.
19These tests are able to be run locally on a developer's workstation without a VM
20or special hardware.
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
27Get started now: :doc:`start`
28
29Why KUnit?
30==========
31
32A unit test is supposed to test a single unit of code in isolation, hence the
33name. A unit test should be the finest granularity of testing and as such should
34allow all possible code paths to be tested in the code under test; this is only
35possible if the code under test is very small and does not have any external
36dependencies outside of the test's control like hardware.
37
38Outside of KUnit, there are no testing frameworks currently
39available for the kernel that do not require installing the kernel on a test
40machine or in a VM and all require tests to be written in userspace running on
41the kernel; this is true for Autotest, and kselftest, disqualifying
42any of them from being considered unit testing frameworks.
43
44KUnit addresses the problem of being able to run tests without needing a virtual
45machine or actual hardware with User Mode Linux. User Mode Linux is a Linux
46architecture, like ARM or x86; however, unlike other architectures it compiles
47to a standalone program that can be run like any other program directly inside
48of a host operating system; to be clear, it does not require any virtualization
49support; it is just a regular program.
50
51KUnit is fast. Excluding build time, from invocation to completion KUnit can run
52several dozen tests in only 10 to 20 seconds; this might not sound like a big
53deal to some people, but having such fast and easy to run tests fundamentally
54changes the way you go about testing and even writing code in the first place.
55Linus himself said in his `git talk at Google
56<https://gist.github.com/lorn/1272686/revisions#diff-53c65572127855f1b003db4064a94573R874>`_:
57
58	"... a lot of people seem to think that performance is about doing the
59	same thing, just doing it faster, and that is not true. That is not what
60	performance is all about. If you can do something really fast, really
61	well, people will start using it differently."
62
63In this context Linus was talking about branching and merging,
64but this point also applies to testing. If your tests are slow, unreliable, are
65difficult to write, and require a special setup or special hardware to run,
66then you wait a lot longer to write tests, and you wait a lot longer to run
67tests; this means that tests are likely to break, unlikely to test a lot of
68things, and are unlikely to be rerun once they pass. If your tests are really
69fast, you run them all the time, every time you make a change, and every time
70someone sends you some code. Why trust that someone ran all their tests
71correctly on every change when you can just run them yourself in less time than
72it takes to read their test log?
73
74How do I use it?
75================
76
77*   :doc:`start` - for new users of KUnit
78*   :doc:`usage` - for a more detailed explanation of KUnit features
79*   :doc:`api/index` - for the list of KUnit APIs used for testing
80