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

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Date:   Wed, 19 Jul 2023 15:40:52 +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 2023/7/19 13:39, Tian, Kevin wrote:
>> 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.

It's fine to add a check here if there's no real case. I was just
thinking that remove_device_pasid could also be invoked when there's no
domain attached.

>>
>> 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. 😊

Okay, let's add a check here. And we can loose it later if any real use
case comes.

Best regards,
baolu