Use mwaitx when busy-waiting signals

Use mwaitx instructions when busy waiting for signals to reduce CPU
energy usage.
This can be disabled by setting HSA_ENABLE_MWAITX=0

Change-Id: Ic207895a491b2bf6dacba47ef0921df3faad5b5a

runtime/hsa-runtime/core/util/flag.h

@@ -163,6 +163,9 @@ class Flag {
     var = os::GetEnvVar("HSA_IMAGE_PRINT_SRD");
     image_print_srd_ = (var == "1") ? true : false;
 
+    var = os::GetEnvVar("HSA_ENABLE_MWAITX");
+    enable_mwaitx_ = (var == "0") ? false : true;
+
     // Temporary environment variable to disable CPU affinity override
     // Will either rename to HSA_OVERRIDE_CPU_AFFINITY later or remove completely.
     var = os::GetEnvVar("HSA_OVERRIDE_CPU_AFFINITY_DEBUG");
@@ -224,6 +227,12 @@ class Flag {
 
   bool image_print_srd() const { return image_print_srd_; }
 
+  bool check_mwaitx(bool mwaitx_supported) {
+    if (enable_mwaitx_ && !mwaitx_supported) enable_mwaitx_ = false;
+
+    return enable_mwaitx_;
+  }
+
   XNACK_REQUEST xnack() const { return xnack_; }
 
   bool debug() const { return debug_; }
@@ -266,6 +275,7 @@ class Flag {
   bool discover_copy_agents_;
   bool override_cpu_affinity_;
   bool image_print_srd_;
+  bool enable_mwaitx_;
 
   SDMA_OVERRIDE enable_sdma_;