docs: add basic CI documentation

Include info how to get access/enable hardware runners and how to select
individual jobs.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

docs/misc/ci.rst

@@ -0,0 +1,35 @@
+.. SPDX-License-Identifier: CC-BY-4.0
+
+Continuous Integration
+======================
+
+Xen Project uses Gitlab-CI for automated testing. Test pipelines for official
+staging branches are at
+`<https://gitlab.com/xen-project/hardware/xen/-/pipelines>`_. Developers can
+schedule test pipelines in their repositories under
+`<https://gitlab.com/xen-project/people/>`_.
+
+Hardware runners
+****************
+
+Some of the tests are using dedicated hardware runners. Those are not available freely, but the access is granted to individual developers. To get access to them, ask on the ``#XenDevel:matrix.org`` Matrix channel.
+After getting access to relevant runners, few extra changes are necessary in settings of the relevant "xen" gitlab project (under your `<https://gitlab.com/xen-project/people/>`_ namespace):
+
+1. Go to Settings -> CI/CD, expand the "Runners" section and enable relevant runners for your project.
+2. Expand "Variables" section and add ``QUBES_JOBS=true`` variable for Qubes runners.
+3. Go to Settings -> Repository, expand "Branch rules" section and add a rule for protected branches - only those branches will get tests on the hardware runners. It's okay to use a pattern for branch name, and it's okay to allow force push.
+
+Selecting individual tests
+**************************
+
+Normally, all build and test jobs are scheduled in a pipeline. When working on a specific patches, it is sometimes useful to run only jobs relevant for the current work - both to save time and to save CI resources. This can be done by seeting ``SELECTED_JOBS_ONLY`` variable when starting the pipeline. The variable holds a regular expression, enclosed with ``/`` that matches jobs to be included. The variable can be set via the gitlab.com web UI or directly when pushing changes to gitlab::
+
+   git push -o ci.variable=SELECTED_JOBS_ONLY="/job1|job2/"
+
+Note if a test job requires some build job, both need to be included in the regex. For example, ``adl-smoke-x86-64-gcc-debug`` requires ``alpine-3.18-gcc-debug``, so to run just this test the command will look like this::
+
+   git push -o ci.variable=SELECTED_JOBS_ONLY="/adl-smoke-x86-64-gcc-debug|alpine-3.18-gcc-debug/"
+
+More details at `<https://docs.gitlab.co.jp/ee/user/project/push_options.html>`_.
+
+Alternatively, irrelevant jobs can be removed from respective yaml files in ``automation/gitlab-ci`` by adding temporary commit on top of the branch.