[PATCH v2 2/4] ArmPkg/CpuDxe ARM: avoid unnecessary cache/TLB maintenance

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

Page and section entries in the page tables are updated using the
helper ArmUpdateTranslationTableEntry(), which cleans the page
table entry to the PoC, and invalidates the TLB entry covering
the page described by the entry being updated.

Since we may be updating section entries, we might be leaving stale
TLB entries at this point (for all pages in the section except the
first one), which will be invalidated wholesale at the end of
SetMemoryAttributes(). At that point, all caches are cleaned *and*
invalidated as well.

This cache maintenance is costly and unnecessary. The TLB maintenance
is only necessary if we updated any section entries, since any page
by page entries that have been updated will have been invalidated
individually by ArmUpdateTranslationTableEntry().

So drop the clean/invalidate of the caches, and only perform the
full TLB flush if UpdateSectionEntries() was called, or if sections
were split by UpdatePageEntries(). Finally, make the cache maintenance
on the remapped regions themselves conditional on whether any memory
type attributes were modified.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
 ArmPkg/Drivers/CpuDxe/Arm/Mmu.c | 60 +++++++++++---------
 1 file changed, 34 insertions(+), 26 deletions(-)

diff --git a/ArmPkg/Drivers/CpuDxe/Arm/Mmu.c b/ArmPkg/Drivers/CpuDxe/Arm/Mmu.c
index ce4d529bda67..26b637e7658f 100644
--- a/ArmPkg/Drivers/CpuDxe/Arm/Mmu.c
+++ b/ArmPkg/Drivers/CpuDxe/Arm/Mmu.c
@@ -347,10 +347,11 @@ SyncCacheConfig (
 
 EFI_STATUS
 UpdatePageEntries (
-  IN EFI_PHYSICAL_ADDRESS      BaseAddress,
-  IN UINT64                    Length,
-  IN UINT64                    Attributes,
-  IN EFI_PHYSICAL_ADDRESS      VirtualMask
+  IN  EFI_PHYSICAL_ADDRESS      BaseAddress,
+  IN  UINT64                    Length,
+  IN  UINT64                    Attributes,
+  IN  EFI_PHYSICAL_ADDRESS      VirtualMask,
+  OUT BOOLEAN                   *FlushTlbs
   )
 {
   EFI_STATUS    Status;
@@ -446,6 +447,9 @@ UpdatePageEntries (
 
       // Re-read descriptor
       Descriptor = FirstLevelTable[FirstLevelIdx];
+      if (FlushTlbs != NULL) {
+        *FlushTlbs = TRUE;
+      }
     }
 
     // Obtain page table base address
@@ -471,15 +475,16 @@ UpdatePageEntries (
 
     if (CurrentPageTableEntry  != PageTableEntry) {
       Mva = (VOID *)(UINTN)((((UINTN)FirstLevelIdx) << TT_DESCRIPTOR_SECTION_BASE_SHIFT) + (PageTableIndex << TT_DESCRIPTOR_PAGE_BASE_SHIFT));
-      if ((CurrentPageTableEntry & TT_DESCRIPTOR_PAGE_CACHEABLE_MASK) == TT_DESCRIPTOR_PAGE_CACHEABLE_MASK) {
-        // The current section mapping is cacheable so Clean/Invalidate the MVA of the page
-        // Note assumes switch(Attributes), not ARMv7 possibilities
-        WriteBackInvalidateDataCacheRange (Mva, TT_DESCRIPTOR_PAGE_SIZE);
-      }
 
       // Only need to update if we are changing the entry
       PageTable[PageTableIndex] = PageTableEntry;
       ArmUpdateTranslationTableEntry ((VOID *)&PageTable[PageTableIndex], Mva);
+
+      // Clean/invalidate the cache for this page, but only
+      // if we are modifying the memory type attributes
+      if (((CurrentPageTableEntry ^ PageTableEntry) & TT_DESCRIPTOR_PAGE_CACHE_POLICY_MASK) != 0) {
+        WriteBackInvalidateDataCacheRange (Mva, TT_DESCRIPTOR_PAGE_SIZE);
+      }
     }
 
