[PATCH for-edk2-stable201905 1/6] Revert "OvmfPkg/PlatformPei: fix MTRR for low-RAM sizes that have many bits clear"

["Laszlo Ersek" <lersek@redhat.com>]

This reverts commit 39b9a5ffe6618b7870be2a54fe7725000249c33a.

The original fix for <https://bugzilla.tianocore.org/show_bug.cgi?id=1814>
triggered a bug / incorrect assumption in QEMU.

QEMU assumes that the PCIEXBAR is below the 32-bit PCI window, not above
it. When the firmware doesn't satisfy this assumption, QEMU generates an
\_SB.PCI0._CRS object in the ACPI DSDT that does not reflect the
firmware's 32-bit MMIO BAR assignments. This causes OSes to re-assign
32-bit MMIO BARs.

Working around the problem in the firmware looks less problematic than
fixing QEMU. Revert the original changes first, before implementing an
alternative fix.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1859
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
---
 OvmfPkg/PlatformPei/Platform.h  |  2 --
 OvmfPkg/PlatformPei/MemDetect.c | 23 +++-----------------
 OvmfPkg/PlatformPei/Platform.c  |  4 +++-
 3 files changed, 6 insertions(+), 23 deletions(-)

diff --git a/OvmfPkg/PlatformPei/Platform.h b/OvmfPkg/PlatformPei/Platform.h
index 4476ddd871cd..81af8b71480f 100644
--- a/OvmfPkg/PlatformPei/Platform.h
+++ b/OvmfPkg/PlatformPei/Platform.h
@@ -114,6 +114,4 @@ extern UINT32 mMaxCpuCount;
 
 extern UINT16 mHostBridgeDevId;
 
-extern UINT32 mQemuUc32Base;
-
 #endif // _PLATFORM_PEI_H_INCLUDED_
diff --git a/OvmfPkg/PlatformPei/MemDetect.c b/OvmfPkg/PlatformPei/MemDetect.c
index ae73c63d27d5..e890e36408a6 100644
--- a/OvmfPkg/PlatformPei/MemDetect.c
+++ b/OvmfPkg/PlatformPei/MemDetect.c
@@ -42,8 +42,6 @@ STATIC UINT32 mS3AcpiReservedMemorySize;
 
 STATIC UINT16 mQ35TsegMbytes;
 
-UINT32 mQemuUc32Base;
-
 VOID
 Q35TsegMbytesInitialization (
   VOID
@@ -665,8 +663,6 @@ QemuInitializeRam (
   // cover it exactly.
   //
   if (IsMtrrSupported ()) {
-    UINT32 Uc32Size;
-
     MtrrGetAllMtrrs (&MtrrSettings);
 
     //
@@ -693,24 +689,11 @@ QemuInitializeRam (
 
     //
     // Set memory range from the "top of lower RAM" (RAM below 4GB) to 4GB as
-    // uncacheable. Make sure one variable MTRR suffices by truncating the size
-    // to a whole power of two. This will round the base *up*, and a gap (not
-    // used for either RAM or MMIO) may stay in the middle, marked as
-    // cacheable-by-default.
+    // uncacheable
     //
-    Uc32Size = GetPowerOfTwo32 ((UINT32)(SIZE_4GB - LowerMemorySize));
-    mQemuUc32Base = (UINT32)(SIZE_4GB - Uc32Size);
-    if (mQemuUc32Base != LowerMemorySize) {
-      DEBUG ((DEBUG_VERBOSE, "%a: rounded UC32 base from 0x%x up to 0x%x, for "
-        "an UC32 size of 0x%x\n", __FUNCTION__, (UINT32)LowerMemorySize,
-        mQemuUc32Base, Uc32Size));
-    }
-
-    Status = MtrrSetMemoryAttribute (mQemuUc32Base, Uc32Size,
-               CacheUncacheable);
+    Status = MtrrSetMemoryAttribute (LowerMemorySize,
+               SIZE_4GB - LowerMemorySize, CacheUncacheable);
     ASSERT_EFI_ERROR (Status);
-  } else {
-    mQemuUc32Base = (UINT32)LowerMemorySize;
   }
 }
 
diff --git a/OvmfPkg/PlatformPei/Platform.c b/OvmfPkg/PlatformPei/Platform.c
index c064b4ed9b8f..fd8eccaf3e50 100644
--- a/OvmfPkg/PlatformPei/Platform.c
+++ b/OvmfPkg/PlatformPei/Platform.c
@@ -174,12 +174,14 @@ MemMapInitialization (
   AddIoMemoryRangeHob (0x0A0000, BASE_1MB);
 
   if (!mXen) {
+    UINT32  TopOfLowRam;
     UINT64  PciExBarBase;
     UINT32  PciBase;
     UINT32  PciSize;
 
+    TopOfLowRam = GetSystemMemorySizeBelow4gb ();
     PciExBarBase = 0;
-    PciBase = (mQemuUc32Base < BASE_2GB) ? BASE_2GB : mQemuUc32Base;
+    PciBase = (TopOfLowRam < BASE_2GB) ? BASE_2GB : TopOfLowRam;
     if (mHostBridgeDevId == INTEL_Q35_MCH_DEVICE_ID) {
       //
       // The 32-bit PCI host aperture is expected to fall between the top of
-- 
2.19.1.3.g30247aa5d201