[PATCH] ArmPkg/ArmMmuLib: support page tables in cacheable memory only

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

Translation table walks are always cache coherent on ARMv8-A, so cache
maintenance on page tables is never needed. Since there is a risk of
loss of coherency when using mismatched attributes, and given that memory
is mapped cacheable except for extraordinary cases (such as non-coherent
DMA), restrict the page table walker to performing cacheable accesses to
the translation tables.

For DEBUG builds, retain some of the logic so that we can double check
that the memory holding the root translation table is indeed located in
memory that is mapped cacheable.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
 ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c | 49 ++++++++++----------
 1 file changed, 24 insertions(+), 25 deletions(-)

diff --git a/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c b/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c
index 1fb3bbec6347..c78297084207 100644
--- a/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c
+++ b/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c
@@ -627,6 +627,19 @@ ArmConfigureMmu (
     return RETURN_UNSUPPORTED;
   }
 
+  //
+  // Translation table walks are always cache coherent on ARMv8-A, so cache
+  // maintenance on page tables is never needed. Since there is a risk of
+  // loss of coherency when using mismatched attributes, and given that memory
+  // is mapped cacheable except for extraordinary cases (such as non-coherent
+  // DMA), have the page table walker perform cached accesses as well, and
+  // assert below that that matches the attributes we use for CPU accesses to
+  // the region.
+  //
+  TCR |= TCR_SH_INNER_SHAREABLE |
+         TCR_RGN_OUTER_WRITE_BACK_ALLOC |
+         TCR_RGN_INNER_WRITE_BACK_ALLOC;
+
   // Set TCR
   ArmSetTCR (TCR);
 
@@ -672,11 +685,15 @@ ArmConfigureMmu (
 
   TranslationTableAttribute = TT_ATTR_INDX_INVALID;
   while (MemoryTable->Length != 0) {
-    // Find the memory attribute for the Translation Table
-    if (((UINTN)TranslationTable >= MemoryTable->PhysicalBase) &&
-        ((UINTN)TranslationTable <= MemoryTable->PhysicalBase - 1 + MemoryTable->Length)) {
-      TranslationTableAttribute = MemoryTable->Attributes;
-    }
+
+    DEBUG_CODE_BEGIN ();
+      // Find the memory attribute for the Translation Table
+      if ((UINTN)TranslationTable >= MemoryTable->PhysicalBase &&
+          (UINTN)TranslationTable + RootTableEntrySize <= MemoryTable->PhysicalBase +
+                                                          MemoryTable->Length) {
+        TranslationTableAttribute = MemoryTable->Attributes;
+      }
+    DEBUG_CODE_END ();
 
     Status = FillTranslationTable (TranslationTable, MemoryTable);
     if (RETURN_ERROR (Status)) {
@@ -685,26 +702,8 @@ ArmConfigureMmu (
     MemoryTable++;
   }
 
-  // Translate the Memory Attributes into Translation Table Register Attributes
-  if ((TranslationTableAttribute == ARM_MEMORY_REGION_ATTRIBUTE_UNCACHED_UNBUFFERED) ||
-      (TranslationTableAttribute == ARM_MEMORY_REGION_ATTRIBUTE_NONSECURE_UNCACHED_UNBUFFERED)) {
-    TCR |= TCR_SH_NON_SHAREABLE | TCR_RGN_OUTER_NON_CACHEABLE | TCR_RGN_INNER_NON_CACHEABLE;
-  } else if ((TranslationTableAttribute == ARM_MEMORY_REGION_ATTRIBUTE_WRITE_BACK) ||
-      (TranslationTableAttribute == ARM_MEMORY_REGION_ATTRIBUTE_NONSECURE_WRITE_BACK)) {
-    TCR |= TCR_SH_INNER_SHAREABLE | TCR_RGN_OUTER_WRITE_BACK_ALLOC | TCR_RGN_INNER_WRITE_BACK_ALLOC;
-  } else if ((TranslationTableAttribute == ARM_MEMORY_REGION_ATTRIBUTE_WRITE_THROUGH) ||
-      (TranslationTableAttribute == ARM_MEMORY_REGION_ATTRIBUTE_NONSECURE_WRITE_THROUGH)) {
-    TCR |= TCR_SH_NON_SHAREABLE | TCR_RGN_OUTER_WRITE_THROUGH | TCR_RGN_INNER_WRITE_THROUGH;
-  } else {
-    // If we failed to find a mapping that contains the root translation table then it probably means the translation table
-    // is not mapped in the given memory map.
-    ASSERT (0);
-    Status = RETURN_UNSUPPORTED;
-    goto FREE_TRANSLATION_TABLE;
-  }
-
-  // Set again TCR after getting the Translation Table attributes
-  ArmSetTCR (TCR);
+  ASSERT (TranslationTableAttribute == ARM_MEMORY_REGION_ATTRIBUTE_WRITE_BACK ||
+          TranslationTableAttribute == ARM_MEMORY_REGION_ATTRIBUTE_NONSECURE_WRITE_BACK);
 
   ArmSetMAIR (MAIR_ATTR(TT_ATTR_INDX_DEVICE_MEMORY, MAIR_ATTR_DEVICE_MEMORY) |                      // mapped to EFI_MEMORY_UC
               MAIR_ATTR(TT_ATTR_INDX_MEMORY_NON_CACHEABLE, MAIR_ATTR_NORMAL_MEMORY_NON_CACHEABLE) | // mapped to EFI_MEMORY_WC
-- 
2.7.4