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

Ethan Zhao
2024-01-31T05:42:40Z

On 1/30/2024 5:24 PM, Tian, Kevin wrote:
>> From: Ethan Zhao <haifeng.zhao@linux.intel.com>
>> Sent: Tuesday, January 30, 2024 5:13 PM
>>
>> 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,
> in this case is pci_dev_is_disconnected() true or false?
>
> how is this patch supposed to work with it?

pci_dev_is_disconnected() is true for safe removal, false for surprise
removal, but it not called in this patch, is used in patch[2/5],
explained in its commit log. This patch use the pci_device_is_present()
to check device present or not. if pci_dev_is_disconnected() returns true, then check its presence by pci vendor
configuration reading (a specific protocal in PCIe spec).

>
>> 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);
>> }
> no matter where driver unload is requested, it needs to wait for aborting
> in-fly request on CPU0.

yes, the progress of driver unloading has complex sync mechanism in
__device_release_driver() to do that.

>
>> So the interval the device is searchable, only applied to those devices
>> not hot plugged, or never be scanned.
>>
> and in the worst case even if pci_dev is not searchable, isn't it already
> an indicator that the device is absent then qi_submit_sync() should
> just exit upon ITE?

Hmmm, pci_dev is not searchable, but that pci_dev instance is just not in
the bus list or device list, not mean is disconnected or not present that
moment. :)

Thanks,
Ethan