Re: [PATCH RFCv2] iommu: Add domain type and flag to domain_alloc_paging()

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

On 2024/8/7 1:32, Jason Gunthorpe wrote:
> On Tue, Aug 06, 2024 at 08:11:19PM +0530, Vasant Hegde wrote:
> 
>> I only have issue with allocating right page table for UNMANAGED domain. In this
>> case just 'dev' param is not sufficient. Ops should send some sort of indication
>> that its UNMANAGED domain (like its non DMA API domain). Otherwise AMD driver
>> cannot allocate right page table.
> Be specific, you mean when VFIO allocates it because VFIO is almost
> the exclusive user of UNMANAGED domains on x86 hardware.
> 
>> My understanding is we essentially have two modes for device passthrough.
>>    1 - Existing interface where vfio driver manages page table (call
>> iommu_paging_domain_alloc()). No PASID support in this mode.
>>    With this RFC I am trying to solve this scenario.
> Honestly I'd prefer you change VFIO type 1 to call domain_alloc_user()
> instead of hacking domain_alloc_paging().
> 
> This is a lot more explicit than trying to push new meaning onto
> UNMANAGED which is trying to get itself removed.
> 
>>    2 - via iommufd - It will use domain_alloc_user() ops. This is the interface
>> where we want to support PASID w/ passthrough device.
>>
>>     This is where you/Yi suggested to start discussing with Alex on Yi's PASID
>> support series. I am running behind. I will start looking into Yi's thread soon.
>>
>> Is my understanding correct?
> Broadly, but I'd rather VFIO exclusively use domain_alloc_user() and
> it can accomodate both cases. So add a flag to indicate if PASID
> should be enabled.
> 
> It makes sense to me that type 1 would just always disable PASID
> support, it should just be done explicitly without the word
> "UNMANAGED".

Convert paging domain to user domain allocation appears to be a
reasonable solution. But I think it's not VFIO-only, vhost-vdpa is
another case, right?

Thanks,
baolu