authors: Jason Lowe-Power
last edited: 2024-06-01 14:26:34 +0000

Building the MSI protocol

The SLICC file

Now that we have finished implementing the protocol, we need to compile it. You can download the complete SLICC files below:

Before building the protocol, we need to create one more file: MSI.slicc. This file tells the SLICC compiler which state machine files to compile for this protocol. The first line contains the name of our protocol. Then, the file has a number of include statements. Each include statement has a file name. This filename can come from any of the env variable PROTOCOL_DIRS directories. We declared the current directory as part of the PROTOCOL_DIRS in the SConsopts file (main.Append(PROTOCOL_DIRS=[Dir('.')])). The other directory is src/mem/protocol/. These files are included like C++ header files. Effectively, all of the files are processed as one large SLICC file. Thus, any files that declare types that are used in other files must come before the files they are used in (e.g., MSI-msg.sm must come before MSI-cache.sm since MSI-cache.sm uses the RequestMsg type).

protocol "MSI";
include "RubySlicc_interfaces.slicc";
include "MSI-msg.sm";
include "MSI-cache.sm";
include "MSI-dir.sm";

You can download the fill file here.

Add new config options RUBY_PROTOCOL_MSI (gem5 >= 23.1)

Note: If users use the gem5 version newer than 23.0, they need to do some additional steps to set up the Kconfig file. Otherwise, users can skip the steps to Compiling a protocol with SCons section.

Then You need to added the MSI protocol in the learning_gem5/part3/Kconfig file to let scons enable build gem5 with MSI protocol.

# Set the PROTOCOL="MSI" if the RUBY_PROTOCOL_MSI=y
config PROTOCOL
    default "MSI" if RUBY_PROTOCOL_MSI

# Add the new choice RUBY_PROTOCOL_MSI
cont_choice "Ruby protocol"
    config RUBY_PROTOCOL_MSI
        bool "MSI"
endchoice

In the src/Kconfig

rsource "base/Kconfig"
rsource "mem/ruby/Kconfig"
rsource "learning_gem5/part3/Kconfig"

Please add the learning_gem5/part3/Kconfig below the mem/ruby/Kconfig.

Compiling a protocol with SCons

In the older gem5 versions (gem5 <= 23.0)

Most SCons defaults (found in build_opts/) specify the protocol as MI_example, an example, but poor performing protocol. Therefore, we cannot simply use a default build name (e.g., X86 or ARM). We have to specify the SCons options on the command line. The command line below will build our new protocol with the X86 ISA.

scons build/X86_MSI/gem5.opt --default=X86 PROTOCOL=MSI SLICC_HTML=True

This command will build gem5.opt in the directory build/X86_MSI. You can specify any directory here. This command line has two new parameters: --default and PROTOCOL. First, --default specifies which file to use in build_opts for defaults for all of the SCons variables (e.g., ISA, CPU_MODELS). Next, PROTOCOL overrides any default for the PROTOCOL SCons variable in the default specified. Thus, we are telling SCons to specifically compile our new protocol, not whichever protocol was specified in build_opts/X86.

There is one more variable on this command line to build gem5: SLICC_HTML=True. When you specify this on the building command line, SLICC will generate the HTML tables for your protocol. You can find the HTML tables in <build directory>/mem/protocol/html. By default, the SLICC compiler skips building the HTML tables because it impacts the performance of compiling gem5, especially when compiling on a network file system.

After gem5 finishes compiling, you will have a gem5 binary with your new protocol! If you want to build another protocol into gem5, you have to change the PROTOCOL SCons variable. Thus, it is a good idea to use a different build directory for each protocol, especially if you will be comparing protocols.

When building your protocol, you will likely encounter errors in your SLICC code reported by the SLICC compiler. Most errors include the file and line number of the error. Sometimes, this line number is the line after the error occurs. In fact, the line number can be far below the actual error. For instance, if the curly brackets do not match correctly, the error will report the last line in the file as the location.

In the newer gem5 version (gem5 >= 23.1)

Most Kconfig defaults (found in build_opts/) specify the protocol as MI_example, an example, but poor performing protocol. Therefore, we cannot simply use a default build name (e.g., X86 or ARM). We have to specify the Kconfig options through menuconfig, setconfig, etc. The command lines below will build our new protocol with the X86 ISA.

scons defconfig build/X86_MSI build_opts/X86
scons setconfig build/X86_MSI RUBY_PROTOCOL_MSI=y SLICC_HTML=y
scons build/X86_MSI/gem5.opt

This command will build gem5.opt in the directory build/X86_MSI. You can specify any directory here. The first command tells SCons to create a new build directory, and use the defaults in build_opts/X86 to configure it. The second command uses the setconfig kconfig tool use RUBY_PROTOCOL_MSI=y to update the PROTOCOL and SLICC_HTML options in the build/X86_MSI directory’s configuration. You can also use other tools like menuconfig to update these settings interactively. Finally, the last command tells SCons to build in our build directory using this new configuration.

There is one more kconfig setting we’re changing: SLICC_HTML=y. When you specify this, SLICC will generate the HTML tables for your protocol. You can find the HTML tables in <build directory>/mem/protocol/html. By default, the SLICC compiler skips building the HTML tables because it impacts the performance of compiling gem5, especially when compiling on a network file system.

After gem5 finishes compiling, you will have a gem5 binary with your new protocol! If you want to build another protocol into gem5, you have to set the RUBY_PROTOCOL_{NAME}=y in setconfig step to change the PROTOCOL kconfig variable. Thus, it is a good idea to use a different build directory for each protocol, especially if you will be comparing protocols.

When building your protocol, you will likely encounter errors in your SLICC code reported by the SLICC compiler. Most errors include the file and line number of the error. Sometimes, this line number is the line after the error occurs. In fact, the line number can be far below the actual error. For instance, if the curly brackets do not match correctly, the error will report the last line in the file as the location.

For gem5 kconfig document, see here