This file explains how to use gem5‘s updated testing infrastructure. Running tests before submitting a patch is incredibly important so unexpected bugs don’t creep into gem5.

gem5's testing infrastructure has the following goals:

  • Simple for all users to run
  • Fast execution in the simple case
  • High coverage of gem5 code

Running unit tests

gem5 comes with unit tests, created using the Google Test framework. These can be built through SCons.

To build and run all the unit tests:

scons build/NULL/unittests.opt

All unit tests should be run prior to posting a patch to

To compile and run just one set of tests (e.g. those declared within src/base/

scons build/NULL/base/bitunion.test.opt

To list the available test functions from a test file:

./build/NULL/base/bitunion.test.opt --gtest_list_tests

To run a specific test function (e.g., BitUnionData.NormalBitfield):

./build/NULL/base/bitunion.test.opt --gtest_filter=BitUnionData.NormalBitfield

Running system-level tests

Within the tests directory we have system-level tests. These tests run the gem5 framework against various hardware configurations, with different ISAs, then verify the simulations execute correctly. These should be seen as high-level, coarse-grained tests to compliment the unit-tests.

Below is the most common way the tests are run. This will run all of the “quick” tests for X86, ARM, and RISC-V. These tests make up our best-supported platforms and use cases. When running these tests, you will likely want to us the option -j <CPUs> where CPUs is as large as you can make it. Additionally, it is often a good idea to run longer tests (e.g., linux boot) before submitting your patch.

cd tests
./ run

The above is the minumum you should run before posting a patch to

Running tests from multiple directories

The command line above will walk the directory tree starting from the cwd (tests), and it will run every test it encounters in its path. It is possible to specify multiple root directories by providing several positional arguments:

./ run <directory1> <directory2> [...]

This will load every test in directory1 and directory2 (and their subdirectories).

Specifying a subset of tests to run

You can use the tag query interface to specify the exact tests you want to run. For instance, if you want to run only with gem5.opt, you can use

./ run --variant opt

Or, if you want to just run X86 tests with the gem5.opt binary:

./ run --length quick --variant opt --isa X86

To view all of the available tags, use

./ list --all-tags

The output is split into tag types (e.g., isa, variant, length) and the tags for each type are listed after the type name.

You can specify “or” between tags within the same type by using the tag flag multiple times. For instance, to run everything that is tagged “opt” or “fast” use

./ run --variant opt --variant fast

You can also specify “and” between different types of tags by specifying more than one type on the command line. For instance, this will only run tests with both the “X86” and “opt” tags.

./ run --isa X86 --variant opt

Running tests in batch

The testing infrastructure provides the two needed methods to run tests in batch. First, you can list all of the tests based on the same tags as above in a machine-readable format by passing the -q flag. This will list all of the suites that match the given tag(s).

./ list -q --suites

Next, you can run a single suite from the command line by passing the option --uid. For instance,

./ run --skip-build \
    --uid SuiteUID:tests/gem5/m5_util/

With this method, you can only run a single suite at a time. If you want to run more than one uid, you must call ./ multiple times.

Currently, you must specify --skip-build if you want to run a single suite or run in batch mode. Otherwise, you will build gem5 for all architectures.

Rerunning failed tests

While developing software a common practice is to run tests, make a change, and assert that the tests still pass. If tests fail you'll likely want to rerun and fix those specific tests without running redundant ones. The testing infrastructure allows you to rerun tests which failed in the last execution by using the rerun command.

./ run
#  Some tests fail...

# Rerun only the failed test suites (not the ones which passed).
./ rerun

If something goes wrong

The first step is to turn up the verbosity of the output using -v. This will allow you to see what tests are running and why a test is failing.

If a test fails, the temporary directory where the gem5 output was saved is kept and the path to the directory is printed in the terminal.

Debugging the testing infrastructure

Every command takes an option for the verbosity. -v, -vv, -vvv will increase the verbosity level. If something isn't working correctly, you can start here.

Most of the code for the testing infrastructure is in ext/testlib. This code contains the base code for tests, suites, fixtures, etc. The code in tests/gem5 is gem5-specific code. For the most part, the code in tests/gem5 extends the structures in ext/testlib.

Common errors

You may see a number of lines of output during test discovery that look like the following:

    Tried to load tests from ... but failed with an exception.

The testing library searches all python files in the tests/ directory. The test library executes each python file it finds searching for tests. It‘s okay if the file causes an exception. This means there are no tests in that file (e.g., it’s not a new-style test).

Binary test applications

The code for some test binaries that are run in the gem5 guest during testing can be found in tests/test-progs. There's one directory per test application. The source code is under the source directory.

You may have a bin directory as well. The bin directory is automatically created when running the test case that uses the test binary. This is not the case when a test is run via the --bin-path option. In that scenario a bin directory will be created in the selected path rather than in tests/test-progs. The binary is downloaded from the gem5 servers the first time it is referenced by a test.

Some other tests (like Linux-boot) don't have sources inside gem5 and are simply downloaded from gem5 servers.

Updating the test binaries

The test infrastructure should check with the gem5 servers to ensure you have the latest binaries. However, if you believe your binaries are out of date, simply delete the bin directory and they will be re-downloaded to your local machine.

Building (new-style) test binaries

In each src/ directory under tests/test-progs, there is a Makefile. This Makefile downloads a docker image and builds the test binary for some ISA (e.g., Makefile.x86 builds the binary for x86). Additionally, if you run make upload it will upload the binaries to the gem5 server, if you have access to modify the binaries. If you need to modify the binaries for updating a test or adding a new test and you don't have access to the gem5 server, contact a maintainer (see MAINTAINERS).

Running Tests in Parallel

Whimsy has support for parallel testing baked in. This system supports running multiple suites at the same time on the same computer. To run suites in parallel, supply the -t <number-tests> flag to the run command.

For example, to run up to three test suites at the same time::

./ run --skip-build -t 3