[PATCH 3/3] OvmfPkg/XenSupport: turn off address decoding before BAR sizing

[Igor Druzhinin <igor.druzhinin@citrix.com>]

On Xen, hvmloader firmware leaves address decoding enabled for
enumerated PCI device before jumping into OVMF. OVMF seems to
expect it to be disabled and tries to size PCI BARs in several places
without disabling it which causes BAR64, for example, being
incorrectly placed by QEMU.

Fix it by disabling PCI address decoding explicitly before the
first attempt to size BARs on Xen.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Igor Druzhinin <igor.druzhinin@citrix.com>
---
 OvmfPkg/Library/PciHostBridgeLib/XenSupport.c | 34 +++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/OvmfPkg/Library/PciHostBridgeLib/XenSupport.c b/OvmfPkg/Library/PciHostBridgeLib/XenSupport.c
index 408fb24..9940335 100644
--- a/OvmfPkg/Library/PciHostBridgeLib/XenSupport.c
+++ b/OvmfPkg/Library/PciHostBridgeLib/XenSupport.c
@@ -55,6 +55,33 @@ PcatPciRootBridgeBarExisted (
   EnableInterrupts ();
 }
 
+#define EFI_PCI_COMMAND_DECODE ((UINT16)(EFI_PCI_COMMAND_IO_SPACE | \
+                                         EFI_PCI_COMMAND_MEMORY_SPACE))
+STATIC
+VOID
+PcatPciRootBridgeDecoding (
+  IN  UINTN                          Address,
+  IN  BOOLEAN                        Enabled,
+  OUT UINT16                         *OriginalValue
+  )
+{
+  UINT16                            Value;
+
+  //
+  // Preserve the original value
+  //
+  Value = *OriginalValue;
+  *OriginalValue = PciRead16 (Address);
+
+  if (!Enabled) {
+    if (*OriginalValue & EFI_PCI_COMMAND_DECODE)
+       PciWrite16 (Address, *OriginalValue & ~EFI_PCI_COMMAND_DECODE);
+  } else {
+    if (Value & EFI_PCI_COMMAND_DECODE)
+      PciWrite16 (Address, Value);
+  }
+}
+
 STATIC
 VOID
 PcatPciRootBridgeParseBars (
@@ -76,6 +103,7 @@ PcatPciRootBridgeParseBars (
   UINT32                            Value;
   UINT32                            OriginalUpperValue;
   UINT32                            UpperValue;
+  UINT16                            OriginalCommand;
   UINT64                            Mask;
   UINTN                             Offset;
   UINT64                            Base;
@@ -83,6 +111,12 @@ PcatPciRootBridgeParseBars (
   UINT64                            Limit;
   PCI_ROOT_BRIDGE_APERTURE          *MemAperture;
 
+  // Disable address decoding for every device before OVMF starts sizing it
+  PcatPciRootBridgeDecoding (
+    PCI_LIB_ADDRESS (Bus, Device, Function, PCI_COMMAND_OFFSET),
+    FALSE, &OriginalCommand
+  );
+
   for (Offset = BarOffsetBase; Offset < BarOffsetEnd; Offset += sizeof (UINT32)) {
     PcatPciRootBridgeBarExisted (
       PCI_LIB_ADDRESS (Bus, Device, Function, Offset),
-- 
2.7.4