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

[Baolu Lu]

On 2023/7/14 11:49, Tian, Kevin wrote:
> > From: Baolu Lu <baolu.lu@xxxxxxxxxxxxxxx>
> > Sent: Friday, July 14, 2023 11:28 AM
> >
> > On 2023/7/13 15:49, Tian, Kevin wrote:
> > > > From: Jacob Pan <jacob.jun.pan@xxxxxxxxxxxxxxx>
> > > > 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.

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.

Best regards,
baolu