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:
> On Mon, Jan 30, 2023 at 12:04:33PM -0800, Nicolin Chen wrote:
> 
> > I recall we've discussed this that SMMU sets up domain when it
> > attaches the device to, so we made a compromise here...
> 
> The ARM driver has a problem that it doesn't know what SMMU instance
> will host the domain when it is allocated so it doesn't know if it
> should select a S1 or S2 page table format - which is determined by
> the capabilities of the specific SMMU HW block.
> 
> However, we don't have this problem when creating the S2. The S2
> should be created by a special alloc_domain_iommufd() asking for the
> S2 format. Not only does the new alloc_domain_iommufd API directly
> request a S2 format table, but it also specifies the struct device so
> any residual details can be determined directly.
> 
> Thus there is no need to do the two stage process when working with
> the S2.

Ah, right! Taking a quick look, we should be able to call that
arm_smmu_domain_finalise when handling alloc_domain_iommufd().

> So fixup the driver to create fully configured iommu_domain's
> immediately and get rid of this problem.

OK. I will draft a patch today.

> 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.

I see. I can try that as well. Hopefully it won't touch too
many places that cam raise a potential big concern/objection..

Thanks
Nicolin