linux-kernel - Re: [PATCH v2 5/7] iommu/vt-d: Save prq descriptors in an internal list

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Date:   Thu, 16 Apr 2020 09:46:21 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        Joerg Roedel <joro@...tes.org>
Cc:     baolu.lu@...ux.intel.com, "Raj, Ashok" <ashok.raj@...el.com>,
        "jacob.jun.pan@...ux.intel.com" <jacob.jun.pan@...ux.intel.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 5/7] iommu/vt-d: Save prq descriptors in an internal
 list

On 2020/4/15 17:30, Tian, Kevin wrote:
>> From: Lu Baolu<baolu.lu@...ux.intel.com>
>> Sent: Wednesday, April 15, 2020 1:26 PM
>>
>> Currently, the page request interrupt thread handles the page
>> requests in the queue in this way:
>>
>> - Clear PPR bit to ensure new interrupt could come in;
>> - Read and record the head and tail registers;
>> - Handle all descriptors between head and tail;
>> - Write tail to head register.
>>
>> This might cause some descriptors to be handles multiple times.
>> An example sequence:
>>
>> - Thread A got scheduled with PRQ_1 and PRQ_2 in the queue;
>> - Thread A clear the PPR bit and record the head and tail;
>> - A new PRQ_3 comes and Thread B gets scheduled;
>> - Thread B record the head and tail which includes PRQ_1
>>    and PRQ_2.
> I may overlook something but isn't the prq interrupt thread
> per iommu then why would two prq threads contend here?

The prq interrupt could be masked by the PPR (Pending Page Request) bit
in Page Request Status Register. In the interrupt handling thread once
this bit is clear, new prq interrupts are allowed to be generated.

So, if a page request is in process and the PPR bit is cleared, another
page request from any devices under the same iommu could trigger another
interrupt thread.

Best regards,
baolu