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authorBrendan Higgins <brendanhiggins@google.com>2019-09-23 02:02:45 -0700
committerShuah Khan <skhan@linuxfoundation.org>2019-09-30 17:35:01 -0600
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Documentation: kunit: add documentation for KUnit
Add documentation for KUnit, the Linux kernel unit testing framework. - Add intro and usage guide for KUnit - Add API reference Signed-off-by: Felix Guo <felixguoxiuping@gmail.com> Signed-off-by: Brendan Higgins <brendanhiggins@google.com> Cc: Jonathan Corbet <corbet@lwn.net> Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Reviewed-by: Logan Gunthorpe <logang@deltatee.com> Reviewed-by: Stephen Boyd <sboyd@kernel.org> Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
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+.. SPDX-License-Identifier: GPL-2.0
+
+===========
+Using KUnit
+===========
+
+The purpose of this document is to describe what KUnit is, how it works, how it
+is intended to be used, and all the concepts and terminology that are needed to
+understand it. This guide assumes a working knowledge of the Linux kernel and
+some basic knowledge of testing.
+
+For a high level introduction to KUnit, including setting up KUnit for your
+project, see :doc:`start`.
+
+Organization of this document
+=============================
+
+This document is organized into two main sections: Testing and Isolating
+Behavior. The first covers what a unit test is and how to use KUnit to write
+them. The second covers how to use KUnit to isolate code and make it possible
+to unit test code that was otherwise un-unit-testable.
+
+Testing
+=======
+
+What is KUnit?
+--------------
+
+"K" is short for "kernel" so "KUnit" is the "(Linux) Kernel Unit Testing
+Framework." KUnit is intended first and foremost for writing unit tests; it is
+general enough that it can be used to write integration tests; however, this is
+a secondary goal. KUnit has no ambition of being the only testing framework for
+the kernel; for example, it does not intend to be an end-to-end testing
+framework.
+
+What is Unit Testing?
+---------------------
+
+A `unit test <https://martinfowler.com/bliki/UnitTest.html>`_ is a test that
+tests code at the smallest possible scope, a *unit* of code. In the C
+programming language that's a function.
+
+Unit tests should be written for all the publicly exposed functions in a
+compilation unit; so that is all the functions that are exported in either a
+*class* (defined below) or all functions which are **not** static.
+
+Writing Tests
+-------------
+
+Test Cases
+~~~~~~~~~~
+
+The fundamental unit in KUnit is the test case. A test case is a function with
+the signature ``void (*)(struct kunit *test)``. It calls a function to be tested
+and then sets *expectations* for what should happen. For example:
+
+.. code-block:: c
+
+ void example_test_success(struct kunit *test)
+ {
+ }
+
+ void example_test_failure(struct kunit *test)
+ {
+ KUNIT_FAIL(test, "This test never passes.");
+ }
+
+In the above example ``example_test_success`` always passes because it does
+nothing; no expectations are set, so all expectations pass. On the other hand
+``example_test_failure`` always fails because it calls ``KUNIT_FAIL``, which is
+a special expectation that logs a message and causes the test case to fail.
+
+Expectations
+~~~~~~~~~~~~
+An *expectation* is a way to specify that you expect a piece of code to do
+something in a test. An expectation is called like a function. A test is made
+by setting expectations about the behavior of a piece of code under test; when
+one or more of the expectations fail, the test case fails and information about
+the failure is logged. For example:
+
+.. code-block:: c
+
+ void add_test_basic(struct kunit *test)
+ {
+ KUNIT_EXPECT_EQ(test, 1, add(1, 0));
+ KUNIT_EXPECT_EQ(test, 2, add(1, 1));
+ }
+
+In the above example ``add_test_basic`` makes a number of assertions about the
+behavior of a function called ``add``; the first parameter is always of type
+``struct kunit *``, which contains information about the current test context;
+the second parameter, in this case, is what the value is expected to be; the
+last value is what the value actually is. If ``add`` passes all of these
+expectations, the test case, ``add_test_basic`` will pass; if any one of these
+expectations fail, the test case will fail.
+
+It is important to understand that a test case *fails* when any expectation is
+violated; however, the test will continue running, potentially trying other
+expectations until the test case ends or is otherwise terminated. This is as
+opposed to *assertions* which are discussed later.
+
+To learn about more expectations supported by KUnit, see :doc:`api/test`.
+
+.. note::
+ A single test case should be pretty short, pretty easy to understand,
+ focused on a single behavior.
