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

[Tian, Kevin]

> From: Jacob Pan <jacob.jun.pan@xxxxxxxxxxxxxxx>
> Sent: Friday, April 17, 2020 11:29 PM
> 
> On Fri, 17 Apr 2020 09:46:55 +0200
> Auger Eric <eric.auger@xxxxxxxxxx> wrote:
> 
> > Hi Kevin,
> > On 4/17/20 4:45 AM, Tian, Kevin wrote:
> > >> 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.
> > Ah OK. So if we can detect the case here it is even better
> >
> I don't understand why VFIO cannot track, since it is mdev aware. if we
> don;t refcount the users, one mdev unbind will result unbind for all
> mdev under the same pdev. That may not be the right thing to do.
> 

The open here is not for mdev, which refcount is still required. Eric's
point is for non-mdev endpoints. It's meaningless and not intuitive 
to allow binding a PASID multiple-times to the same device. 

Thanks
Kevin