diff --git a/MdePkg/Include/Register/Amd/Ghcb.h b/MdePkg/Include/Register/Amd/Ghcb.h
index bd7bf986d0..ae1486b526 100644
--- a/MdePkg/Include/Register/Amd/Ghcb.h
+++ b/MdePkg/Include/Register/Amd/Ghcb.h
@@ -197,13 +197,14 @@ typedef struct {
   UINT32    Reserved;
 } SNP_PAGE_STATE_HEADER;
 
-#define SNP_PAGE_STATE_MAX_ENTRY  253
-
 typedef struct {
   SNP_PAGE_STATE_HEADER    Header;
-  SNP_PAGE_STATE_ENTRY     Entry[SNP_PAGE_STATE_MAX_ENTRY];
+  SNP_PAGE_STATE_ENTRY     Entry[];
 } SNP_PAGE_STATE_CHANGE_INFO;
 
+#define SNP_PAGE_STATE_MAX_ENTRY  \
+  ((sizeof (((GHCB *)0)->SharedBuffer) - sizeof (SNP_PAGE_STATE_HEADER)) / sizeof (SNP_PAGE_STATE_ENTRY))
+
 //
 // Get APIC IDs
 //