config: Add the ability to read a config file using C++ and Python

This patch adds the ability to load in config.ini files generated from
gem5 into another instance of gem5 built without Python configuration
support. The intended use case is for configuring gem5 when it is a
library embedded in another simulation system.

A parallel config file reader is also provided purely in Python to
demonstrate the approach taken and to provided similar functionality
for as-yet-unknown use models. The Python configuration file reader
can read both .ini and .json files.

C++ configuration file reading:

A command line option has been added for scons to enable C++ configuration
file reading: --with-cxx-config

There is an example in util/cxx_config that shows C++ configuration in action.
util/cxx_config/README explains how to build the example.

Configuration is achieved by the object CxxConfigManager. It handles
reading object descriptions from a CxxConfigFileBase object which
wraps a config file reader. The wrapper class CxxIniFile is provided
which wraps an IniFile for reading .ini files. Reading .json files
from C++ would be possible with a similar wrapper and a JSON parser.

After reading object descriptions, CxxConfigManager creates
SimObjectParam-derived objects from the classes in the (generated with this
patch) directory build/ARCH/cxx_config

CxxConfigManager can then build SimObjects from those SimObjectParams (in an
order dictated by the SimObject-value parameters on other objects) and bind
ports of the produced SimObjects.

A minimal set of instantiate-replacing member functions are provided by
CxxConfigManager and few of the member functions of SimObject (such as drain)
are extended onto CxxConfigManager.

Python configuration file reading (configs/example/read_config.py):

A Python version of the reader is also supplied with a similar interface to
CxxConfigFileBase (In Python: ConfigFile) to config file readers.

The Python config file reading will handle both .ini and .json files.

The object construction strategy is slightly different in Python from the C++
reader as you need to avoid objects prematurely becoming the children of other
objects when setting parameters.

Port binding also needs to be strictly in the same port-index order as the
original instantiation.

1 files changed, 44 insertions, 0 deletions

diff --git a/util/cxx_config/README b/util/cxx_config/README
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index 000000000..a80f7ad8f
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+++ b/util/cxx_config/README
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+This directory contains a demo of C++ configuration of gem5.  The intention
+is to provide a mechanism to allow pre-generated config.ini files generated
+by Python-based gem5 to be reloaded in library-base versions of gem5
+embedded in other systems using C++ calls for simulation control.
+
+This demo implements a few of the simulation control mechanisms of the Python
+gem5 on top of a C++ configured system.
+
+Read main.cc for more details of the implementation.
+
+To build:
+
+First build gem5 as a library with cxx-config support and (optionally)
+without python.  Also build a normal gem5 (cxx-config not needed, Python
+needed):
+
+> cd ../..
+> scons build/ARM/gem5.opt
+> scons --with-cxx-config --without-python build/ARM/libgem5_opt.so
+> cd util/cxx_config
+
+Then edit Makefile to set the paths for PYTHON and run make
+
+> make
+
+Make a config file for the C++-configured gem5 using normal gem5
+
+> ../../build/ARM/gem5.opt ../../configs/example/se.py -c \
+>       ../../tests/test-progs/hello/bin/arm/linux/hello
+
+The binary 'gem5.opt.cxx' can now be used to load in the generated config
+file from the previous normal gem5 run.
+
+Try:
+
+> ./gem5.opt.cxx m5out/config.ini
+
+This should print:
+
+> Hello world!
+
+The .ini file can also be read by the Python .ini file reader example:
+
+> ../../build/ARM/gem5.opt ../../configs/example/read_config.py m5out/config.ini