Adding plugin tests to QEMU

ADDING_PLUGIN_TESTS_TO_QEMU

# Adding Plugin Tests to QEMU

I've been working on some enhancements to the QEMU TCG plugin API recently, and wouldn't you know it, when you change the
plugin API it's usually helpful to test your changes. So I've had to figure out how to add TCG plugin tests, which is not a particularly
self-explanatory process.

## Makefile System

If you run `make help` in your QEMU build directory after building QEMU with e.g. `./configure --enable-plugins --enable-capstone && make -C build`
you'll see a `check-tcg` target. This will run a lot of tests, but it won't run the ones you want. As it turns out, the
Makefiles for TCG eventually include `tests/tcg/Makefile.target`, which in turn includes all the specific targets like
`i386` and `x86_64` as well as the `multiarch` tests. It also defines the top-level make targets to actually run those
tests, like `run-plugin-%`. As it turns out, there are also targets for running all the tests for a specific QEMU target,
like `build-tcg-tests-x86_64-softmmu` and `run-tcg-tests-x86_64-softmmu`.

## Adding a test target

A test target should be a small program that just does the required thing, and it can be either linux-user or softmmu. For linux-user targets, just put them either in the multiarch/ directory or the directory for the target arch you care about. For
system targets, they'll need to go in the system subdirectory of multiarch or the target arch you care about. Both .S and .c files will get picked up and built, although keep in mind for system targets you can only use `minilib.h`.

## Adding test target rules

To add args or an output check command for your target and plugin, add something like this to e.g. multiarch/system/Makefile.softmmu-target:

```
# Test plugin instruction patching
run-plugin-patch-target-with-libpatch.so:		\
	PLUGIN_ARGS=$(COMMA)use_hwaddr=true$(COMMA)debug_insns=true
run-plugin-patch-target-with-libpatch.so:		\
	CHECK_PLUGIN_OUTPUT_COMMAND=$(MULTIARCH_SYSTEM_SRC)/validate-patch.py [email protected]
```

## Skipping a test

If you want a test built so people can manually run it but it only applies to specific architectures or some other special scenario, you can explicily skip it by setting this in tests/tcg/Makefile.target:

```
run-plugin-%-with-libhypercalls.so:
	$(call skip-test, $<, "MANUAL ONLY libhypercalls.so only valid on certain arches")
```