     Status = EFI_SUCCESS;
@@ -581,7 +586,12 @@ UpdateSectionEntries (
     // has this descriptor already been coverted to pages?
     if (TT_DESCRIPTOR_SECTION_TYPE_IS_PAGE_TABLE(CurrentDescriptor)) {
       // forward this 1MB range to page table function instead
-      Status = UpdatePageEntries ((FirstLevelIdx + i) << TT_DESCRIPTOR_SECTION_BASE_SHIFT, TT_DESCRIPTOR_SECTION_SIZE, Attributes, VirtualMask);
+      Status = UpdatePageEntries (
+                 (FirstLevelIdx + i) << TT_DESCRIPTOR_SECTION_BASE_SHIFT,
+                 TT_DESCRIPTOR_SECTION_SIZE,
+                 Attributes,
+                 VirtualMask,
+                 NULL);
     } else {
       // still a section entry
 
@@ -596,15 +606,16 @@ UpdateSectionEntries (
 
       if (CurrentDescriptor  != Descriptor) {
         Mva = (VOID *)(UINTN)(((UINTN)FirstLevelTable) << TT_DESCRIPTOR_SECTION_BASE_SHIFT);
-        if ((CurrentDescriptor & TT_DESCRIPTOR_SECTION_CACHEABLE_MASK) == TT_DESCRIPTOR_SECTION_CACHEABLE_MASK) {
-          // The current section mapping is cacheable so Clean/Invalidate the MVA of the section
-          // Note assumes switch(Attributes), not ARMv7 possabilities
-          WriteBackInvalidateDataCacheRange (Mva, SIZE_1MB);
-        }
 
         // Only need to update if we are changing the descriptor
         FirstLevelTable[FirstLevelIdx + i] = Descriptor;
         ArmUpdateTranslationTableEntry ((VOID *)&FirstLevelTable[FirstLevelIdx + i], Mva);
+
+        // Clean/invalidate the cache for this section, but only
+        // if we are modifying the memory type attributes
+        if (((CurrentDescriptor ^ Descriptor) & TT_DESCRIPTOR_SECTION_CACHE_POLICY_MASK) != 0) {
+          WriteBackInvalidateDataCacheRange (Mva, SIZE_1MB);
+        }
       }
 
       Status = EFI_SUCCESS;
@@ -680,6 +691,7 @@ SetMemoryAttributes (
 {
   EFI_STATUS    Status;
   UINT64        ChunkLength;
+  BOOLEAN       FlushTlbs;
 
   //
   // Ignore invocations that only modify permission bits
@@ -688,6 +700,7 @@ SetMemoryAttributes (
     return EFI_SUCCESS;
   }
 
+  FlushTlbs = FALSE;
   while (Length > 0) {
     if ((BaseAddress % TT_DESCRIPTOR_SECTION_SIZE == 0) &&
         Length >= TT_DESCRIPTOR_SECTION_SIZE) {
@@ -700,6 +713,8 @@ SetMemoryAttributes (
 
       Status = UpdateSectionEntries (BaseAddress, ChunkLength, Attributes,
                  VirtualMask);
+
+      FlushTlbs = TRUE;
     } else {
 
       //
@@ -717,7 +732,7 @@ SetMemoryAttributes (
         BaseAddress, ChunkLength, Attributes));
 
       Status = UpdatePageEntries (BaseAddress, ChunkLength, Attributes,
-                 VirtualMask);
+                 VirtualMask, &FlushTlbs);
     }
 
     if (EFI_ERROR (Status)) {
@@ -728,16 +743,9 @@ SetMemoryAttributes (
     Length -= ChunkLength;
   }
 
-  // Flush d-cache so descriptors make it back to uncached memory for subsequent table walks
-  // flush and invalidate pages
-  //TODO: Do we really need to invalidate the caches everytime we change the memory attributes ?
-  ArmCleanInvalidateDataCache ();
-
-  ArmInvalidateInstructionCache ();
-
-  // Invalidate all TLB entries so changes are synced
-  ArmInvalidateTlb ();
-
+  if (FlushTlbs) {
+    ArmInvalidateTlb ();
+  }
   return Status;
 }
 
-- 
2.7.4