[PATCH for-next] MdeModulePkg/PciBusDxe: catch unimplemented extended config space reads

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

When assigning a physical PCIe device to a QEMU/KVM guest, PciBusDxe may
find that the extended config space is not (fully) implemented. In
LocatePciExpressCapabilityRegBlock(), "CapabilityEntry" may be read as
0xFFFF_FFFF at a given config space offset, after which the loop gets
stuck spinning on offset 0xFFC (the read at offset 0xFFC returns
0xFFFF_FFFF most likely as well).

Another scenario (not related to virtualization) for triggering the above
is when a Conventional PCI bus -- exposed by a PCIe-to-PCI bridge in the
topology -- intervenes between a PCI Express Root Port and a PCI Express
Endpoint. The Conventional PCI bus limits the accessible config space of
the PCI Express Endpoint, even though the endpoint advertizes the PCI
Express capability. Here's a diagram, courtesy of Alex Williamson:

  [PCIe Root Port]--[PCIe-to-PCI]--[PCI-to-PCIe]--[PCIe EP]
                              ->|  |<- Conventional PCI bus

Catch reads of 0xFFFF_FFFF in LocatePciExpressCapabilityRegBlock(), and
break out of the scan with a warning message. The function will return
EFI_NOT_FOUND.

Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Hao A Wu <hao.a.wu@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
---

Notes:
    Repo:   https://github.com/lersek/edk2.git
    Branch: pcibus_no_ext_conf

 MdeModulePkg/Bus/Pci/PciBusDxe/PciCommand.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/MdeModulePkg/Bus/Pci/PciBusDxe/PciCommand.c b/MdeModulePkg/Bus/Pci/PciBusDxe/PciCommand.c
index 214aeecdd40a..6283d602207c 100644
--- a/MdeModulePkg/Bus/Pci/PciBusDxe/PciCommand.c
+++ b/MdeModulePkg/Bus/Pci/PciBusDxe/PciCommand.c
@@ -236,6 +236,19 @@ LocatePciExpressCapabilityRegBlock (
       break;
     }
 
+    if (CapabilityEntry == MAX_UINT32) {
+      DEBUG ((
+        DEBUG_WARN,
+        "%a: [%02x|%02x|%02x] failed to access config space at offset 0x%x\n",
+        __FUNCTION__,
+        PciIoDevice->BusNumber,
+        PciIoDevice->DeviceNumber,
+        PciIoDevice->FunctionNumber,
+        CapabilityPtr
+        ));
+      break;
+    }
+
     CapabilityID = (UINT16) CapabilityEntry;
 
     if (CapabilityID == CapId) {
-- 
2.19.1.3.g30247aa5d201