linux-kernel - Re: [PATCH v2 5/5] iommu/vt-d: Move PRI handling to IOPF feature path

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Date:   Thu, 16 Mar 2023 16:17:00 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>
Cc:     baolu.lu@...ux.intel.com, Joerg Roedel <joro@...tes.org>,
        Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        Vinod Koul <vkoul@...nel.org>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 5/5] iommu/vt-d: Move PRI handling to IOPF feature path

On 2023/3/16 15:17, Tian, Kevin wrote:
>> From: Lu Baolu <baolu.lu@...ux.intel.com>
>> Sent: Thursday, March 9, 2023 10:57 AM
>>
>> @@ -4689,17 +4704,21 @@ static int intel_iommu_disable_iopf(struct device
>> *dev)
>>   {
>>   	struct device_domain_info *info = dev_iommu_priv_get(dev);
>>   	struct intel_iommu *iommu = info->iommu;
>> -	int ret;
>>
>> -	ret = iommu_unregister_device_fault_handler(dev);
>> -	if (ret)
>> -		return ret;
>> +	if (!info->pri_enabled)
>> +		return -EINVAL;
>>
>> -	ret = iopf_queue_remove_device(iommu->iopf_queue, dev);
>> -	if (ret)
>> -		iommu_register_device_fault_handler(dev,
>> iommu_queue_iopf, dev);
>> +	pci_disable_pri(to_pci_dev(dev));
>> +	info->pri_enabled = 0;
>>
>> -	return ret;
>> +	/*
>> +	 * With pri_enabled checked, unregistering fault handler and
>> +	 * removing device from iopf queue should never fail.
>> +	 */
>> +	iommu_unregister_device_fault_handler(dev);
>> +	iopf_queue_remove_device(iommu->iopf_queue, dev);
>> +
>> +	return 0;
>>   }
> 
> PCIe spec says that clearing the enable bit doesn't mean in-fly
> page requests are completed:
> --
> Enable (E) - This field, when set, indicates that the Page Request
> Interface is allowed to make page requests. If this field is Clear,
> the Page Request Interface is not allowed to issue page requests.
> If both this field and the Stopped field are Clear, then the Page
> Request Interface will not issue new page requests, but has
> outstanding page requests that have been transmitted or are
> queued for transmission

Yes. So the iommu driver should drain the in-fly PRQs.

The Intel VT-d implementation drains the PRQs when any PASID is unbound
from the iommu domain (see intel_svm_drain_prq()) before reuse. Before
disabling iopf, the device driver should unbind pasid and disable sva,
so when it comes here, the PRQ should have been drained.

Perhaps I can add below comments to make this clear:

         /*
          * PCIe spec states that by clearing PRI enable bit, the Page
          * Request Interface will not issue new page requests, but has
          * outstanding page requests that have been transmitted or are
          * queued for transmission. This is supposed to be called after
          * the device driver has stopped DMA, all PASIDs have been
          * unbound and the outstanding PRQs have been drained.
          */

Best regards,
baolu