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Subject: Re: [RFC v2] /dev/iommu uAPI proposal
To:     "Tian, Kevin" <kevin.tian@intel.com>
CC:     Jason Gunthorpe <jgg@nvidia.com>,
        "Alex Williamson (alex.williamson@redhat.com)" 
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From:   Shenming Lu <lushenming@huawei.com>
Message-ID: <a8edb2c1-9c9c-6204-072c-4f1604b7dace@huawei.com>
Date:   Thu, 15 Jul 2021 14:29:00 +0800
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On 2021/7/15 11:55, Tian, Kevin wrote:
>> From: Shenming Lu <lushenming@huawei.com>
>> Sent: Thursday, July 15, 2021 11:21 AM
>>
>> On 2021/7/9 15:48, Tian, Kevin wrote:
>>> 4.6. I/O page fault
>>> +++++++++++++++++++
>>>
>>> uAPI is TBD. Here is just about the high-level flow from host IOMMU driver
>>> to guest IOMMU driver and backwards. This flow assumes that I/O page
>> faults
>>> are reported via IOMMU interrupts. Some devices report faults via device
>>> specific way instead of going through the IOMMU. That usage is not
>> covered
>>> here:
>>>
>>> -   Host IOMMU driver receives a I/O page fault with raw fault_data {rid,
>>>     pasid, addr};
>>>
>>> -   Host IOMMU driver identifies the faulting I/O page table according to
>>>     {rid, pasid} and calls the corresponding fault handler with an opaque
>>>     object (registered by the handler) and raw fault_data (rid, pasid, addr);
>>>
>>> -   IOASID fault handler identifies the corresponding ioasid and device
>>>     cookie according to the opaque object, generates an user fault_data
>>>     (ioasid, cookie, addr) in the fault region, and triggers eventfd to
>>>     userspace;
>>>
>>
>> Hi, I have some doubts here:
>>
>> For mdev, it seems that the rid in the raw fault_data is the parent device's,
>> then in the vSVA scenario, how can we get to know the mdev(cookie) from
>> the
>> rid and pasid?
>>
>> And from this point of view，would it be better to register the mdev
>> (iommu_register_device()) with the parent device info?
>>
> 
> This is what is proposed in this RFC. A successful binding generates a new
> iommu_dev object for each vfio device. For mdev this object includes 
> its parent device, the defPASID marking this mdev, and the cookie 
> representing it in userspace. Later it is iommu_dev being recorded in
> the attaching_data when the mdev is attached to an IOASID:
> 
> 	struct iommu_attach_data *__iommu_device_attach(
> 		struct iommu_dev *dev, u32 ioasid, u32 pasid, int flags);
> 
> Then when a fault is reported, the fault handler just needs to figure out 
> iommu_dev according to {rid, pasid} in the raw fault data.
> 

Yeah, we have the defPASID that marks the mdev and refers to the default
I/O address space, but how about the non-default I/O address spaces?
Is there a case that two different mdevs (on the same parent device)
are used by the same process in the guest, thus have a same pasid route
in the physical IOMMU? It seems that we can't figure out the mdev from
the rid and pasid in this case...

Did I misunderstand something?... :-)

Thanks,
Shenming