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    SubjectRE: [PATCH v11 05/10] iommu/vt-d: Add bind guest PASID support
    Date
    > From: Jacob Pan <jacob.jun.pan@linux.intel.com>
    > Sent: Friday, April 17, 2020 11:29 PM
    >
    > On Fri, 17 Apr 2020 09:46:55 +0200
    > Auger Eric <eric.auger@redhat.com> 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

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