[lldb][docs] Add section on testing with QEMU user (#149057)

This is not recommended to basically anyone but on occasion it's useful
and could be used for testing with other simulator programs for example
bare metal simulators.

It is not something we do officially support or make any quality
guarantees for.

Adding this is also an excuse to document the limitations and make the
time spent setting up system mode look more worthwhile and might be good
to cite in future discussions about testing in simulation.

lldb/docs/resources/qemu-testing.rst

@@ -167,3 +167,50 @@ The result of this is that:
 
 Your VM configuration should have ports ``54321`` and ``49140`` forwarded for
 this to work.
+
+QEMU user mode emulation
+------------------------
+
+Serious testing of LLDB should be done using system mode emulation. The following
+is presented for information only and is not a supported testing configuration
+supported by the LLDB project.
+
+However, it is possible to run the test suite against user mode QEMU if you just
+want to test a specific aspect of ``lldb`` and are ok ignoring a lot of expected
+failures. This method can also be adapted for simulators with a qemu-like command
+line interface.
+
+(``lldb-server`` cannot be tested using user mode QEMU because that does not
+emulate the debugging system calls that ``lldb-server`` tries to make)
+
+Change ``LLDB_TEST_USER_ARGS`` to choose the ``qemu-user`` platform and
+configure it for your architecture. The example below is for AArch64 and assumes
+that ``qemu-aarch64`` is installed and on your path.
+
+If you need to override how the ``qemu-user`` platform finds the QEMU binary,
+look up the rest of the platform's settings in LLDB.
+
+::
+
+  -DLLDB_TEST_USER_ARGS="--platform-name;qemu-user;--setting;platform.plugin.qemu-user.architecture=aarch64;--arch;aarch64"
+
+Also set ``LLDB_TEST_COMPILER`` to something that can target the emulated
+architecture. Then you should be able to run ``ninja check-lldb`` and it will
+run the tests on QEMU user automatically.
+
+You will see a number of failures compared to a normal test run. Reasons for
+this can be, but are not limited to:
+
+* QEMU's built-in debug stub acting differently and supporting different
+  features to different extents, when compared to ``lldb-server``. We try to
+  be compatible but LLDB is not regularly tested with QEMU user.
+
+* Tests that spawn new processes to attach to. QEMU user only emulates a single
+  process.
+
+* Watchpoints. Either these are not emulated or behave differently to real
+  hardware. Add ``--skip-category;watchpoint`` to ``-DLLDB_TEST_USER_ARGS`` to
+  skip those.
+
+* Lack of memory region information due to QEMU communicating this in the
+  GDB server format which LLDB does not use.