[Lldb-commits] [lldb] [lldb][docs] Add section on testing with QEMU user (PR #149057)

[David Spickett via lldb-commits]

https://github.com/DavidSpickett created https://github.com/llvm/llvm-project/pull/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.

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.

>From eab94453a4275cdbc5fe60607fb18a7869a2d3a8 Mon Sep 17 00:00:00 2001
From: David Spickett <david.spickett at linaro.org>
Date: Wed, 16 Jul 2025 10:20:59 +0000
Subject: [PATCH] [lldb][docs] Add section on testing with QEMU user

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

Adding this is also an excuse to document the limitations
and make the time spent setting up system mode look more
worthwhile.
---
 lldb/docs/resources/qemu-testing.rst | 46 ++++++++++++++++++++++++++++
 1 file changed, 46 insertions(+)

diff --git a/lldb/docs/resources/qemu-testing.rst b/lldb/docs/resources/qemu-testing.rst
index 8571287a04262..36b586113d31d 100644
--- a/lldb/docs/resources/qemu-testing.rst
+++ b/lldb/docs/resources/qemu-testing.rst
@@ -167,3 +167,49 @@ 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 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.