[PATCH 6/6] Dont always remove XenFilt from UpperFilters

[Owen Smith <owen.smith@xxxxxxxxxx>]

There is a race which can prevent XenFilt from having AddDevice
correctly called on some devices. This seems to happen more frequently
after driver upgrades.
It seems this is due to XenBus!DriverEntry removing XenFilt before
XenBus!AddDevice inserts XenFilt into the UpperFilters value. In this
gap, PnP manager can sample the UpperFilters key and fail to bind
XenFilt to the required device stacks. This leads to XenFilt failing
to determine the active device, which prevents the device unplugs
from occuring, leading to a constant cycle of reboot prompts from
xenbus_monitor.

Signed-off-by: Owen Smith <owen.smith@xxxxxxxxxx>
---
 src/xenbus/driver.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/src/xenbus/driver.c b/src/xenbus/driver.c
index 1b621fa..e66bf45 100644
--- a/src/xenbus/driver.c
+++ b/src/xenbus/driver.c
@@ -296,6 +296,9 @@ DriverRemoveFunctionDeviceObject(
     RemoveEntryList(&Dx->ListEntry);
     ASSERT3U(Driver.References, !=, 0);
     References = --Driver.References;
+
+    if (References == 1)
+        FiltersUninstall();
 }
 
 //
@@ -859,16 +862,15 @@ DriverEntry(
                       MICRO_VERSION,
                       BUILD_NUMBER);
     if (!NT_SUCCESS(status)) {
-        if (status == STATUS_INCOMPATIBLE_DRIVER_BLOCKED)
+        if (status == STATUS_INCOMPATIBLE_DRIVER_BLOCKED) {
+            // Re-instate XenFilt to avoid a second required reboot
+            FiltersInstall();
             __DriverRequestReboot();
+        }
 
         goto done;
     }
 
-    // Remove the filters from the registry. They will be re-instated by
-    // the first successful AddDevice.
-    FiltersUninstall();
-
     DriverObject->DriverExtension->AddDevice = DriverAddDevice;
 
     for (Index = 0; Index <= IRP_MJ_MAXIMUM_FUNCTION; Index++) {
-- 
2.31.1.windows.1