[PATCH for-edk2-stable201905 5/6] OvmfPkg: raise the PCIEXBAR base to 2816 MB on Q35

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

(This is a replacement for commit 75136b29541b, "OvmfPkg/PlatformPei:
reorder the 32-bit PCI window vs. the PCIEXBAR on q35", 2019-05-16).

Commit 7b8fe63561b4 ("OvmfPkg: PlatformPei: enable PCIEXBAR (aka MMCONFIG
/ ECAM) on Q35", 2016-03-10) claimed that,

  On Q35 machine types that QEMU intends to support in the long term, QEMU
  never lets the RAM below 4 GB exceed 2 GB.

Alas, this statement came from a misunderstanding that occurred while we
worked out the interface contract. In fact QEMU does allow the 32-bit RAM
extend up to 0xB000_0000 (exclusive), in case the RAM size falls in the
range (0x8000_0000, 0xB000_0000) (i.e., the RAM size is greater than
2048MB and smaller than 2816MB).

In turn, such a RAM size (justifiedly) triggers

  ASSERT (TopOfLowRam <= PciExBarBase);

in MemMapInitialization(), because we placed the 256MB PCIEXBAR at
0x8000_0000 (2GB) exactly, relying on the interface contract. (And, the
32-bit PCI window would follow the PCIEXBAR, covering the [0x9000_0000,
0xFC00_0000) range.)

In order to fix this, place the PCIEXBAR at 2816MB (0xB000_0000), and
start the 32-bit PCI window at 3 GB (0xC000_0000). This shrinks the 32-bit
PCI window to

  0xFC00_0000 - 0xC000_0000 = 0x3C00_0000 = 960 MB.

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/OvmfPkgIa32.dsc    | 4 ++--
 OvmfPkg/OvmfPkgIa32X64.dsc | 4 ++--
 OvmfPkg/OvmfPkgX64.dsc     | 4 ++--
 3 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/OvmfPkg/OvmfPkgIa32.dsc b/OvmfPkg/OvmfPkgIa32.dsc
index 578fc6c98ec8..e74a9d5a5149 100644
--- a/OvmfPkg/OvmfPkgIa32.dsc
+++ b/OvmfPkg/OvmfPkgIa32.dsc
@@ -492,8 +492,8 @@ [PcdsFixedAtBuild]
   # the PCIEXBAR register.
   #
   # On Q35 machine types that QEMU intends to support in the long term, QEMU
-  # never lets the RAM below 4 GB exceed 2 GB.
-  gEfiMdePkgTokenSpaceGuid.PcdPciExpressBaseAddress|0x80000000
+  # never lets the RAM below 4 GB exceed 2816 MB.
+  gEfiMdePkgTokenSpaceGuid.PcdPciExpressBaseAddress|0xB0000000
 
 !ifdef $(SOURCE_DEBUG_ENABLE)
   gEfiSourceLevelDebugPkgTokenSpaceGuid.PcdDebugLoadImageMethod|0x2
diff --git a/OvmfPkg/OvmfPkgIa32X64.dsc b/OvmfPkg/OvmfPkgIa32X64.dsc
index eade8f62d3de..67ac015991fd 100644
--- a/OvmfPkg/OvmfPkgIa32X64.dsc
+++ b/OvmfPkg/OvmfPkgIa32X64.dsc
@@ -497,8 +497,8 @@ [PcdsFixedAtBuild]
   # the PCIEXBAR register.
   #
   # On Q35 machine types that QEMU intends to support in the long term, QEMU
-  # never lets the RAM below 4 GB exceed 2 GB.
-  gEfiMdePkgTokenSpaceGuid.PcdPciExpressBaseAddress|0x80000000
+  # never lets the RAM below 4 GB exceed 2816 MB.
+  gEfiMdePkgTokenSpaceGuid.PcdPciExpressBaseAddress|0xB0000000
 
 !ifdef $(SOURCE_DEBUG_ENABLE)
   gEfiSourceLevelDebugPkgTokenSpaceGuid.PcdDebugLoadImageMethod|0x2
diff --git a/OvmfPkg/OvmfPkgX64.dsc b/OvmfPkg/OvmfPkgX64.dsc
index 733a4c9d8a43..68073ef55b4d 100644
--- a/OvmfPkg/OvmfPkgX64.dsc
+++ b/OvmfPkg/OvmfPkgX64.dsc
@@ -497,8 +497,8 @@ [PcdsFixedAtBuild]
   # the PCIEXBAR register.
   #
   # On Q35 machine types that QEMU intends to support in the long term, QEMU
-  # never lets the RAM below 4 GB exceed 2 GB.
-  gEfiMdePkgTokenSpaceGuid.PcdPciExpressBaseAddress|0x80000000
+  # never lets the RAM below 4 GB exceed 2816 MB.
+  gEfiMdePkgTokenSpaceGuid.PcdPciExpressBaseAddress|0xB0000000
 
 !ifdef $(SOURCE_DEBUG_ENABLE)
   gEfiSourceLevelDebugPkgTokenSpaceGuid.PcdDebugLoadImageMethod|0x2
-- 
2.19.1.3.g30247aa5d201