configs,gpu-compute: Support fetch from system pages

The amdgpu driver supports fetching instructions from pages which reside
in system memory rather than device memory. This changeset adds support
to do this by adding the system hub object added in a prior changeset to
the fetch unit and issues requests to the system hub if the system bit
in the memory page's PTE is set. Otherwise, the requestor ID is set to
be device memory and the request is routed through the Ruby network /
GPU caches to fetch the instructions.

Change-Id: Ib2fb47c589fdd5e544ab6493d7dbd8f2d9d7b0e8
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/57652
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

configs/example/gpufs/system/system.py

@@ -133,6 +133,10 @@ def makeGpuFSSystem(args):
     gpu_mem_mgr = AMDGPUMemoryManager()
     system.pc.south_bridge.gpu.memory_manager = gpu_mem_mgr
 
+    # CPU data path (SystemHub)
+    system_hub = AMDGPUSystemHub()
+    shader.system_hub = system_hub
+
     # GPU, HSAPP, and GPUCommandProc are DMA devices
     system._dma_ports.append(gpu_hsapp)
     system._dma_ports.append(gpu_cmd_proc)
@@ -141,6 +145,7 @@ def makeGpuFSSystem(args):
     system._dma_ports.append(sdma1)
     system._dma_ports.append(device_ih)
     system._dma_ports.append(pm4_pkt_proc)
+    system._dma_ports.append(system_hub)
     system._dma_ports.append(gpu_mem_mgr)
     system._dma_ports.append(hsapp_pt_walker)
     system._dma_ports.append(cp_pt_walker)
@@ -154,6 +159,7 @@ def makeGpuFSSystem(args):
     sdma1.pio = system.iobus.mem_side_ports
     device_ih.pio = system.iobus.mem_side_ports
     pm4_pkt_proc.pio = system.iobus.mem_side_ports
+    system_hub.pio = system.iobus.mem_side_ports
 
     # Full system needs special TLBs for SQC, Scalar, and vector data ports
     args.full_system = True