[PATCH 1/7] ArmPkg/ArmMmuLib: don't replace table entries with block entries

["Ard Biesheuvel" <ardb@kernel.org>]

Drop the optimization that replaces table entries with block entries and
frees the page tables in the subhierarchy that is being replaced. This
rarely occurs in practice anyway, and will require more elaborate TLB
maintenance once we switch to a different approach when running at EL1,
where we no longer disable the MMU and nuke the TLB entirely every time
we update a descriptor in a way that requires break-before-make (BBM).

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c | 20 ++------------------
 1 file changed, 2 insertions(+), 18 deletions(-)

diff --git a/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c b/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c
index 4a0513392a75..b85ae5a30768 100644
--- a/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c
+++ b/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c
@@ -197,12 +197,9 @@ UpdateRegionMappingRecursive (
     // than a block, and recurse to create the block or page entries at
     // the next level. No block mappings are allowed at all at level 0,
     // so in that case, we have to recurse unconditionally.
-    // If we are changing a table entry and the AttributeClearMask is non-zero,
-    // we cannot replace it with a block entry without potentially losing
-    // attribute information, so keep the table entry in that case.
     //
     if ((Level == 0) || (((RegionStart | BlockEnd) & BlockMask) != 0) ||
-        (IsTableEntry (*Entry, Level) && (AttributeClearMask != 0)))
+        IsTableEntry (*Entry, Level))
     {
       ASSERT (Level < 3);
 
@@ -294,20 +291,7 @@ UpdateRegionMappingRecursive (
       EntryValue |= (Level == 3) ? TT_TYPE_BLOCK_ENTRY_LEVEL3
                                  : TT_TYPE_BLOCK_ENTRY;
 
-      if (IsTableEntry (*Entry, Level)) {
-        //
-        // We are replacing a table entry with a block entry. This is only
-        // possible if we are keeping none of the original attributes.
-        // We can free the table entry's page table, and all the ones below
-        // it, since we are dropping the only possible reference to it.
-        //
-        ASSERT (AttributeClearMask == 0);
-        TranslationTable = (VOID *)(UINTN)(*Entry & TT_ADDRESS_MASK_BLOCK_ENTRY);
-        ReplaceTableEntry (Entry, EntryValue, RegionStart, TRUE);
-        FreePageTablesRecursive (TranslationTable, Level + 1);
-      } else {
-        ReplaceTableEntry (Entry, EntryValue, RegionStart, FALSE);
-      }
+      ReplaceTableEntry (Entry, EntryValue, RegionStart, FALSE);
     }
   }
 
-- 
2.35.1