Allow PciBus to tolerate a CRS response by ignoring the device (#269)

# Preface

## Description

If there is a slow device on the PCI Bus, and the HostBridge is
programmed to allow CRS, the slow device may return 0x0001 to inform the
config space reader of the Vendor Id that pci device is not ready. The
current PciBus enumerator will treat 0001 as a valid Vendor Id, but it
is not. It indicates that all other config space is invalid. This code
changes that operation to skip slow devices.

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- [x] Impacts functionality?
- **Functionality** - Does the change ultimately impact how firmware
functions?
- Examples: Add a new library, publish a new PPI, update an algorithm,
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- [ ] Impacts security?
- **Security** - Does the change have a direct security impact on an
application,
    flow, or firmware?
  - Examples: Crypto algorithm change, buffer overflow fix, parameter
    validation improvement, ...
- [ ] Breaking change?
- **Breaking change** - Will anyone consuming this change experience a
break
    in build or boot behavior?
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    a function in a new library class in a pre-existing module, ...
- [ ] Includes tests?
  - **Tests** - Does the change include any explicit test code?
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- **Documentation** - Does the change contain explicit documentation
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    outside direct code modifications (and comments)?
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## How This Was Tested

Verified that a platform without CRS still boots.

## Integration Instructions

N/A

MdeModulePkg/Bus/Pci/PciBusDxe/PciEnumeratorSupport.c

@@ -59,21 +59,32 @@ PciDevicePresent (
                                   Pci
                                   );
 
-  if (!EFI_ERROR (Status) && ((Pci->Hdr).VendorId != 0xffff)) {
-    //
-    // Read the entire config header for the device
-    //
-    Status = PciRootBridgeIo->Pci.Read (
-                                    PciRootBridgeIo,
-                                    EfiPciWidthUint32,
-                                    Address,
-                                    sizeof (PCI_TYPE00) / sizeof (UINT32),
-                                    Pci
-                                    );
+  // MU_CHANGE Begin
+  // The host bridge may be programmed to accept CRS.  If the PCI device is slow, and CRS is enabled,
+  // the VendorId may read as 0x0001 when not ready. This value is assigned to the PCI-SIG for this,
+  // and other purposes.  Skip the device, as all the other data read will be invalid.
+  //
+  if (!EFI_ERROR (Status)) {
+    if (((Pci->Hdr).VendorId != 0xffff) && ((Pci->Hdr).VendorId != 0x0001)) {
+      //
+      // Read the entire config header for the device
+      //
+      Status = PciRootBridgeIo->Pci.Read (
+                                      PciRootBridgeIo,
+                                      EfiPciWidthUint32,
+                                      Address,
+                                      sizeof (PCI_TYPE00) / sizeof (UINT32),
+                                      Pci
+                                      );
 
-    return EFI_SUCCESS;
+      return EFI_SUCCESS;
+    } else if ((Pci->Hdr).VendorId == 0x0001) {
+      DEBUG ((DEBUG_WARN, "CRS response detected.  Devices that return a CRS response during enumeration are currently ignored\n"));
+    }
   }
 
+  // MU_CHANGE End
+
   return EFI_NOT_FOUND;
 }