[PATCH] ArmPkg/ArmMmuLib ARM: remove cache maintenance of block mapping contents

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

Peculiarly enough, the current page table manipulation code takes it
upon itself to write back and invalidate the memory contents covered
by section mappings when their memory attributes change. It is not
generally the case that data must be written back when such a change
occurs, even when switching from cacheable to non-cacheable attributes,
and in some cases, it is actually causing problems. (The cache
maintenance is also performed on the PCIe MMIO regions as they get
mapped by the PCI bus driver, and under virtualization, each cache
maintenance operation on an emulated MMIO region triggers a round
trip to the host and back)

So let's just drop this code.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
 ArmPkg/Library/ArmMmuLib/Arm/ArmMmuLibCore.c | 6 ------
 1 file changed, 6 deletions(-)

diff --git a/ArmPkg/Library/ArmMmuLib/Arm/ArmMmuLibCore.c b/ArmPkg/Library/ArmMmuLib/Arm/ArmMmuLibCore.c
index 9bf4ba03fd5b..d1bca4c601b8 100644
--- a/ArmPkg/Library/ArmMmuLib/Arm/ArmMmuLibCore.c
+++ b/ArmPkg/Library/ArmMmuLib/Arm/ArmMmuLibCore.c
@@ -718,12 +718,6 @@ UpdateSectionEntries (
       if (CurrentDescriptor  != Descriptor) {
         Mva = (VOID *)(UINTN)(((UINTN)FirstLevelIdx + i) << TT_DESCRIPTOR_SECTION_BASE_SHIFT);
 
-        // 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);
-        }
-
         // Only need to update if we are changing the descriptor
         FirstLevelTable[FirstLevelIdx + i] = Descriptor;
         ArmUpdateTranslationTableEntry ((VOID *)&FirstLevelTable[FirstLevelIdx + i], Mva);
-- 
2.17.1