Re: [PATCH v5 7/9] iommufd: Associate fault object with iommufd_hw_pgtable

[Baolu Lu <baolu.lu@linux.intel.com>]

On 5/27/24 9:33 AM, Tian, Kevin wrote:
>> From: Jason Gunthorpe <jgg@ziepe.ca>
>> Sent: Friday, May 24, 2024 10:25 PM
>>
>> On Mon, May 20, 2024 at 03:39:54AM +0000, Tian, Kevin wrote:
>>>> From: Baolu Lu <baolu.lu@linux.intel.com>
>>>> Sent: Monday, May 20, 2024 10:19 AM
>>>>
>>>> On 5/15/24 4:50 PM, Tian, Kevin wrote:
>>>>>> From: Lu Baolu <baolu.lu@linux.intel.com>
>>>>>> Sent: Tuesday, April 30, 2024 10:57 PM
>>>>>>
>>>>>> @@ -308,6 +314,19 @@ int iommufd_hwpt_alloc(struct
>> iommufd_ucmd
>>>>>> *ucmd)
>>>>>>    		goto out_put_pt;
>>>>>>    	}
>>>>>>
>>>>>> +	if (cmd->flags & IOMMU_HWPT_FAULT_ID_VALID) {
>>>>>> +		struct iommufd_fault *fault;
>>>>>> +
>>>>>> +		fault = iommufd_get_fault(ucmd, cmd->fault_id);
>>>>>> +		if (IS_ERR(fault)) {
>>>>>> +			rc = PTR_ERR(fault);
>>>>>> +			goto out_hwpt;
>>>>>> +		}
>>>>>> +		hwpt->fault = fault;
>>>>>> +		hwpt->domain->iopf_handler = iommufd_fault_iopf_handler;
>>>>>> +		hwpt->domain->fault_data = hwpt;
>>>>>> +	}
>>>>>
>>>>> this is nesting specific. why not moving it to the nested_alloc()?
>>>>
>>>> Nesting is currently a use case for userspace I/O page faults, but this
>>>> design should be general enough to support other scenarios as well.
>>>
>>> Do we allow user page table w/o nesting?
>>>
>>> What would be a scenario in which the user doesn't manage the
>>> page table but still want to handle the I/O page fault? The fault
>>> should always be delivered to the owner managing the page table...
>>
>> userspace always manages the page table, either it updates the IOPTE
>> directly in a nest or it calls iommufd map operations.
>>
>> Ideally the driver will allow PRI on normal cases, although it will
>> probably never be used.
>>
> 
> But now it's done in a half way.
> 
> valid_flags in normal cases doesn't accept a fault ID. but we then
> handle the fault ID flag generally above.
> 
> I'd like to see a consistent message throughout the path.

Okay, I see. I think valid_flags logic is doing the right thing. It
indicates that user space page fault on a paging hwpt is not supported
yet, but it leaves the room to grow it in the future.

I will post v6 of this series soon to address some obvious issues
identified during this v5 review cycle. Thanks to all review comments.

Best regards,
baolu