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

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Date:   Fri, 17 Apr 2020 11:25:18 +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

Hi Kevin,

On 2020/4/16 9:46, Lu Baolu wrote:
> 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.

Rechecked the code. You are right. As long as the interrupt thread is
per iommu, there will only single prq thread scheduled. I will change
this accordingly in the new version. Thank you for pointing this out.

Best regards,
baolu