Re: [PATCH v2 1/3] iommufd: Add devices_users to track the hw_pagetable usage by device

[Nicolin Chen <nicolinc@nvidia.com>]

On Mon, Jan 30, 2023 at 04:35:35PM -0400, Jason Gunthorpe wrote:
 
> IMHO I would structure the smmu driver so that all the different
> iommu_domain formats have their own ops pointer. The special
> "undecided" format would have a special ops with only attach_dev and
> at first attach it would switch the ops to whatever format it
> selected.
> 
> I think this could get rid of a lot of the 'if undecided/S1/S2/CD'
> complexity all over the place. You know what type it is because you
> were called on a op that is only called on its type.

An auto/unmanaged domain allocation via iommu_domain_alloc() would
be S1, while an allocation via ops->domain_alloc_user can be S1 or
S2 with a given parameter/flag. So, actually the format is always
decided. The trouble we have with the iommu_domain_alloc() path is
that we don't pass the dev pointer down to ops->domain_alloc. So,
the SMMU driver can't know which SMMU device the device is behind,
resulting in being unable to finalizing the domain. Robin mentioned
that he has a patch "iommu: Pass device through ops->domain_alloc".
Perhaps that is required for us to entirely fix the add_domain()
problem?

Thanks
Nic