RE: [PATCH v11 05/10] iommu/vt-d: Add bind guest PASID support

[Tian, Kevin]

> From: Auger Eric
> Sent: Thursday, April 16, 2020 6:43 PM
> 
[...]
> >>> +	if (svm) {
> >>> +		/*
> >>> +		 * If we found svm for the PASID, there must be at
> >>> +		 * least one device bond, otherwise svm should be
> >>> freed.
> >>> +		 */
> >>> +		if (WARN_ON(list_empty(&svm->devs))) {
> >>> +			ret = -EINVAL;
> >>> +			goto out;
> >>> +		}
> >>> +
> >>> +		for_each_svm_dev(sdev, svm, dev) {
> >>> +			/* In case of multiple sub-devices of the
> >>> same pdev
> >>> +			 * assigned, we should allow multiple bind
> >>> calls with
> >>> +			 * the same PASID and pdev.
> >>> +			 */
> >>> +			sdev->users++;
> >> What if this is not an mdev device. Is it also allowed?
> > Yes. IOMMU and VT-d driver is not mdev aware. Here mdev is just an
> > example of normal use case. You can bind the same PCI device (PF or
> > SRIOV VF) more than once to the same PASID. Just need to unbind also.
> 
> I don't get the point of binding a non mdev device several times with
> the same PASID. Do you intend to allow that at userspace level or
> prevent this from happening in VFIO?

I feel it's better to prevent this from happening, otherwise VFIO also
needs to track the bind count and do multiple unbinds at mm_exit.
But it's not necessary to prevent it in VFIO. We can check here
upon whether aux_domain is valid, and if not return -EBUSY.

> 
> Besides, the comment is a bit misleading as it gives the impression it
> is only true for mdev and there is no associated check.

Thanks
Kevin