+
+For example, if we wanted to properly test the add function above, we would
+create additional tests cases which would each test a different property that an
+add function should have like this:
+
+.. code-block:: c
+
+ void add_test_basic(struct kunit *test)
+ {
+ KUNIT_EXPECT_EQ(test, 1, add(1, 0));
+ KUNIT_EXPECT_EQ(test, 2, add(1, 1));
+ }
+
+ void add_test_negative(struct kunit *test)
+ {
+ KUNIT_EXPECT_EQ(test, 0, add(-1, 1));
+ }
+
+ void add_test_max(struct kunit *test)
+ {
+ KUNIT_EXPECT_EQ(test, INT_MAX, add(0, INT_MAX));
+ KUNIT_EXPECT_EQ(test, -1, add(INT_MAX, INT_MIN));
+ }
+
+ void add_test_overflow(struct kunit *test)
+ {
+ KUNIT_EXPECT_EQ(test, INT_MIN, add(INT_MAX, 1));
+ }
+
+Notice how it is immediately obvious what all the properties that we are testing
+for are.
+
+Assertions
+~~~~~~~~~~
+
+KUnit also has the concept of an *assertion*. An assertion is just like an
+expectation except the assertion immediately terminates the test case if it is
+not satisfied.
+
+For example:
+
+.. code-block:: c
+
+ static void mock_test_do_expect_default_return(struct kunit *test)
+ {
+ struct mock_test_context *ctx = test->priv;
+ struct mock *mock = ctx->mock;
+ int param0 = 5, param1 = -5;
+ const char *two_param_types[] = {"int", "int"};
+ const void *two_params[] = {&param0, &param1};
+ const void *ret;
+
+ ret = mock->do_expect(mock,
+ "test_printk", test_printk,
+ two_param_types, two_params,
+ ARRAY_SIZE(two_params));
+ KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ret);
+ KUNIT_EXPECT_EQ(test, -4, *((int *) ret));
+ }
+
+In this example, the method under test should return a pointer to a value, so
+if the pointer returned by the method is null or an errno, we don't want to
+bother continuing the test since the following expectation could crash the test
+case. `ASSERT_NOT_ERR_OR_NULL(...)` allows us to bail out of the test case if
+the appropriate conditions have not been satisfied to complete the test.
+
+Test Suites
+~~~~~~~~~~~
+
+Now obviously one unit test isn't very helpful; the power comes from having
+many test cases covering all of your behaviors. Consequently it is common to
+have many *similar* tests; in order to reduce duplication in these closely
+related tests most unit testing frameworks provide the concept of a *test
+suite*, in KUnit we call it a *test suite*; all it is is just a collection of
+test cases for a unit of code with a set up function that gets invoked before
+every test cases and then a tear down function that gets invoked after every
+test case completes.
+
+Example:
+
+.. code-block:: c
+
+ static struct kunit_case example_test_cases[] = {
+ KUNIT_CASE(example_test_foo),
+ KUNIT_CASE(example_test_bar),
+ KUNIT_CASE(example_test_baz),
+ {}
+ };
+
+ static struct kunit_suite example_test_suite = {
+ .name = "example",
+ .init = example_test_init,
+ .exit = example_test_exit,
+ .test_cases = example_test_cases,
+ };
+ kunit_test_suite(example_test_suite);
+
+In the above example the test suite, ``example_test_suite``, would run the test
+cases ``example_test_foo``, ``example_test_bar``, and ``example_test_baz``,
+each would have ``example_test_init`` called immediately before it and would
+have ``example_test_exit`` called immediately after it.
+``kunit_test_suite(example_test_suite)`` registers the test suite with the
+KUnit test framework.
+
+.. note::
+ A test case will only be run if it is associated with a test suite.
+
+For a more information on these types of things see the :doc:`api/test`.
+
+Isolating Behavior
+==================
+
+The most important aspect of unit testing that other forms of testing do not
+provide is the ability to limit the amount of code under test to a single unit.
+In practice, this is only possible by being able to control what code gets run
+when the unit under test calls a function and this is usually accomplished
+through some sort of indirection where a function is exposed as part of an API
+such that the definition of that function can be changed without affecting the
+rest of the code base. In the kernel this primarily comes from two constructs,
+classes, structs that contain function pointers that are provided by the
+implementer, and architecture specific functions which have definitions selected
+at compile time.
+
+Classes
+-------
+
+Classes are not a construct that is built into the C programming language;
+however, it is an easily derived concept. Accordingly, pretty much every project
+that does not use a standardized object oriented library (like GNOME's GObject)
+has their own slightly different way of doing object oriented programming; the
+Linux kernel is no exception.
