Seeking Guidance on State Preservation and Configuration for Switched CPUs

[30ZHANGjin]

1. Understanding State Preservation during the Switch
I'm working to build a clear mental model of what happens to the system state when the CPU switch occurs. Based on my research, here is my current understanding:

Preserved States: I believe the Caches, the Branch Predictor, and the Architectural Register File are preserved across the switch to maintain system-level history and program correctness.

Reset States: I believe core-specific microarchitectural states, like the ROB, IQ, and internal pipeline stages, are flushed and reset.

Could you please confirm if this understanding is generally correct? Any details or nuances I might be missing would be very helpful.

2、Challenge with Configuring the Switched CPU
I've also encountered a practical challenge. When I try to pass command-line arguments like --l1d_size to configure the caches for the detailed O3 CPU, I've noticed they only seem to affect the initial CPU (system.cpu) and not the target one (system.switch_cpu_1).

I've considered using multiple --param "system.switch_cpu_1.dcache.size = '...'" flags, but I'm not sure if this is the standard or most robust way to handle this.

Could you offer any advice or point me toward the recommended practice for configuring the switched-to CPU from the command line in this scenario?

[powerjg]

Preserved States: I believe the Caches, the Branch Predictor, and the Architectural Register File are preserved across the switch to maintain system-level history and program correctness.

Correct.

Reset States: I believe core-specific microarchitectural states, like the ROB, IQ, and internal pipeline stages, are flushed and reset.

Correct. I would add that the idea in switching CPUs is that you're going to a new microarchitecture. So, the new microarchitecture (pipeline) will be cold. Also, on the old microarchitecture we need to make sure that nothing is in-flight before switching.

Could you offer any advice or point me toward the recommended practice for configuring the switched-to CPU from the command line in this scenario?

There is no good answer here. If you want to have a complex difference in configuration you are not going to be able to get that using the "command line" as the interface to gem5. You should write a python script from scratch (e.g., using the standard library) which has the behavior you want. Simply, this is much too complex to do via command line parameters and needs to be done using a full programming language.

That said, the standard library SimpleSwitchableProcessor may also be too simple for you. You may need to write your own version of that class depending on exactly what you want to do.

Finally, I'll say that the AI coding tools can be very helpful in learning how to use the python interface to gem5. However, they can sometimes be out of date.

[30ZHANGjin]

Hi @powerjg,

Thanks for the helpful response. As you suggested, I looked into the standard library to configure the switched-out O3 CPU via a Python script.

My findings align with your recommendation. While the caches can sometimes be set from the command line, the core O3 CPU parameters (e.g., issueWidth, numROBEntries) and the branch predictor are best handled in the script. I found two approaches:

Modifying the default values in source files like src/cpu/o3/BaseO3CPU.py. This requires recompiling gem5, making it inflexible.

Setting the parameters on the switch_cpu object directly within the simulation script. This is more flexible as it doesn't require a recompile.

For example:

# Import the required objects
form m5.objects.BranchPredictor import *
# ...

 if options.standard_switch:
        switch_cpus = [
            TimingSimpleCPU(switched_out=True, cpu_id=(i)) for i in range(np)
        ]
        switch_cpus_1 = [
            DerivO3CPU(switched_out=True, cpu_id=(i),
                decodeWidth=8,
                renameWidth=8,
                commitWidth=8,
                branchPred=TAGE()) for i in range(np)
        ]

This is far more flexible than the alternative of modifying the default values in source files (like src/cpu/o3/BaseO3CPU.py), as it doesn't require a recompile for each configuration change. Thanks again for the guidance.