Re: [PATCH v12 5/5] iommu/vt-d: improve ITE fault handling if target device isn't present

[Ethan Zhao <haifeng.zhao@linux.intel.com>]

On 1/30/2024 4:43 PM, Tian, Kevin wrote:
>> From: Ethan Zhao <haifeng.zhao@linux.intel.com>
>> Sent: Tuesday, January 30, 2024 4:16 PM
>>
>> On 1/30/2024 2:22 PM, Tian, Kevin wrote:
>>> Here we need consider two situations.
>>>
>>> One is that the device is not bound to a driver or bound to a driver
>>> which doesn't do active work to the device when it's removed. In
>>> that case one may observe the timeout situation only in the removal
>>> path as the stack dump in your patch02 shows.
>> When iommu_bus_notifier() got called for hotplug removal cases to
>> flush devTLB (ATS invalidation), driver was already unloaded.
>> whatever safe removal or surprise removal. so in theory no active
>> driver working there.
>>
>> pciehp_ist()
>>    pciehp_disable_slot()
>>     remove_board()
>>      pciehp_unconfigure_device()
>>       pci_stop_and_remove_bus_device()
>>        pci_stop_bus_device()--->here unload driver
>>        pci_remove_bus_device()->here qi_flush_dev_iotlb() got called.
> yes, so patch02 can fix this case.
>
>>> patch02 can fix that case by checking whether the device is present
>>> to skip sending the invalidation requests. So the logic being discussed
>>> here doesn't matter.
>>>
>>> The 2nd situation is more tricky. The device might be bound to
>>> a driver which is doing active work to the device with in-fly
>>> ATS invalidation requests. In this case in-fly requests must be aborted
>>> before the driver can be detached from the removed device. Conceptually
>>> a device is removed from the bus only after its driver is detached.
>> Some tricky situations:
>>
>> 1. The ATS invalidation request is issued from driver driver, while it is
>> in handling, device is removed. this momment, the device instance still
>> exists in the bus list. yes, if searching it by BDF, could get it.
> it's searchable between the point where the device is removed and the
> point where the driver is unloaded:
>
>          CPU0                                CPU1
>    (Driver is active)                    (pciehp handler)
>    qi_submit_sync()                      pciehp_ist()
>      ...                                   ...
>      loop for completion() {               pciehp_unconfigure_device()
>        ...                                   pci_dev_set_disconnected()
>        if (ITE) {                            ...
>          //find pci_dev from sid             pci_remove_bus_device()
>          if (pci_dev_is_connected())           device_del()
>            break;                                bus_remove_device()
>        }                                           device_remove_driver()

If the device was hot plugin or re-scanned, the device has a PCI_DEV_ADDED flag,
if so the driver unloading work isn't defered to the tail of device_del(), it
is unloaded before pci_remove_bus_device()->device_del(), in pci_stop_dev

pci_stop_bus_device()
  pci_stop_dev()
  {
   if (pci_dev_is_added(dev)) {
       device_release_driver(&dev->dev);
  }

So the interval the device is searchable, only applied to those devices
not hot plugged, or never be scanned.


Thanks,
Ethan

>        ..                                            //wait for driver unload
>      }
>      ..
>      return;
>
>                                                    BUS_NOTIFY_REMOVED_DEVICE;
>                                                list_del(&dev->bus_list);
>
> (I didn’t draw the full calling stack on the right hand side)

>
>> 2. The ATS invalidation request is issued from iommu_bus_notifier()
>> for surprise removal reason, as shown in above calltrace, device was
>> already removed from bus list. if searching it by BDF, return NULL.
>>
>> 3. The ATS invlidation request is issued from iommu_bus_notifier()
>> for safe removal, when is in handling, device is removed or link
>> is down. also as #2, device was already removed from bus list.
>> if searching it by BDF. got NULL.
>> ...
>>
>> so, searching device by BDF, only works for the ATS invalidation
>> request is from device driver.
>>
> anything related to bus notifier has been fixed by patch02.
>
> the remaining logic is really for fixing the race invalidation from
> device driver.