Re: [PATCH v3] PCI: hv: Warn when wait_for_response() waits indefinitely

[Naman Jain <namjain@linux.microsoft.com>]

On 9/8/2026 5:00 PM, Sahil Chandna wrote:
> A guest can wait indefinitely in wait_for_response() for the host to
> send either a rescind message or a packet completion. If the
> host does not send either, the guest can remain blocked with no
> diagnostic indicating a reason.
> This was observed during a guest kernel upgrade in which the
> host-side application handling the PCI channel faulted, causing the
> guest to never receive the completion request.
> Add a warning in wait_for_response() when the wait exceeds
> a timeout so that such a hang is visible in the guest's kernel log
> and can be correlated with host-side state.
> 
> Suggested-by: Michael Kelley <mhklinux@outlook.com>
> Signed-off-by: Sahil Chandna <sahilchandna@linux.microsoft.com>
> ---
> Changes since v2:
> - Add counter based timeout instead of introducing another timer
> Link to v2: https://lore.kernel.org/all/20260902115854.2629164-1-sahilchandna@linux.microsoft.com/
> 
> Changes since v1:
> - Removed periodic warning to one time warning in 2 minutes
> - Include vmbus relid and stuck PCI msg.
> Link to v1: https://lore.kernel.org/all/20260825051850.2438816-1-sahilchandna@linux.microsoft.com/
>   drivers/pci/controller/pci-hyperv.c | 46 ++++++++++++++++++++++-------
>   1 file changed, 36 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/pci/controller/pci-hyperv.c b/drivers/pci/controller/pci-hyperv.c
> index 89816a2bd7cd..bd07402c47ed 100644
> --- a/drivers/pci/controller/pci-hyperv.c
> +++ b/drivers/pci/controller/pci-hyperv.c
> @@ -1040,19 +1040,38 @@ static void put_pcichild(struct hv_pci_dev *hpdev)
> 
>   /*
>    * There is no good way to get notified from vmbus_onoffer_rescind(),
> - * so let's use polling here, since this is not a hot path.
> + * so let's use polling here, since this is not a hot path. If
> + * wait_for_response() has been polling for 2 minutes
> + * without either a rescind or completion, add a warning.
>    */
> +#define PCI_RESPONSE_HANG_TICKS 1200
> +

Nit, I think this variable name could be misleading, as this represents 
a loop counter for a retry logic, but not exactly *ticks* from kernel 
terminology.

I would have preferred something like: PCI_RESPONSE_WARN_POLL_COUNT.

Rest LGTM.

Reviewed-by: Naman Jain <namjain@linux.microsoft.com>

Regards,
Naman Jain