+
+The central concept in kernel object oriented programming is the class. In the
+kernel, a *class* is a struct that contains function pointers. This creates a
+contract between *implementers* and *users* since it forces them to use the
+same function signature without having to call the function directly. In order
+for it to truly be a class, the function pointers must specify that a pointer
+to the class, known as a *class handle*, be one of the parameters; this makes
+it possible for the member functions (also known as *methods*) to have access
+to member variables (more commonly known as *fields*) allowing the same
+implementation to have multiple *instances*.
+
+Typically a class can be *overridden* by *child classes* by embedding the
+*parent class* in the child class. Then when a method provided by the child
+class is called, the child implementation knows that the pointer passed to it is
+of a parent contained within the child; because of this, the child can compute
+the pointer to itself because the pointer to the parent is always a fixed offset
+from the pointer to the child; this offset is the offset of the parent contained
+in the child struct. For example:
+
+.. code-block:: c
+
+ struct shape {
+ int (*area)(struct shape *this);
+ };
+
+ struct rectangle {
+ struct shape parent;
+ int length;
+ int width;
+ };
+
+ int rectangle_area(struct shape *this)
+ {
+ struct rectangle *self = container_of(this, struct shape, parent);
+
+ return self->length * self->width;
+ };
+
+ void rectangle_new(struct rectangle *self, int length, int width)
+ {
+ self->parent.area = rectangle_area;
+ self->length = length;
+ self->width = width;
+ }
+
+In this example (as in most kernel code) the operation of computing the pointer
+to the child from the pointer to the parent is done by ``container_of``.
+
+Faking Classes
+~~~~~~~~~~~~~~
+
+In order to unit test a piece of code that calls a method in a class, the
+behavior of the method must be controllable, otherwise the test ceases to be a
+unit test and becomes an integration test.
+
+A fake just provides an implementation of a piece of code that is different than
+what runs in a production instance, but behaves identically from the standpoint
+of the callers; this is usually done to replace a dependency that is hard to
+deal with, or is slow.
+
+A good example for this might be implementing a fake EEPROM that just stores the
+"contents" in an internal buffer. For example, let's assume we have a class that
+represents an EEPROM:
+
+.. code-block:: c
+
+ struct eeprom {
+ ssize_t (*read)(struct eeprom *this, size_t offset, char *buffer, size_t count);
+ ssize_t (*write)(struct eeprom *this, size_t offset, const char *buffer, size_t count);
+ };
+
+And we want to test some code that buffers writes to the EEPROM:
+
+.. code-block:: c
+
+ struct eeprom_buffer {
+ ssize_t (*write)(struct eeprom_buffer *this, const char *buffer, size_t count);
+ int flush(struct eeprom_buffer *this);
+ size_t flush_count; /* Flushes when buffer exceeds flush_count. */
+ };
+
+ struct eeprom_buffer *new_eeprom_buffer(struct eeprom *eeprom);
+ void destroy_eeprom_buffer(struct eeprom *eeprom);
+
+We can easily test this code by *faking out* the underlying EEPROM:
+
+.. code-block:: c
+
+ struct fake_eeprom {
+ struct eeprom parent;
+ char contents[FAKE_EEPROM_CONTENTS_SIZE];
+ };
+
+ ssize_t fake_eeprom_read(struct eeprom *parent, size_t offset, char *buffer, size_t count)
+ {
+ struct fake_eeprom *this = container_of(parent, struct fake_eeprom, parent);
+
+ count = min(count, FAKE_EEPROM_CONTENTS_SIZE - offset);
+ memcpy(buffer, this->contents + offset, count);
+
+ return count;
+ }
+
+ ssize_t fake_eeprom_write(struct eeprom *this, size_t offset, const char *buffer, size_t count)
+ {
