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

[Tian, Kevin]

> From: Tian, Kevin
> Sent: Wednesday, August 30, 2023 3:44 PM
> 
> > From: Baolu Lu <baolu.lu@xxxxxxxxxxxxxxx>
> > 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.

to be clear it's correct to drain request queues from bottom to top as the
lower level queue is the input to the higher level queue. But for response
the lowest draining needs to wait for response from higher levels. It's
interesting that intel-iommu driver combines draining hw page requests
and responses in one step in intel_svm_drain_prq(). this also needs some
consideration regarding to iommufd...

> 
> 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?