[PATCH v3 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.

Signed-off-by: Igor Druzhinin <igor.druzhinin@citrix.com>
---
Changes in v3:
- dropped unused arguments and rename PcatPciRootBridgeDecoding function
- make mask application more clear as suggested
---
 OvmfPkg/Library/PciHostBridgeLib/XenSupport.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/OvmfPkg/Library/PciHostBridgeLib/XenSupport.c b/OvmfPkg/Library/PciHostBridgeLib/XenSupport.c
index 76fca53..b41bd6b 100644
--- a/OvmfPkg/Library/PciHostBridgeLib/XenSupport.c
+++ b/OvmfPkg/Library/PciHostBridgeLib/XenSupport.c
@@ -49,6 +49,22 @@ PcatPciRootBridgeBarExisted (
   EnableInterrupts ();
 }
 
+#define PCI_COMMAND_DECODE ((UINT16)(EFI_PCI_COMMAND_IO_SPACE | \
+                                     EFI_PCI_COMMAND_MEMORY_SPACE))
+STATIC
+VOID
+PcatPciRootBridgeDecodingDisable (
+  IN  UINTN                          Address
+  )
+{
+  UINT16                             Value;
+
+  Value = PciRead16 (Address);
+  if (Value & PCI_COMMAND_DECODE) {
+    PciWrite16 (Address, Value & ~(UINT32)PCI_COMMAND_DECODE);
+  }
+}
+
 STATIC
 VOID
 PcatPciRootBridgeParseBars (
@@ -75,6 +91,11 @@ PcatPciRootBridgeParseBars (
   UINT64                            Limit;
   PCI_ROOT_BRIDGE_APERTURE          *MemAperture;
 
+  // Disable address decoding for every device before OVMF starts sizing it
+  PcatPciRootBridgeDecodingDisable (
+    PCI_LIB_ADDRESS (Bus, Device, Function, PCI_COMMAND_OFFSET)
+  );
+
   for (Offset = BarOffsetBase; Offset < BarOffsetEnd; Offset += sizeof (UINT32)) {
     PcatPciRootBridgeBarExisted (
       PCI_LIB_ADDRESS (Bus, Device, Function, Offset),
-- 
2.7.4