+ struct fake_eeprom *this = container_of(parent, struct fake_eeprom, parent);
+
+ count = min(count, FAKE_EEPROM_CONTENTS_SIZE - offset);
+ memcpy(this->contents + offset, buffer, count);
+
+ return count;
+ }
+
+ void fake_eeprom_init(struct fake_eeprom *this)
+ {
+ this->parent.read = fake_eeprom_read;
+ this->parent.write = fake_eeprom_write;
+ memset(this->contents, 0, FAKE_EEPROM_CONTENTS_SIZE);
+ }
+
+We can now use it to test ``struct eeprom_buffer``:
+
+.. code-block:: c
+
+ struct eeprom_buffer_test {
+ struct fake_eeprom *fake_eeprom;
+ struct eeprom_buffer *eeprom_buffer;
+ };
+
+ static void eeprom_buffer_test_does_not_write_until_flush(struct kunit *test)
+ {
+ struct eeprom_buffer_test *ctx = test->priv;
+ struct eeprom_buffer *eeprom_buffer = ctx->eeprom_buffer;
+ struct fake_eeprom *fake_eeprom = ctx->fake_eeprom;
+ char buffer[] = {0xff};
+
+ eeprom_buffer->flush_count = SIZE_MAX;
+
+ eeprom_buffer->write(eeprom_buffer, buffer, 1);
+ KUNIT_EXPECT_EQ(test, fake_eeprom->contents[0], 0);
+
+ eeprom_buffer->write(eeprom_buffer, buffer, 1);
+ KUNIT_EXPECT_EQ(test, fake_eeprom->contents[1], 0);
+
+ eeprom_buffer->flush(eeprom_buffer);
+ KUNIT_EXPECT_EQ(test, fake_eeprom->contents[0], 0xff);
+ KUNIT_EXPECT_EQ(test, fake_eeprom->contents[1], 0xff);
+ }
+
+ static void eeprom_buffer_test_flushes_after_flush_count_met(struct kunit *test)
+ {
+ struct eeprom_buffer_test *ctx = test->priv;
+ struct eeprom_buffer *eeprom_buffer = ctx->eeprom_buffer;
+ struct fake_eeprom *fake_eeprom = ctx->fake_eeprom;
+ char buffer[] = {0xff};
+
+ eeprom_buffer->flush_count = 2;
+
+ eeprom_buffer->write(eeprom_buffer, buffer, 1);
+ KUNIT_EXPECT_EQ(test, fake_eeprom->contents[0], 0);
+
+ eeprom_buffer->write(eeprom_buffer, buffer, 1);
+ KUNIT_EXPECT_EQ(test, fake_eeprom->contents[0], 0xff);
+ KUNIT_EXPECT_EQ(test, fake_eeprom->contents[1], 0xff);
+ }
+
+ static void eeprom_buffer_test_flushes_increments_of_flush_count(struct kunit *test)
+ {
+ struct eeprom_buffer_test *ctx = test->priv;
+ struct eeprom_buffer *eeprom_buffer = ctx->eeprom_buffer;
+ struct fake_eeprom *fake_eeprom = ctx->fake_eeprom;
+ char buffer[] = {0xff, 0xff};
+
+ eeprom_buffer->flush_count = 2;
+
+ eeprom_buffer->write(eeprom_buffer, buffer, 1);
+ KUNIT_EXPECT_EQ(test, fake_eeprom->contents[0], 0);
+
+ eeprom_buffer->write(eeprom_buffer, buffer, 2);
+ KUNIT_EXPECT_EQ(test, fake_eeprom->contents[0], 0xff);
+ KUNIT_EXPECT_EQ(test, fake_eeprom->contents[1], 0xff);
+ /* Should have only flushed the first two bytes. */
+ KUNIT_EXPECT_EQ(test, fake_eeprom->contents[2], 0);
+ }
+
+ static int eeprom_buffer_test_init(struct kunit *test)
+ {
+ struct eeprom_buffer_test *ctx;
+
+ ctx = kunit_kzalloc(test, sizeof(*ctx), GFP_KERNEL);
+ KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ctx);
+
+ ctx->fake_eeprom = kunit_kzalloc(test, sizeof(*ctx->fake_eeprom), GFP_KERNEL);
+ KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ctx->fake_eeprom);
+ fake_eeprom_init(ctx->fake_eeprom);
+
+ ctx->eeprom_buffer = new_eeprom_buffer(&ctx->fake_eeprom->parent);
+ KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ctx->eeprom_buffer);
+
+ test->priv = ctx;
+
+ return 0;
+ }
+
+ static void eeprom_buffer_test_exit(struct kunit *test)
+ {
+ struct eeprom_buffer_test *ctx = test->priv;
+
+ destroy_eeprom_buffer(ctx->eeprom_buffer);
+ }
+
+.. _kunit-on-non-uml:
+
+KUnit on non-UML architectures
+==============================
+
+By default KUnit uses UML as a way to provide dependencies for code under test.
+Under most circumstances KUnit's usage of UML should be treated as an
+implementation detail of how KUnit works under the hood. Nevertheless, there
+are instances where being able to run architecture specific code, or test
+against real hardware is desirable. For these reasons KUnit supports running on
+other architectures.
