UefiCpuPkg CpuPageTableMap: Fixing page table handling for non-presen…

…t pages.

This change will add logic to populate the address and propagate bits
for the non-present pages.

Without this change, the issue can be seen as when a smaller page is
marked present from a big non-present page, the remainder of the
non-present pages will return `EFI_UNSUPPORTED` when using get
attributes function though protocols like CPU arch.

UefiCpuPkg/Library/CpuPageTableLib/CpuPageTableMap.c

@@ -390,6 +390,16 @@ PageTableLibMapInLevel (
       if (RETURN_ERROR (Status)) {
         return Status;
       }
+
+      // MU_CHANGE Start - Populate base address bits for non present pages
+      OneOfPagingEntry.Pnle.Uint64 = 0;
+      if ((Level != 1) && (Level != 2) && (Level != 3)) {
+        PageTableLibSetPnle (&OneOfPagingEntry.Pnle, &PleBAttribute, &AllOneMask);
+      } else {
+        PageTableLibSetPle (Level, &OneOfPagingEntry, 0, &PleBAttribute, &AllOneMask);
+      }
+
+      // MU_CHANGE End - Populate base address bits for non present pages
     } else {
       PageTableLibSetPle (Level, &OneOfPagingEntry, 0, &PleBAttribute, &AllOneMask);
     }
@@ -426,16 +436,17 @@ PageTableLibMapInLevel (
       PagingEntry = (IA32_PAGING_ENTRY *)((UINTN)Buffer + *BufferSize);
       ZeroMem (PagingEntry, SIZE_4KB);
 
-      if (ParentPagingEntry->Pce.Present) {
-        //
-        // Create 512 child-level entries that map to 2M/4K.
-        //
-        for (SubOffset = 0, Index = 0; Index < 512; Index++) {
-          PagingEntry[Index].Uint64 = OneOfPagingEntry.Uint64 + SubOffset;
-          SubOffset                += RegionLength;
-        }
+      // MU_CHANGE Start - Populate contents for both present and non-present pages
+      //
+      // Create 512 child-level entries that map to 2M/4K.
+      //
+      for (SubOffset = 0, Index = 0; Index < 512; Index++) {
+        PagingEntry[Index].Uint64 = OneOfPagingEntry.Uint64 + SubOffset;
+        SubOffset                += RegionLength;
       }
 
+      // MU_CHANGE End - Populate contents for both present and non-present pages
+
       //
       // Set NOP attributes
       // Note: Should NOT inherit the attributes from the original entry because a zero RW bit
@@ -519,9 +530,11 @@ PageTableLibMapInLevel (
         // e.g.: Set PDE[0-255].ReadWrite = 0
         //
         for (Index = 0; Index < 512; Index++) {
-          if (PagingEntry[Index].Pce.Present == 0) {
-            continue;
-          }
+          // MU_CHANGE Start - Count the space for non-present pages as well.
+          // if (PagingEntry[Index].Pce.Present == 0) {
+          //   continue;
+          // }
+          // MU_CHANGE End - Count the space for non-present pages as well.
 
           if (IsPle (&PagingEntry[Index], Level)) {
             PageTableLibSetPle (Level, &PagingEntry[Index], 0, &ChildAttribute, &ChildMask);