--- a/hotspot/src/share/vm/runtime/arguments.cpp	Wed Sep 19 08:48:10 2012 -0700
+++ b/hotspot/src/share/vm/runtime/arguments.cpp	Wed Sep 19 15:48:02 2012 -0700
@@ -1978,6 +1978,19 @@
   status = status && verify_min_value(ClassMetaspaceSize, 1*M,
                                       "ClassMetaspaceSize");
 
+#ifdef SPARC
+  if (UseConcMarkSweepGC || UseG1GC) {
+    // Issue a stern warning if the user has explicitly set
+    // UseMemSetInBOT (it is known to cause issues), but allow
+    // use for experimentation and debugging.
+    if (VM_Version::is_sun4v() && UseMemSetInBOT) {
+      assert(!FLAG_IS_DEFAULT(UseMemSetInBOT), "Error");
+      warning("Experimental flag -XX:+UseMemSetInBOT is known to cause instability"
+          " on sun4v; please understand that you are using at your own risk!");
+    }
+  }
+#endif // SPARC
+
   return status;
 }