sim: Squash a long standing warning from pybind11.

The module_ constructor which takes a module name and an optional
docstring is deprecated. This change replaces it with the
create_extension_module call that it would result in, which gets rid of
the warning.

Change-Id: I700b4afcf1e5e2548af18e2eb2a7b1214c989807
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/50587
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/sim/init.cc

@@ -193,7 +193,17 @@ EmbeddedPyBind::initAll()
 {
     std::list<EmbeddedPyBind *> pending;
 
-    py::module_ m_m5 = py::module_("_m5");
+    // The PyModuleDef structure needs to live as long as the module it
+    // defines, so we'll leak it here so it lives forever. This is what
+    // pybind11 does internally in the module_ constructor we were using. We
+    // could theoretically keep track of the lifetime of the _m5 module
+    // somehow and clean this up when it goes away, but that doesn't seem
+    // worth the effort. The docs recommend statically allocating it, but that
+    // could be unsafe on the very slim chance this method is called more than
+    // once.
+    auto *py_mod_def = new py::module_::module_def;
+    py::module_ m_m5 = py::module_::create_extension_module(
+            "_m5", nullptr, py_mod_def);
     m_m5.attr("__package__") = py::cast("_m5");
 
     pybind_init_core(m_m5);