mem: Minor refactor of how the abstract mem backdoor is exposed.

Previously the SimpleMem depended on the fact that it inherited from the
AbstractMem in order to access and export it's back door. Now, the
AbstractMem has a method which will set a back door pointer if
appropriate, which the SimpleMem can use, or anything else which uses an
AbstractMem as its backing store.

Also, make the AbstractMem invalidate any existing back doors and refuse
to give out any new ones while some bit of memory is locked. That's
because if the storage is accessed directly, the AbstractMem will have
no change to manage its bookkeeping, and locking won't work properly.

Change-Id: If8c2a63e0827bb88b583f27ab4151d6b761e116e
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/36977
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>

src/mem/abstract_mem.cc

@@ -269,6 +269,7 @@ AbstractMemory::trackLoadLocked(PacketPtr pkt)
     DPRINTF(LLSC, "Adding lock record: context %d addr %#x\n",
             req->contextId(), paddr);
     lockedAddrList.push_front(LockedAddr(req));
+    backdoor.invalidate();
 }
 
 

src/mem/abstract_mem.hh

@@ -227,16 +227,31 @@ class AbstractMemory : public ClockedObject
      */
     void setBackingStore(uint8_t* pmem_addr);
 
+    void
+    getBackdoor(MemBackdoorPtr &bd_ptr)
+    {
+        if (lockedAddrList.empty() && backdoor.ptr())
+            bd_ptr = &backdoor;
+    }
+
     /**
      * Get the list of locked addresses to allow checkpointing.
      */
-    const std::list<LockedAddr>& getLockedAddrList() const
-    { return lockedAddrList; }
+    const std::list<LockedAddr> &
+    getLockedAddrList() const
+    {
+        return lockedAddrList;
+    }
 
     /**
      * Add a locked address to allow for checkpointing.
      */
-    void addLockedAddr(LockedAddr addr) { lockedAddrList.push_back(addr); }
+    void
+    addLockedAddr(LockedAddr addr)
+    {
+        backdoor.invalidate();
+        lockedAddrList.push_back(addr);
+    }
 
     /** read the system pointer
      * Implemented for completeness with the setter

src/mem/simple_mem.cc

@@ -80,9 +80,7 @@ Tick
 SimpleMemory::recvAtomicBackdoor(PacketPtr pkt, MemBackdoorPtr &_backdoor)
 {
     Tick latency = recvAtomic(pkt);
-
-    if (backdoor.ptr())
-        _backdoor = &backdoor;
+    getBackdoor(_backdoor);
     return latency;
 }