mem: Maintain a back door into the AbstractMem's backing store.

The backing store pointer is added to the back door when it's set,
assuming that the range isn't interleaved. If it is interleaved, then
there isn't a way to get a flat pointer to the backing store.

Depending on how the backing store is set up, it may be possible to
return a larger backdoor which applies to all interleaved memories at
the same time and to avoid problems with interleaving. I'm leaving this
as a todo.

Change-Id: I0e531c22835ec10954ab39f761b3d87666b59220
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17668
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

src/mem/abstract_mem.cc

@@ -58,6 +58,9 @@ using namespace std;
 
 AbstractMemory::AbstractMemory(const Params *p) :
     MemObject(p), range(params()->range), pmemAddr(NULL),
+    backdoor(params()->range, nullptr,
+             (MemBackdoor::Flags)(MemBackdoor::Readable |
+                                  MemBackdoor::Writeable)),
     confTableReported(p->conf_table_reported), inAddrMap(p->in_addr_map),
     kvmMap(p->kvm_map), _system(NULL)
 {
@@ -75,6 +78,13 @@ AbstractMemory::init()
 void
 AbstractMemory::setBackingStore(uint8_t* pmem_addr)
 {
+    // If there was an existing backdoor, let everybody know it's going away.
+    if (backdoor.ptr())
+        backdoor.invalidate();
+
+    // The back door can't handle interleaved memory.
+    backdoor.ptr(range.interleaved() ? nullptr : pmem_addr);
+
     pmemAddr = pmem_addr;
 }
 

src/mem/abstract_mem.hh

@@ -49,6 +49,7 @@
 #ifndef __MEM_ABSTRACT_MEMORY_HH__
 #define __MEM_ABSTRACT_MEMORY_HH__
 
+#include "mem/backdoor.hh"
 #include "mem/mem_object.hh"
 #include "params/AbstractMemory.hh"
 #include "sim/stats.hh"
@@ -110,6 +111,9 @@ class AbstractMemory : public MemObject
     // Pointer to host memory used to implement this memory
     uint8_t* pmemAddr;
 
+    // Backdoor to access this memory.
+    MemBackdoor backdoor;
+
     // Enable specific memories to be reported to the configuration table
     const bool confTableReported;