Documentation: kunit: document support for QEMU in kunit_tool

Document QEMU support, what it does, and how to use it in kunit_tool.
Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

Documentation/dev-tools/kunit/kunit-tool.rst

@@ -145,6 +145,54 @@ to run KUnit resource tests, you could use:
 
 This uses the standard glob format for wildcards.
 
+Running Tests on QEMU
+=====================
+
+kunit_tool supports running tests on QEMU as well as via UML (as mentioned
+elsewhere). The default way of running tests on QEMU requires two flags:
+
+``--arch``
+	Selects a collection of configs (Kconfig as well as QEMU configs
+	options, etc) that allow KUnit tests to be run on the specified
+	architecture in a minimal way; this is usually not much slower than
+	using UML. The architecture argument is the same as the name of the
+	option passed to the ``ARCH`` variable used by Kbuild. Not all
+	architectures are currently supported by this flag, but can be handled
+	by the ``--qemu_config`` discussed later. If ``um`` is passed (or this
+	this flag is ignored) the tests will run via UML. Non-UML architectures,
+	e.g. i386, x86_64, arm, um, etc. Non-UML run on QEMU.
+
+``--cross_compile``
+	Specifies the use of a toolchain by Kbuild. The argument passed here is
+	the same passed to the ``CROSS_COMPILE`` variable used by Kbuild. As a
+	reminder this will be the prefix for the toolchain binaries such as gcc
+	for example ``sparc64-linux-gnu-`` if you have the sparc toolchain
+	installed on your system, or
+	``$HOME/toolchains/microblaze/gcc-9.2.0-nolibc/microblaze-linux/bin/microblaze-linux-``
+	if you have downloaded the microblaze toolchain from the 0-day website
+	to a directory in your home directory called ``toolchains``.
+
+In many cases it is likely that you may want to run an architecture which is
+not supported by the ``--arch`` flag, or you may want to just run KUnit tests
+on QEMU using a non-default configuration. For this use case, you can write
+your own QemuConfig. These QemuConfigs are written in Python. They must have an
+import line ``from ..qemu_config import QemuArchParams`` at the top of the file
+and the file must contain a variable called ``QEMU_ARCH`` that has an instance
+of ``QemuArchParams`` assigned to it. An example can be seen in
+``tools/testing/kunit/qemu_configs/x86_64.py``.
+
+Once you have a QemuConfig you can pass it into kunit_tool using the
+``--qemu_config`` flag; when used this flag replaces the ``--arch`` flag. If we
+were to do this with the ``x86_64.py`` example from above, the invocation would
+look something like this:
+
+.. code-block:: bash
+
+	./tools/testing/kunit/kunit.py run \
+		--timeout=60 \
+		--jobs=12 \
+		--qemu_config=./tools/testing/kunit/qemu_configs/x86_64.py
+
 Other Useful Options
 ====================
 

Documentation/dev-tools/kunit/usage.rst

@@ -609,17 +609,45 @@ non-UML architectures:
 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!
+Currently, the KUnit Wrapper (``tools/testing/kunit/kunit.py``) (aka
+kunit_tool) only fully supports running tests inside of UML and QEMU; however,
+this is only due to our own time limitations as humans working on KUnit. It is
+entirely possible to support other emulators and even actual hardware, but for
+now QEMU and UML is what is fully supported within the KUnit Wrapper. Again, to
+be clear, this is just the Wrapper. The actualy KUnit tests and the KUnit
+library they are written in is fully architecture agnostic and can be used in
+virtually any setup, you just won't have the benefit of typing a single command
+out of the box and having everything magically work perfectly.
+
+Again, all core KUnit framework features are fully supported on all
+architectures, and using them is straightforward: Most popular architectures
+are supported directly in the KUnit Wrapper via QEMU. Currently, supported
+architectures on QEMU include:
+
+*   i386
+*   x86_64
+*   arm
+*   arm64
+*   alpha
+*   powerpc
+*   riscv
+*   s390
+*   sparc
+
+In order to run KUnit tests on one of these architectures via QEMU with the
+KUnit wrapper, all you need to do is specify the flags ``--arch`` and
+``--cross_compile`` when invoking the KUnit Wrapper. For example, we could run
+the default KUnit tests on ARM in the following manner (assuming we have an ARM
+toolchain installed):
+
+.. code-block:: bash
+
+	tools/testing/kunit/kunit.py run --timeout=60 --jobs=12 --arch=arm --cross_compile=arm-linux-gnueabihf-
+
+Alternatively, if you want to run your tests on real hardware or in some other
+emulation environment, 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: