linux-kernel - RE: [PATCH v10 5/7] iommu/vt-d: Make prq draining code generic

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Date:   Wed, 19 Jul 2023 05:39:30 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Baolu Lu <baolu.lu@...ux.intel.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        Joerg Roedel <joro@...tes.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        "Robin Murphy" <robin.murphy@....com>
CC:     Jason Gunthorpe <jgg@...dia.com>, Will Deacon <will@...nel.org>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        "Luck, Tony" <tony.luck@...el.com>
Subject: RE: [PATCH v10 5/7] iommu/vt-d: Make prq draining code generic

> From: Baolu Lu <baolu.lu@...ux.intel.com>
> Sent: Friday, July 14, 2023 1:47 PM
> 
> On 2023/7/14 11:49, Tian, Kevin wrote:
> >> From: Baolu Lu <baolu.lu@...ux.intel.com>
> >> Sent: Friday, July 14, 2023 11:28 AM
> >>
> >> On 2023/7/13 15:49, Tian, Kevin wrote:
> >>>> From: Jacob Pan <jacob.jun.pan@...ux.intel.com>
> >>>> Sent: Thursday, July 13, 2023 12:34 AM
> >>>>
> >>>> -	/* Domain type specific cleanup: */
> >>>>    	domain = iommu_get_domain_for_dev_pasid(dev, pasid, 0);
> >>>> -	if (domain) {
> >>>> -		switch (domain->type) {
> >>>> -		case IOMMU_DOMAIN_SVA:
> >>>> -			intel_svm_remove_dev_pasid(dev, pasid);
> >>>> -			break;
> >>>> -		default:
> >>>> -			/* should never reach here */
> >>>> -			WARN_ON(1);
> >>>> -			break;
> >>>> -		}
> >>>> +	if (!domain)
> >>>> +		goto out_tear_down;
> >>>
> >>> WARN_ON()
> >>
> >> Why?
> >>
> >> My understanding is that remve_device_pasid could be call in any context
> >> including no domain attached.
> >>
> >
> > oh I'm not aware of that. Can you elaborate the usage which uses a pasid
> > w/o domain? pasid needs to point to a page table. Presumably every
> > page table should be wrapped by a iommu domain...
> 
> A case I can think of is error rewinding. A domain is being attached to
> multiple pasids. When one of them is failed, remove_device_pasid should
> be called on all pasids so that they are parked at a determinant state.

Can you elaborate what is the association among those pasid's so failing
one would lead to failing all?

Just like a domain can be attached to multiple devices. I don't think there
is an unwinding policy forcing to detach all devices just because there is
a failure attaching the domain to a new one.

> 
> On the other hand, I don't want the remove_device_pasid to be the
> counterpart of attach_dev_pasid. remove_device_pasid simply denotes:
> 
> - The pasid will be parked in blocking state;
> - If any domain that has been attached to this pasid, stop reference to
>    it any more. Otherwise, there might be use-after-free issues.
> 
> Hence, remove_device_pasid should never fail.
> 

It should never fail. But could warn if there is a condition which shouldn't
be hit. 😊