From 5febf432df1cfa5b25d99b54c32103fafbdd0eb9 Mon Sep 17 00:00:00 2001 From: Miguel Ojeda Date: Sun, 16 Aug 2026 15:32:33 +0200 Subject: kbuild: rust: keep Rust objects out of Clang LTO with inline helpers Under `CONFIG_LTO_CLANG` + `CONFIG_RUST_INLINE_HELPERS`, one may hit `objtool` errors such as: vmlinux.o: error: objtool: _R..._3Gsp4boot+0xd6a: can't find jump dest instruction at .text._R..._3Gsp4boot+0x1dfd The reason is that in such builds, the Clang invocation that compiles the combined Rust plus helpers bitcode emits LLVM bitcode (again) -- the final code generation happens in the linker's LTO step, which the `-mllvm` trap options passed to Clang do not reach. This, in turn, means that unreachable traps are missing, and the impossible paths do not merely fallthrough to the next symbol, but past the end of their own section, since LTO builds place each function in its own section. Thus filter `CC_FLAGS_LTO` out of the Clang invocation, so that it always emits machine code directly, with the traps in place. Assisted-by: LLM Cc: Gary Guo Cc: Boqun Feng Cc: Alice Ryhl Cc: Matthew Maurer Cc: Josh Poimboeuf Cc: Peter Zijlstra Cc: stable@vger.kernel.org Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust") Acked-by: Gary Guo Link: https://patch.msgid.link/20260816133233.197500-2-ojeda@kernel.org Signed-off-by: Miguel Ojeda --- Documentation/dev-tools/kunit/faq.rst | 109 ++++++++++++++++++++++++++++++++++ 1 file changed, 109 insertions(+) create mode 100644 Documentation/dev-tools/kunit/faq.rst (limited to 'Documentation/dev-tools/kunit/faq.rst') diff --git a/Documentation/dev-tools/kunit/faq.rst b/Documentation/dev-tools/kunit/faq.rst new file mode 100644 index 000000000..b1341c1a6 --- /dev/null +++ b/Documentation/dev-tools/kunit/faq.rst @@ -0,0 +1,109 @@ +.. SPDX-License-Identifier: GPL-2.0 + +========================== +Frequently Asked Questions +========================== + +How is this different from Autotest, kselftest, and so on? +========================================================== +KUnit is a unit testing framework. Autotest, kselftest (and some others) are +not. + +A `unit test `_ is supposed to +test a single unit of code in isolation and hence the name *unit test*. A unit +test should be the finest granularity of testing and should allow all possible +code paths to be tested in the code under test. This is only possible if the +code under test is small and does not have any external dependencies outside of +the test's control like hardware. + +There are no testing frameworks currently available for the kernel that do not +require installing the kernel on a test machine or in a virtual machine. All +testing frameworks require tests to be written in userspace and run on the +kernel under test. This is true for Autotest, kselftest, and some others, +disqualifying any of them from being considered unit testing frameworks. + +Does KUnit support running on architectures other than UML? +=========================================================== + +Yes. KUnit can run on any architecture, though the kunit.py tool can only +build and run kernels for some architectures (of which UML is the default). + +You can build and run tests without kunit.py at all on any architecture by +enabling ``CONFIG_KUNIT=y`` and booting the kernel. +See Documentation/dev-tools/kunit/run_manual.rst for more details. + +Alternatively, kunit.py supports many common architectures using +cross-compilers and the qemu emulator. This can be done using the ``--arch`` +parameter when running the tests, and the ``--cross_compile`` parameter +when building (if the architecture is not supported by the host compiler). +See :ref:`kunit-on-qemu` for more details. + +When writing tests targeting other architectures, it's worth keeping the tips +on the :ref:`kunit-on-non-uml` page in mind. + +.. _kinds-of-tests: + +What is the difference between a unit test and other kinds of tests? +==================================================================== +Most existing tests for the Linux kernel would be categorized as an integration +test, or an end-to-end test. + +- A unit test is supposed to test a single unit of code in isolation. A unit + test should be the finest granularity of testing and, as such, allows all + possible code paths to be tested in the code under test. This is only possible + if the code under test is small and does not have any external dependencies + outside of the test's control like hardware. +- An integration test tests the interaction between a minimal set of components, + usually just two or three. For example, someone might write an integration + test to test the interaction between a driver and a piece of hardware, or to + test the interaction between the userspace libraries the kernel provides and + the kernel itself. However, one of these tests would probably not test the + entire kernel along with hardware interactions and interactions with the + userspace. +- An end-to-end test usually tests the entire system from the perspective of the + code under test. For example, someone might write an end-to-end test for the + kernel by installing a production configuration of the kernel on production + hardware with a production userspace and then trying to exercise some behavior + that depends on interactions between the hardware, the kernel, and userspace. + +KUnit is not working, what should I do? +======================================= + +Unfortunately, there are a number of things which can break, but here are some +things to try. + +1. Run ``./tools/testing/kunit/kunit.py run`` with the ``--raw_output`` + parameter. This might show details or error messages hidden by the kunit_tool + parser. +2. Instead of running ``kunit.py run``, try running ``kunit.py config``, + ``kunit.py build``, and ``kunit.py exec`` independently. This can help track + down where an issue is occurring. (If you think the parser is at fault, you + can run it manually against ``stdin`` or a file with ``kunit.py parse``.) +3. Running the UML kernel directly can often reveal issues or error messages, + ``kunit_tool`` ignores. This should be as simple as runningi the ``vmlinux`` + binary in the output directory (by default ``./.kunit/vmlinux``) after + building the UML kernel (for example, by using ``kunit.py build``). + Note that UML has some unusual requirements (such as the host having a tmpfs + filesystem mounted), and has had issues in the past when built statically and + the host has KASLR enabled. (On older host kernels, you may need to run + ``setarch `uname -m` -R ./vmlinux`` to disable KASLR.) +4. Try running KUnit on a different architecture by using the ``--arch`` + option. On an x86_64 host, using ``--arch=x86_64`` is a good first step. +5. Make sure the kernel .config has ``CONFIG_KUNIT=y`` and at least one test + (e.g. ``CONFIG_KUNIT_EXAMPLE_TEST=y``). kunit_tool will keep its .config + around, so you can see what config was used after running ``kunit.py run``. + It also preserves any config changes you might make, so you can + enable/disable things with ``make ARCH=um menuconfig`` or similar, and then + re-run kunit_tool. +6. Try to run ``make ARCH=um defconfig`` before running ``kunit.py run``. This + may help clean up any residual config items which could be causing problems. +7. Finally, try running KUnit manually, instead of via ``kunit.py``. KUnit can + be built into any kernel, or can be built as a module and loaded at runtime. + When tests are built-in, they will execute when the kernel boots, and + modules will automatically execute associated tests when loaded. Test results + can be collected from ``/sys/kernel/debug/kunit//results``, and + can be parsed with ``kunit.py parse``. For more details, see + Documentation/dev-tools/kunit/run_manual.rst + +If none of the above tricks help, you are always welcome to email any issues to +kunit-dev@googlegroups.com. -- cgit v1.3.1