[edk2] [PATCH v2] ArmPkg/ArmV7Mmu: prefer non shareable memory on non-coherent hardware

[Ard Biesheuvel]

Commit SVN r18778 made all mappings of normal memory (inner) shareable,
even on hardware that implements shareability as uncached accesses.
The original concerns that prompted the change, regarding coherent DMA
and virt guests migrating between CPUs, do not apply to such hardware,
so revert to the original behavior in that case.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheu...@linaro.org>
---
v2: use symbolic constants for the various shift and mask values
    default to shareable on unexpected values, but ASSERT() as well

 ArmPkg/Library/ArmLib/ArmV7/ArmV7Lib.h | 14 +++++++
 ArmPkg/Library/ArmLib/ArmV7/ArmV7Mmu.c | 42 ++++++++++++++++++--
 2 files changed, 53 insertions(+), 3 deletions(-)

diff --git a/ArmPkg/Library/ArmLib/ArmV7/ArmV7Lib.h 
b/ArmPkg/Library/ArmLib/ArmV7/ArmV7Lib.h
index e138613ca548..cbd3d6f654a6 100644
--- a/ArmPkg/Library/ArmLib/ArmV7/ArmV7Lib.h
+++ b/ArmPkg/Library/ArmLib/ArmV7/ArmV7Lib.h
@@ -15,6 +15,20 @@
 #ifndef __ARM_V7_LIB_H__
 #define __ARM_V7_LIB_H__
 
+#define ID_MMFR0_SHARELVL_SHIFT       12
+#define ID_MMFR0_SHARELVL_MASK       0xf
+#define ID_MMFR0_SHARELVL_ONE          0
+#define ID_MMFR0_SHARELVL_TWO          1
+
+#define ID_MMFR0_INNERSHR_SHIFT       28
+#define ID_MMFR0_INNERSHR_MASK       0xf
+#define ID_MMFR0_OUTERSHR_SHIFT        8
+#define ID_MMFR0_OUTERSHR_MASK       0xf
+
+#define ID_MMFR0_SHR_IMP_UNCACHED      0
+#define ID_MMFR0_SHR_IMP_HW_COHERENT   1
+#define ID_MMFR0_SHR_IGNORED         0xf
+
 typedef VOID (*ARM_V7_CACHE_OPERATION)(UINT32);
 
 VOID
diff --git a/ArmPkg/Library/ArmLib/ArmV7/ArmV7Mmu.c 
b/ArmPkg/Library/ArmLib/ArmV7/ArmV7Mmu.c
index 23d2e43beba0..fc8ea42843b3 100644
--- a/ArmPkg/Library/ArmLib/ArmV7/ArmV7Mmu.c
+++ b/ArmPkg/Library/ArmLib/ArmV7/ArmV7Mmu.c
@@ -42,6 +42,42 @@ ConvertSectionAttributesToPageAttributes (
 }
 
 STATIC
+BOOLEAN
+PreferNonshareableMemory (
+  VOID
+  )
+{
+  UINTN   Mmfr;
+  UINTN   Val;
+
+  if (FeaturePcdGet (PcdNormalMemoryNonshareableOverride)) {
+    return TRUE;
+  }
+
+  //
+  // Check whether the innermost level of shareability (the level we will use
+  // by default to map normal memory) is implemented with hardware coherency
+  // support. Otherwise, revert to mapping as non-shareable.
+  //
+  Mmfr = ArmReadIdMmfr0 ();
+  switch ((Mmfr >> ID_MMFR0_SHARELVL_SHIFT) & ID_MMFR0_SHARELVL_MASK) {
+  case ID_MMFR0_SHARELVL_ONE:
+    // one level of shareability
+    Val = (Mmfr >> ID_MMFR0_OUTERSHR_SHIFT) & ID_MMFR0_OUTERSHR_MASK;
+    break;
+  case ID_MMFR0_SHARELVL_TWO:
+    // two levels of shareability
+    Val = (Mmfr >> ID_MMFR0_INNERSHR_SHIFT) & ID_MMFR0_INNERSHR_MASK;
+    break;
+  default:
+    // unexpected value -> shareable is the safe option
+    ASSERT (FALSE);
+    return FALSE;
+  }
+  return Val != ID_MMFR0_SHR_IMP_HW_COHERENT;
+}
+
+STATIC
 VOID
 PopulateLevel2PageTable (
   IN UINT32                         *SectionEntry,
@@ -80,7 +116,7 @@ PopulateLevel2PageTable (
       break;
   }
 
-  if (FeaturePcdGet(PcdNormalMemoryNonshareableOverride)) {
+  if (PreferNonshareableMemory ()) {
     PageAttributes &= ~TT_DESCRIPTOR_PAGE_S_SHARED;
   }
 
@@ -189,7 +225,7 @@ FillTranslationTable (
       break;
   }
 
-  if (FeaturePcdGet(PcdNormalMemoryNonshareableOverride)) {
+  if (PreferNonshareableMemory ()) {
     Attributes &= ~TT_DESCRIPTOR_SECTION_S_SHARED;
   }
 
@@ -281,7 +317,7 @@ ArmConfigureMmu (
   }
 
   if (TTBRAttributes & TTBR_SHAREABLE) {
-    if (FeaturePcdGet(PcdNormalMemoryNonshareableOverride)) {
+    if (PreferNonshareableMemory ()) {
       TTBRAttributes ^= TTBR_SHAREABLE;
     } else {
       //
-- 
1.9.1

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