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

[Owen Smith <owen.smith@xxxxxxxxxx>]

-----Original Message-----
From: win-pv-devel <win-pv-devel-bounces@xxxxxxxxxxxxxxxxxxxx> On Behalf Of 
Paul Durrant
Sent: Monday, June 21, 2021 3:03 PM
To: win-pv-devel@xxxxxxxxxxxxxxxxxxxx
Subject: Re: [PATCH 6/6] Dont always remove XenFilt from UpperFilters

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On 17/06/2021 13:33, Owen Smith wrote:
> 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>

This is a partial reversion of 9d28a9e9b798f "Avoid removing filters on upgrade 
installations". I think you need to state why that commit was wrong (assuming 
it was).

   Paul


I have encountered upgrade issues, particularly involving Win10 in-place OS 
upgrade, that can affect the ordering the PnP manager handles driver load. I 
will see if I can pin down exactly the conditions that trigger the race here in 
9d28a9e9b798f.

Owen

> ---
>   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++) {
>