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

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Date:   Fri, 17 Apr 2020 09:46:55 +0200
From:   Auger Eric <eric.auger@...hat.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:     Yi L <yi.l.liu@...ux.intel.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        David Woodhouse <dwmw2@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Jonathan Cameron <jic23@...nel.org>
Subject: Re: [PATCH v11 05/10] iommu/vt-d: Add bind guest PASID support

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

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
>