Re: [PATCH v6 09/14] iommu/vt-d: Add IOMMU_HWPT_ALLOC_PASID support

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

On 2024/12/25 12:30, Yi Liu wrote:
> On 2024/12/25 09:02, Baolu Lu wrote:
>> On 12/24/24 19:35, Yi Liu wrote:
>>>> Another related consideration is the support for page faults in nested
>>>> domains once PASID is available in user space. Would it be 
>>>> reasonable to
>>>> support page faults for nested domains?
>>>
>>> yeah, it's good to discuss it.
>>>
>>>> If so, perhaps it's time to open intel_iommu_domain_alloc_nested() to
>>>> support IOMMU_HWPT_FAULT_ID_VALID when PRI is enabled on the device?
>>>
>>> IMHO, the PRQ support for nested domains requires some more facility. 
>>> PRQ
>>> can happen at either stage-1 or stage-2, iommu driver may need to tell
>>> it and forward to the correct domain (nested domain or parent 
>>> domain). Or
>>> the stage-2 is always pinned just as the VFIO/iommufd does. Hence, 
>>> any PRQ
>>> happens under nested translation should be due to stage-1.
>>
>> The parent domain is currently always pinned and does not yet support
>> page faults. Therefore, when a page fault occurs within a nested domain,
>> it apparently should be routed to user space...
>>
>> If we decide to support page faults on the stage-2 domain in the future,
>> we will need to figure out the correct destination of each page fault
>> and route it accordingly, either to the parent domain or the user space
>> nested domain. Hardware assistance would be beneficial, otherwise the
>> software may need to traverse the parent domain, which is not
>> performance friendly.
> 
> this is my question. Is it still true the parent domain is always pinned
> after the below series? If yes, then it's fine to enable IOPF for nested
> domain.
> 
> https://lore.kernel.org/linux-iommu/20241015-jag-iopfv8-v4-0- 
> b696ca89ba29@kernel.org/

Then, perhaps we could enforce this in iommufd for a short-term purpose?

When allocating a hwpt in iommufd, we should enforce that the flags
IOMMU_HWPT_ALLOC_NEST_PARENT and IOMMU_HWPT_FAULT_ID_VALID cannot be set
at the same time.

---
baolu