On 25-08-2026 12:16, Naman Jain wrote:
>
>
> On 8/25/2026 10:47 AM, 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 periodic 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: Hamza Mahfooz <[email protected]>
>> Signed-off-by: Sahil Chandna <[email protected]>
>> ---
>> This was sent earlier upstream [1]
>> [1]
>> https://lore.kernel.org/linux-hyperv/[email protected]/
>> ---
>> drivers/pci/controller/pci-hyperv.c | 14 +++++++++++++-
>> 1 file changed, 13 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/pci/controller/pci-hyperv.c
>> b/drivers/pci/controller/pci-hyperv.c
>> index cfc8fa403dad..c4fba0039164 100644
>> --- a/drivers/pci/controller/pci-hyperv.c
>> +++ b/drivers/pci/controller/pci-hyperv.c
>> @@ -1040,11 +1040,18 @@ 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 PCI_RESPONSE_HANG_TIMEOUT_SEC
>> + * without either a rescind or completion, add a periodic warning.
>> */
>> +#define PCI_RESPONSE_HANG_TIMEOUT_SEC 300
>> +
>> static int wait_for_response(struct hv_device *hdev,
>> struct completion *comp)
>> {
>> + unsigned long delay = secs_to_jiffies(PCI_RESPONSE_HANG_TIMEOUT_SEC);
>> + u64 timeout = get_jiffies_64() + delay;
>> +
>> while (true) {
>> if (hdev->channel->rescind) {
>> dev_warn_once(&hdev->device, "The device is gone.\n");
>> @@ -1053,6 +1060,11 @@ static int wait_for_response(struct hv_device *hdev,
>>
>> if (wait_for_completion_timeout(comp, HZ / 10))
>> break;
>> +
>> + if (time_after64(get_jiffies_64(), timeout)) {
>> + dev_warn(&hdev->device, "PCI stuck waiting for
>> response.\n");
>> + timeout = get_jiffies_64() + delay;
>> + }
>> }
>
> There can be some enhancements in above patch to address these problems:
> 1. Logging forever every 5 minutes in case of no completion or rescind.
> 2. If we now print warning once, not knowing if completion ever arrived.
> 3. On solving pt. 1 and 2 by adding a print for completion, one should
> avoid adding a print by default for regular timely completions.
>
Ack.
> Basically something like this:
>
> #define PCI_RESPONSE_WARN_TIMEOUT_SEC 300
>
> static int wait_for_response(struct hv_device *hdev,
> struct completion *comp)
> {
> unsigned long warn_at =
> jiffies + secs_to_jiffies(PCI_RESPONSE_WARN_TIMEOUT_SEC);
> bool warned = false;
>
> 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)) {
> if (warned || time_after_eq(jiffies, warn_at))
> dev_warn(&hdev->device,
> "PCI response received after prolonged wait.\n");
> return 0;
> }
>
> if (!warned && time_after_eq(jiffies, warn_at)) {
> dev_warn(&hdev->device,
> "PCI still waiting for response.\n");
> warned = true;
> }
> }
> }
>
Thanks for review, I align on repeated warning every 5 minutes for
stalled guest would add to dmesg noise.I will wait for other review
comments as well and share v2 addressing this.
> Regards,
> Naman
