Re: [PATCH v4 09/10] iommu: Make iommu_queue_iopf() more generic

[Vasant Hegde <vasant.hegde@amd.com>]

Tian, Baolu,

On 8/30/2023 1:13 PM, Tian, Kevin wrote:
>> From: Baolu Lu <baolu.lu@linux.intel.com>
>> Sent: Saturday, August 26, 2023 4:01 PM
>>
>> On 8/25/23 4:17 PM, Tian, Kevin wrote:
>>>> +
>>>>   /**
>>>>    * iopf_queue_flush_dev - Ensure that all queued faults have been
>>>> processed
>>>>    * @dev: the endpoint whose faults need to be flushed.
>>> Presumably we also need a flush callback per domain given now
>>> the use of workqueue is optional then flush_workqueue() might
>>> not be sufficient.
>>>
>>
>> The iopf_queue_flush_dev() function flushes all pending faults from the
>> IOMMU queue for a specific device. It has no means to flush fault queues
>> out of iommu core.
>>
>> The iopf_queue_flush_dev() function is typically called when a domain is
>> detaching from a PASID. Hence it's necessary to flush the pending faults
>> from top to bottom. For example, iommufd should flush pending faults in
>> its fault queues after detaching the domain from the pasid.
>>
> 
> Is there an ordering problem? The last step of intel_svm_drain_prq()
> in the detaching path issues a set of descriptors to drain page requests
> and responses in hardware. It cannot complete if not all software queues
> are drained and it's counter-intuitive to drain a software queue after 
> the hardware draining has already been completed.
> 
> btw just flushing requests is probably insufficient in iommufd case since
> the responses are received asynchronously. It requires an interface to
> drain both requests and responses (presumably with timeouts in case
> of a malicious guest which never responds) in the detach path.
> 
> it's not a problem for sva as responses are synchrounsly delivered after
> handling mm fault. So fine to not touch it in this series but certainly
> this area needs more work when moving to support iommufd. 😊
> 
> btw why is iopf_queue_flush_dev() called only in intel-iommu driver?
> Isn't it a common requirement for all sva-capable drivers?

I had same question when we did SVA implementation for AMD IOMMU [1]. Currently
we call queue_flush from remove_dev_pasid() path. Since PASID can be enabled
without ATS/PRI, I thought its individual drivers responsibility.
But looking this series, does it make sense to handle queue_flush in core layer?

[1]
https://lore.kernel.org/linux-iommu/20230823140415.729050-1-vasant.hegde@amd.com/T/#t

-Vasant