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

[Jason Gunthorpe <jgg@ziepe.ca>]

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".

Jason