[PATCH] MdeModulePkg/MdeModulePkg.dec: Clarify usage of PcdHeapGuardPropertyMask

[Jian J Wang <jian.j.wang@intel.com>]

Cc: Star Zeng <star.zeng@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
---
 MdeModulePkg/MdeModulePkg.dec | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/MdeModulePkg/MdeModulePkg.dec b/MdeModulePkg/MdeModulePkg.dec
index 8efad57766..d134ec798a 100644
--- a/MdeModulePkg/MdeModulePkg.dec
+++ b/MdeModulePkg/MdeModulePkg.dec
@@ -939,13 +939,17 @@
   gEfiMdeModulePkgTokenSpaceGuid.PcdHeapGuardPoolType|0x0|UINT64|0x30001053
 
   ## This mask is to control Heap Guard behavior.
+  # Note that due to the limit in pool memory implementation and the alignment
+  # requirement of UEFI spec, BIT7 is a try-best setting which cannot guarantee
+  # that the returned pool is exactly adjacent to head guard page or tail guard
+  # page.
   #   BIT0 - Enable UEFI page guard.<BR>
   #   BIT1 - Enable UEFI pool guard.<BR>
   #   BIT2 - Enable SMM page guard.<BR>
   #   BIT3 - Enable SMM pool guard.<BR>
   #   BIT7 - The direction of Guard Page for Pool Guard.
-  #          0 - The returned pool is adjacent to the bottom guard page.<BR>
-  #          1 - The returned pool is adjacent to the top guard page.<BR>
+  #          0 - The returned pool is near the tail guard page.<BR>
+  #          1 - The returned pool is near the head guard page.<BR>
   # @Prompt The Heap Guard feature mask
   gEfiMdeModulePkgTokenSpaceGuid.PcdHeapGuardPropertyMask|0x0|UINT8|0x30001054
 
-- 
2.15.1.windows.2