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Date:   Fri, 17 Apr 2020 08:28:39 -0700
From:   Jacob Pan <jacob.jun.pan@linux.intel.com>
To:     Auger Eric <eric.auger@redhat.com>
Cc:     "Tian, Kevin" <kevin.tian@intel.com>,
        Yi L <yi.l.liu@linux.intel.com>,
        Alex Williamson <alex.williamson@redhat.com>,
        "Raj, Ashok" <ashok.raj@intel.com>,
        David Woodhouse <dwmw2@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        "iommu@lists.linux-foundation.org" <iommu@lists.linux-foundation.org>,
        Jean-Philippe Brucker <jean-philippe@linaro.com>,
        Jonathan Cameron <jic23@kernel.org>,
        jacob.jun.pan@linux.intel.com
Subject: Re: [PATCH v11 05/10] iommu/vt-d: Add bind guest PASID support
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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.

> Thanks
> 
> Eric
> >   
> >>
> >> 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
> >   
> 

[Jacob Pan]