MdeModulePkg/DxeIpl: Implement NULL pointer detection

NULL pointer detection is done by making use of paging mechanism of CPU.
During page table setup, if enabled, the first 4-K page (0-4095) will be
marked as NOT PRESENT. Any code which unintentionally access memory between
0-4095 will trigger a Page Fault exception which warns users that there's
potential illegal code in BIOS.

This also means that legacy code which has to access memory between 0-4095
should be cautious to temporarily disable this feature before the access
and re-enable it afterwards; or disalbe this feature at all.

Cc: Star Zeng <star.zeng@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Michael Kinney <michael.d.kinney@intel.com>
Cc: Ayellet Wolman <ayellet.wolman@intel.com>
Suggested-by: Ayellet Wolman <ayellet.wolman@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>

MdeModulePkg/Core/DxeIplPeim/DxeIpl.h

@@ -240,4 +240,29 @@ Decompress (
   OUT       UINTN                   *OutputSize
   );
 
+/**
+   Clear legacy memory located at the first 4K-page.
+
+   This function traverses the whole HOB list to check if memory from 0 to 4095
+   exists and has not been allocated, and then clear it if so.
+
+   @param HoStart         The start of HobList passed to DxeCore.
+
+**/
+VOID
+ClearFirst4KPage (
+  IN  VOID *HobStart
+  );
+
+/**
+   Return configure status of NULL pointer detection feature
+
+   @return TRUE   NULL pointer detection feature is enabled
+   @return FALSE  NULL pointer detection feature is disabled
+**/
+BOOLEAN
+IsNullDetectionEnabled (
+  VOID
+  );
+
 #endif