by Ethan Zhao

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Subject: Re: [RFC PATCH v9 2/5] iommu/vt-d: break out ATS Invalidation if target device is gone

On 12/29/2023 4:06 PM, Tian, Kevin wrote:
>> From: Ethan Zhao <[email protected]>
>> Sent: Thursday, December 28, 2023 9:03 PM
>>
>> On 12/28/2023 4:30 PM, Tian, Kevin wrote:
>>>> From: Ethan Zhao <[email protected]>
>>>> Sent: Thursday, December 28, 2023 8:17 AM
>>>>
>>>> For those endpoint devices connect to system via hotplug capable ports,
>>>> users could request a warm reset to the device by flapping device's link
>>>> through setting the slot's link control register, as pciehp_ist() DLLSC
>>>> interrupt sequence response, pciehp will unload the device driver and
>>>> then power it off. thus cause an IOMMU device-TLB invalidation (Intel
>>>> VT-d spec, or ATS Invalidation in PCIe spec r6.1) request for device to
>>>> be sent and a long time completion/timeout waiting in interrupt context.
>>> is above describing the behavior of safe removal or surprise removal?
>> bring the link down is a kind of surprise removal for hotplug capable
>>
>> device.
> then it's better to make it clear from beginning that this is about surprise
> removal in which device is removed and cannot respond to on-going
> ATS invalidation request incurred in the removal process.
>
> safe removal should be immune from this problem as the device is still
> responsive in the whole removal process.
>
>>>> [ 4223.822628] Call Trace:
>>>> [ 4223.822628] qi_flush_dev_iotlb+0xb1/0xd0
>>>> [ 4223.822628] __dmar_remove_one_dev_info+0x224/0x250
>>>> [ 4223.822629] dmar_remove_one_dev_info+0x3e/0x50
>>>> [ 4223.822629] intel_iommu_release_device+0x1f/0x30
>>>> [ 4223.822629] iommu_release_device+0x33/0x60
>>>> [ 4223.822629] iommu_bus_notifier+0x7f/0x90
>>>> [ 4223.822630] blocking_notifier_call_chain+0x60/0x90
>>>> [ 4223.822630] device_del+0x2e5/0x420
>>>> [ 4223.822630] pci_remove_bus_device+0x70/0x110
>>>> [ 4223.822630] pciehp_unconfigure_device+0x7c/0x130
> I'm curious why this doesn't occur earlier when the device is
> detached from the driver. At that point presumably the device
> should be detached from the DMA domain which involves
> ATS invalidation too.
>
>>>> while (qi->desc_status[wait_index] != QI_DONE) {
>>>> + /*
>>>> + * if the device-TLB invalidation target device is gone, don't
>>>> + * wait anymore, it might take up to 1min+50%, causes
>>>> system
>>>> + * hang. (see Implementation Note in PCIe spec r6.1 sec
>>>> 10.3.1)
>>>> + */
>>>> + if ((type == QI_DIOTLB_TYPE || type == QI_DEIOTLB_TYPE)
>>>> && pdev)
>>>> + if (!pci_device_is_present(pdev))
>>>> + break;
>>> I'm not sure it's the right thing to do. Such check should be put in the
>>> caller which has the device pointer and can already know it's absent
>>> to not call those cache invalidation helpers.
>> Here is to handle such case, the invalidation request is sent, but the
>>
>> device is just pulled out at that moment.
>>
> one problem - the caller could pass multiple descriptors while type
> only refers to the 1st descriptor.

If the other invalidation request mixed together with ATS invalidation

in the descriptors passed to qi_submit_sync(), would be problem,

so far to Intel VT-d driver, I didn't see such kind of usage, perhaps

will see it later, no one could prevent that.

>
> btw is it an Intel specific problem? A quick glance at smmu driver
> suggests the same problem too:
>
> arm_smmu_atc_inv_domain()
> arm_smmu_cmdq_batch_submit()
> arm_smmu_cmdq_issue_cmdlist()
> arm_smmu_cmdq_poll_until_sync()
> __arm_smmu_cmdq_poll_until_consumed()
>
> /*
> * Wait until the SMMU cons index passes llq->prod.
> * Must be called with the cmdq lock held in some capacity.
> */
> static int __arm_smmu_cmdq_poll_until_consumed(struct arm_smmu_device *smmu,
> struct arm_smmu_ll_queue *llq)
>
> is there a more general way to solve it?

Surprise removal could happen anytime, depends on user,

no preparation could be done, so called 'surprise' :(

Thanks,

Ethan

2023-12-29 09:24:58

by Ethan Zhao

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Subject: Re: [RFC PATCH v9 5/5] iommu/vt-d: don't loop for timeout ATS Invalidation request forever

On 12/29/2023 4:17 PM, Tian, Kevin wrote:
>> From: Ethan Zhao <[email protected]>
>> Sent: Thursday, December 28, 2023 9:10 PM
>>
>> On 12/28/2023 4:38 PM, Tian, Kevin wrote:
>>>> From: Ethan Zhao <[email protected]>
>>>> Sent: Thursday, December 28, 2023 8:17 AM
>>>>
>>>>
>>>> - if (rc == -EAGAIN)
>>>> + if (rc == -EAGAIN && type !=QI_DIOTLB_TYPE && type !=
>>>> QI_DEIOTLB_TYPE)
>>>> goto restart;
>>>>
>>> this change is moot.
>>>
>>> -EAGAIN is set only when hardware detects a ATS invalidation completion
>>> timeout in qi_check_fault(). so above just essentially kills the restart logic.
>> This change is intended to break the restar login when device-TLB
>>
>> invalidation timeout happens, we don't know how long the ITE took
>>
>> if the device is just no reponse.
> if in the end the agreement is to remove the restart logic, then do it.
>
> it's not good to introduce a change which essentially kills the restart
> logic but still keeps the related code.

Here, the device-TLB invalidation, depends on devcies response,

no one could make sure the what the third party adapters will act.

but for those invalidation issued to iommu itself, should be more

likely to survive ?

Anyway, would like to see more comments.

Thanks,

Ethan