docs/devel: add "check-tcg" to testing.rst

It was pointed out we haven't documented the check-tcg part of the
build system. Attempt to rectify that now.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

docs/devel/testing.rst

@@ -266,6 +266,8 @@ another application on the host may have locked the file, possibly leading to a
 test failure.  If using such devices are explicitly desired, consider adding
 ``locking=off`` option to disable image locking.
 
+.. _docker-ref:
+
 Docker based tests
 ==================
 
@@ -799,3 +801,77 @@ And remove any package you want with::
 
 If you've used ``make check-acceptance``, the Python virtual environment where
 Avocado is installed will be cleaned up as part of ``make check-clean``.
+
+Testing with "make check-tcg"
+=============================
+
+The check-tcg tests are intended for simple smoke tests of both
+linux-user and softmmu TCG functionality. However to build test
+programs for guest targets you need to have cross compilers available.
+If your distribution supports cross compilers you can do something as
+simple as::
+
+  apt install gcc-aarch64-linux-gnu
+
+The configure script will automatically pick up their presence.
+Sometimes compilers have slightly odd names so the availability of
+them can be prompted by passing in the appropriate configure option
+for the architecture in question, for example::
+
+  $(configure) --cross-cc-aarch64=aarch64-cc
+
+There is also a ``--cross-cc-flags-ARCH`` flag in case additional
+compiler flags are needed to build for a given target.
+
+If you have the ability to run containers as the user you can also
+take advantage of the build systems "Docker" support. It will then use
+containers to build any test case for an enabled guest where there is
+no system compiler available. See :ref: `_docker-ref` for details.
+
+Running subset of tests
+-----------------------
+
+You can build the tests for one architecture::
+
+  make build-tcg-tests-$TARGET
+
+And run with::
+
+  make run-tcg-tests-$TARGET
+
+Adding ``V=1`` to the invocation will show the details of how to
+invoke QEMU for the test which is useful for debugging tests.
+
+TCG test dependencies
+---------------------
+
+The TCG tests are deliberately very light on dependencies and are
+either totally bare with minimal gcc lib support (for softmmu tests)
+or just glibc (for linux-user tests). This is because getting a cross
+compiler to work with additional libraries can be challenging.
+
+Other TCG Tests
+---------------
+
+There are a number of out-of-tree test suites that are used for more
+extensive testing of processor features.
+
+KVM Unit Tests
+~~~~~~~~~~~~~~
+
+The KVM unit tests are designed to run as a Guest OS under KVM but
+there is no reason why they can't exercise the TCG as well. It
+provides a minimal OS kernel with hooks for enabling the MMU as well
+as reporting test results via a special device::
+
+  https://git.kernel.org/pub/scm/virt/kvm/kvm-unit-tests.git
+
+Linux Test Project
+~~~~~~~~~~~~~~~~~~
+
+The LTP is focused on exercising the syscall interface of a Linux
+kernel. It checks that syscalls behave as documented and strives to
+exercise as many corner cases as possible. It is a useful test suite
+to run to exercise QEMU's linux-user code::
+
+  https://linux-test-project.github.io/