OcCpuLib: Added privilege detection to CPU frequency detection

Include/Library/OcCpuLib.h

@@ -20,8 +20,11 @@
 
 /**
   Assumed CPU frequency when it cannot be detected.
+  Can be overridden by e.g. emulator.
 **/
+#ifndef OC_FALLBACK_CPU_FREQUENCY
 #define OC_FALLBACK_CPU_FREQUENCY 1000000000
+#endif
 
 typedef struct {
   //

Library/OcCpuLib/FrequencyDetect.c

@@ -140,6 +140,13 @@ InternalCalculateTSCFromPMTimer (
   EFI_TPL     PrevTpl;
   EFI_STATUS  Status;
 
+  //
+  // Do not use ACPI PM timer in ring 3 (e.g. emulator).
+  //
+  if ((AsmReadCs () & 3U) == 3) {
+    return EFI_UNSUPPORTED;
+  }
+
   //
   // Decide whether we need to store the frequency.
   //