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

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