mem: Optionally share the backing store

This patch adds the ability for a host-OS process external to gem5
to access the backing store via POSIX shared memory.
The new param shared_backstore of the System object is the filename
of the shared memory (i.e., the first argument to shm_open()).

Change-Id: I98c948a32a15049a4515e6c02a14595fb5fe379f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/30994
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/sim/System.py

@@ -78,6 +78,10 @@ class System(SimObject):
     # I/O bridge or cache
     mem_ranges = VectorParam.AddrRange([], "Ranges that constitute main memory")
 
+    shared_backstore = Param.String("", "backstore's shmem segment filename, "
+        "use to directly address the backstore from another host-OS process. "
+        "Leave this empty to unset the MAP_SHARED flag.")
+
     cache_line_size = Param.Unsigned(64, "Cache line size in bytes")
 
     redirect_paths = VectorParam.RedirectPath([], "Path redirections")

src/sim/system.cc

@@ -213,7 +213,8 @@ System::System(Params *p)
 #else
       kvmVM(nullptr),
 #endif
-      physmem(name() + ".physmem", p->memories, p->mmap_using_noreserve),
+      physmem(name() + ".physmem", p->memories, p->mmap_using_noreserve,
+              p->shared_backstore),
       memoryMode(p->mem_mode),
       _cacheLineSize(p->cache_line_size),
       workItemsBegin(0),