linux-kernel - Re: [PATCH 6/7] iommu: Use right way to retrieve iommu

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Message-ID: <07ecce72-1904-d695-6ae9-bfaa2fcfe0cc@linux.intel.com>
Date:   Tue, 25 Jan 2022 11:31:50 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        Robin Murphy <robin.murphy@....com>,
        Jason Gunthorpe <jgg@...dia.com>
Cc:     baolu.lu@...ux.intel.com, "Raj, Ashok" <ashok.raj@...el.com>,
        David Airlie <airlied@...ux.ie>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Christoph Hellwig <hch@...radead.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Ben Skeggs <bskeggs@...hat.com>,
        Daniel Vetter <daniel@...ll.ch>, Will Deacon <will@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>
Subject: Re: [PATCH 6/7] iommu: Use right way to retrieve iommu_ops

On 1/25/22 10:08 AM, Tian, Kevin wrote:
>> From: Robin Murphy <robin.murphy@....com>
>> Sent: Tuesday, January 25, 2022 9:52 AM
>>
>> On 2022-01-25 01:11, Tian, Kevin wrote:
>>>> From: Jason Gunthorpe via iommu
>>>> Sent: Tuesday, January 25, 2022 1:37 AM
>>>>> @@ -1295,7 +1298,7 @@ int iommu_page_response(struct device *dev,
>>>>>    			msg->pasid = 0;
>>>>>    		}
>>>>>
>>>>> -		ret = domain->ops->page_response(dev, evt, msg);
>>>>> +		ret = ops->page_response(dev, evt, msg);
>>>>>    		list_del(&evt->list);
>>>>>    		kfree(evt);
>>>>>    		break;
>>>>
>>>> Feels weird that page_response is not connected to a domain, the fault
>>>> originated from a domain after all. I would say this op should be
>>>> moved to the domain and the caller should provide the a pointer to the
>>>> domain that originated the fault.
>>>>
>>>
>>> In concept yes.
>>
>> Not even that, really. It's true that the "fault" itself is logically
>> associated with the domain, but we never see that - the ATS request and
>> response which encapsulate it all happen automatically on the PCI side.
>> It's the endpoint that then decides to handle ATS translation failure
>> via PRI, so all we actually get is a page request message from a
>> RID/PASID, which most definitely represents the "device" (and in fact
>> having to work backwards from there to figure out which domain/context
>> it is currently attached to can be a bit of a pain). Similarly the
>> response is a message directly back to the device itself - an operation
>> on a domain may (or may not) have happened off the back of receiving the
>> initial request, but even if the content of the response is to reflect
>> that, the operation of responding is clearly focused on the device.
>>
>> I fully agree that it's a weird-looking model, but that's how PCI SIG
>> made it - and no IOMMU architecture seems to have tried to wrap it up in
>> anything nicer either - so I don't see that we'd gain much from trying
>> to pretend otherwise :)
>>
> 
> I think the point here is that although page requests are received
> per device from the wire the low level iommu driver should convert
> those requests into domain-wide requests (with RID/PASID recorded
> as private data in the request) which then can be handled by domain
> ops in iommu core. Once a domain-wide request is completed by
> the iommu core, the low level iommu driver then retrieves RID/PASID
> information from private data of the completed request and triggers
> page response per RID/PASID in bus specific way.

I also have a pending series to associate the sva with an iommu domain
and make the existing I/O page fault framework generic (vs. sva
specific). Perhaps we can discuss the page fault handle/response there
with the real code.

> 
> Does it sound reasonable?
> 
> Thanks
> Kevin
> 

Best regards,
baolu