TheRock/rocm-systems/emulation/rocjitsu/docs/configuration.md

Configuration

Simulation topologies are defined declaratively in JSON. The config specifies the component hierarchy, link connectivity, and simulation parameters.

Config files

Pre-built configs are in configs/:

File	Description
gfx950_cdna4.json	Single CDNA4 GPU (standalone simulation)
gfx950_cdna4_kmd.json	Single CDNA4 GPU (daemon/KFD mode)
gfx950_cdna4_kmd_2gpu.json	Two CDNA4 GPUs (multi-GPU daemon mode)
gfx942_cdna3.json	Single CDNA3 GPU (standalone simulation)
gfx942_cdna3_kmd.json	Single CDNA3 GPU (daemon/KFD mode)
gfx1250.json	Single gfx1250 GPU (standalone simulation, no KMD)

JSON structure

{
  "max_ticks": 100000,
  "num_threads": 1,
  "exec_mode": "functional",
  "vm": { "arch": "cdna4" },
  "topology": {
    "root": {
      "name": "soc", "type": "soc",
      "children": [
        { "name": "vram", "type": "gpu_memory" },
        { "name": "xcd[0:8]", "type": "xcd", "children": [...] }
      ]
    },
    "links": [
      {
        "pattern": "xcd[i].se[j].cu[k].req -> xcd[i].l2.cpl_[j*8+k]",
        "for_ranges": [
          { "var_name": "i", "start": 0, "end": 8 },
          { "var_name": "j", "start": 0, "end": 4 },
          { "var_name": "k", "start": 0, "end": 8 }
        ],
        "latency": 1, "weight": 10
      }
    ]
  }
}

The example above is intentionally minimal and single-threaded.

Top-level fields

Field	Type	Description
max_ticks	int	Maximum simulation ticks (0 = unlimited)
num_threads	int	Number of PDES engine partitions/worker threads.
exec_mode	string	"functional" or "cycle"
vm.arch	string	Architecture: cdna3, cdna4, etc.

Topology

Components are defined hierarchically under topology.root. Range expansion (xcd[0:8]) creates multiple instances. Links connect component ports using pattern expressions with loop variables.

KFD device section

KFD-mode configs include a vm.gpu.device section that defines the properties reported through the simulated sysfs topology (GPU ID, vendor/device IDs, CU counts, memory sizes, etc.). These must match the component hierarchy defined in topology.

FlatBuffers schema

The JSON config is validated against FlatBuffers schemas in schemas/:

• simulation_config.fbs — topology and simulation parameters
• checkpoint.fbs — simulation state checkpointing

Multi-GPU

Multi-GPU configs define multiple SoCs with distinct GPU IDs and location IDs. Each GPU gets its own command processor, memory, and cache hierarchy. The daemon manages all GPUs and routes KFD ioctls to the correct device based on gpu_id.

See configs/gfx950_cdna4_kmd_2gpu.json for a working two-GPU configuration used by the RCCL collective tests.