+
+Running existing KUnit tests on non-UML architectures
+-----------------------------------------------------
+
+There are some special considerations when running existing KUnit tests on
+non-UML architectures:
+
+* Hardware may not be deterministic, so a test that always passes or fails
+ when run under UML may not always do so on real hardware.
+* Hardware and VM environments may not be hermetic. KUnit tries its best to
+ provide a hermetic environment to run tests; however, it cannot manage state
+ that it doesn't know about outside of the kernel. Consequently, tests that
+ may be hermetic on UML may not be hermetic on other architectures.
+* Some features and tooling may not be supported outside of UML.
+* Hardware and VMs are slower than UML.
+
+None of these are reasons not to run your KUnit tests on real hardware; they are
+only things to be aware of when doing so.
+
+The biggest impediment will likely be that certain KUnit features and
+infrastructure may not support your target environment. For example, at this
+time the KUnit Wrapper (``tools/testing/kunit/kunit.py``) does not work outside
+of UML. Unfortunately, there is no way around this. Using UML (or even just a
+particular architecture) allows us to make a lot of assumptions that make it
+possible to do things which might otherwise be impossible.
+
+Nevertheless, all core KUnit framework features are fully supported on all
+architectures, and using them is straightforward: all you need to do is to take
+your kunitconfig, your Kconfig options for the tests you would like to run, and
+merge them into whatever config your are using for your platform. That's it!
+
+For example, let's say you have the following kunitconfig:
+
+.. code-block:: none
+
+ CONFIG_KUNIT=y
+ CONFIG_KUNIT_EXAMPLE_TEST=y
+
+If you wanted to run this test on an x86 VM, you might add the following config
+options to your ``.config``:
+
+.. code-block:: none
+
+ CONFIG_KUNIT=y
+ CONFIG_KUNIT_EXAMPLE_TEST=y
+ CONFIG_SERIAL_8250=y
+ CONFIG_SERIAL_8250_CONSOLE=y
+
+All these new options do is enable support for a common serial console needed
+for logging.
+
+Next, you could build a kernel with these tests as follows:
+
+
+.. code-block:: bash
+
+ make ARCH=x86 olddefconfig
+ make ARCH=x86
+
+Once you have built a kernel, you could run it on QEMU as follows:
+
+.. code-block:: bash
+
+ qemu-system-x86_64 -enable-kvm \
+ -m 1024 \
+ -kernel arch/x86_64/boot/bzImage \
+ -append 'console=ttyS0' \
+ --nographic
+
+Interspersed in the kernel logs you might see the following:
+
+.. code-block:: none
+
+ TAP version 14
+ # Subtest: example
+ 1..1
+ # example_simple_test: initializing
+ ok 1 - example_simple_test
+ ok 1 - example
+
+Congratulations, you just ran a KUnit test on the x86 architecture!
+
+Writing new tests for other architectures
+-----------------------------------------
+
+The first thing you must do is ask yourself whether it is necessary to write a
+KUnit test for a specific architecture, and then whether it is necessary to
+write that test for a particular piece of hardware. In general, writing a test
+that depends on having access to a particular piece of hardware or software (not
+included in the Linux source repo) should be avoided at all costs.
+
+Even if you only ever plan on running your KUnit test on your hardware
+configuration, other people may want to run your tests and may not have access
+to your hardware. If you write your test to run on UML, then anyone can run your
+tests without knowing anything about your particular setup, and you can still
+run your tests on your hardware setup just by compiling for your architecture.
+
+.. important::
+ Always prefer tests that run on UML to tests that only run under a particular
+ architecture, and always prefer tests that run under QEMU or another easy
+ (and monitarily free) to obtain software environment to a specific piece of
+ hardware.
+
+Nevertheless, there are still valid reasons to write an architecture or hardware
+specific test: for example, you might want to test some code that really belongs
+in ``arch/some-arch/*``. Even so, try your best to write the test so that it
+does not depend on physical hardware: if some of your test cases don't need the
+hardware, only require the hardware for tests that actually need it.
+
+Now that you have narrowed down exactly what bits are hardware specific, the
+actual procedure for writing and running the tests is pretty much the same as
+writing normal KUnit tests. One special caveat is that you have to reset
+hardware state in between test cases; if this is not possible, you may only be
+able to run one test case per invocation.
+
+.. TODO(brendanhiggins@google.com): Add an actual example of an architecture
+ dependent KUnit test.