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

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

[Sahil Chandna]

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
+
 static int wait_for_response(struct hv_device *hdev,
-			     struct completion *comp)
+			     struct completion *comp,
+			     const char *msg_type)
 {
+	u64 counter = 0;
+
 	while (true) {
 		if (hdev->channel->rescind) {
 			dev_warn_once(&hdev->device, "The device is gone.\n");
 			return -ENODEV;
 		}

-		if (wait_for_completion_timeout(comp, HZ / 10))
+		counter++;
+
+		if (wait_for_completion_timeout(comp, HZ / 10)) {
+			if (counter > PCI_RESPONSE_HANG_TICKS)
+				dev_warn(&hdev->device,
+					 "Late %s completion arrived.\n", msg_type);
 			break;
+		}
+
+		if (counter == PCI_RESPONSE_HANG_TICKS) {
+			dev_err(&hdev->device,
+				"%s stuck waiting for response, relid = %u\n",
+				msg_type, hdev->channel->offermsg.child_relid);
+		}
 	}

 	return 0;
@@ -1518,7 +1537,8 @@ static int hv_read_config_block(struct pci_dev *pdev, void *buf,
 	if (ret)
 		return ret;

-	ret = wait_for_response(hbus->hdev, &comp_pkt.comp_pkt.host_event);
+	ret = wait_for_response(hbus->hdev, &comp_pkt.comp_pkt.host_event,
+				"PCI_READ_BLOCK");
 	if (ret)
 		return ret;

@@ -1607,7 +1627,8 @@ static int hv_write_config_block(struct pci_dev *pdev, void *buf,
 	if (ret)
 		return ret;

-	ret = wait_for_response(hbus->hdev, &comp_pkt.host_event);
+	ret = wait_for_response(hbus->hdev, &comp_pkt.host_event,
+				"PCI_WRITE_BLOCK");
 	if (ret)
 		return ret;

@@ -2624,7 +2645,8 @@ static struct hv_pci_dev *new_pcichild_device(struct hv_pcibus_device *hbus,
 	if (ret)
 		goto error;

-	if (wait_for_response(hbus->hdev, &comp_pkt.host_event))
+	if (wait_for_response(hbus->hdev, &comp_pkt.host_event,
+			      "PCI_QUERY_RESOURCE_REQUIREMENTS"))
 		goto error;

 	hpdev->desc = *desc;
@@ -3256,7 +3278,8 @@ static int hv_pci_protocol_negotiation(struct hv_device *hdev,
 				(unsigned long)pkt, VM_PKT_DATA_INBAND,
 				VMBUS_DATA_PACKET_FLAG_COMPLETION_REQUESTED);
 		if (!ret)
-			ret = wait_for_response(hdev, &comp_pkt.host_event);
+			ret = wait_for_response(hdev, &comp_pkt.host_event,
+						"PCI_QUERY_PROTOCOL_VERSION");

 		if (ret) {
 			dev_err(&hdev->device,
@@ -3476,7 +3499,8 @@ static int hv_pci_enter_d0(struct hv_device *hdev)
 			       (unsigned long)pkt, VM_PKT_DATA_INBAND,
 			       VMBUS_DATA_PACKET_FLAG_COMPLETION_REQUESTED);
 	if (!ret)
-		ret = wait_for_response(hdev, &comp_pkt.host_event);
+		ret = wait_for_response(hdev, &comp_pkt.host_event,
+					"PCI_BUS_D0ENTRY");

 	if (ret)
 		goto exit;
@@ -3553,7 +3577,8 @@ static int hv_pci_query_relations(struct hv_device *hdev)
 	ret = vmbus_sendpacket(hdev->channel, &message, sizeof(message),
 			       0, VM_PKT_DATA_INBAND, 0);
 	if (!ret)
-		ret = wait_for_response(hdev, &comp);
+		ret = wait_for_response(hdev, &comp,
+					"PCI_QUERY_BUS_RELATIONS");

 	/*
 	 * In the case of fast device addition/removal, it's possible that
@@ -3644,7 +3669,8 @@ static int hv_send_resources_allocated(struct hv_device *hdev)
 				VM_PKT_DATA_INBAND,
 				VMBUS_DATA_PACKET_FLAG_COMPLETION_REQUESTED);
 		if (!ret)
-			ret = wait_for_response(hdev, &comp_pkt.host_event);
+			ret = wait_for_response(hdev, &comp_pkt.host_event,
+						"PCI_RESOURCE_ASSIGNED");
 		if (ret)
 			break;

--
2.53.0

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

[Naman Jain]

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

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

[Manivannan Sadhasivam]

On Wed, Sep 09, 2026 at 03:01:22PM +0530, Naman Jain wrote:
> 
> 
> 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.
> 

PCI_RESPONSE_POLL_COUNT?

- Mani

-- 
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