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

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Date:   Fri, 14 Jul 2023 13:47:18 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     "Tian, Kevin" <kevin.tian@...el.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:     baolu.lu@...ux.intel.com, 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

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.

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