Re: [PATCH v2 21/22] iommu/tegra241-cmdqv: Add user-space use support

[Nicolin Chen <nicolinc@nvidia.com>]

On Wed, Apr 30, 2025 at 03:39:22PM -0700, Nicolin Chen wrote:
> On Wed, Apr 30, 2025 at 09:59:13PM +0000, Pranjal Shrivastava wrote:
> > >  enum iommu_viommu_type {
> > >  	IOMMU_VIOMMU_TYPE_DEFAULT = 0,
> > >  	IOMMU_VIOMMU_TYPE_ARM_SMMUV3 = 1,
> > > +	IOMMU_VIOMMU_TYPE_TEGRA241_CMDQV = 2,
> > > +};
> > 
> > This is a little confusing.. I understand that we need a new viommu type
> > to copy the new struct iommu_viommu_tegra241_cmdqv b/w the user & kernel
> > 
> > But, in a previous patch (Add vsmmu_alloc impl op), we add a check to
> > fallback to the standard type SMMUv3, if the impl_ops->vsmmu_alloc 
> > returns -EOPNOTSUPP:
> > 
> > 	if (master->smmu->impl_ops && master->smmu->impl_ops->vsmmu_alloc)
> > 		vsmmu = master->smmu->impl_ops->vsmmu_alloc(
> > 			master->smmu, s2_parent, ictx, viommu_type, user_data);
> > 	if (PTR_ERR(vsmmu) == -EOPNOTSUPP) {
> > 		if (viommu_type != IOMMU_VIOMMU_TYPE_ARM_SMMUV3)
> > 			return ERR_PTR(-EOPNOTSUPP);
> > 		/* Fallback to standard SMMUv3 type if viommu_type matches */
> > 		vsmmu = iommufd_viommu_alloc(ictx, struct arm_vsmmu, core,
> > 					     &arm_vsmmu_ops);
> > 
> > Now, if we'll ALWAYS try to allocate an impl-specified vsmmu first, even
> > when the viommu_type == IOMMU_VIOMMU_TYPE_ARM_SMMUV3, we are anyways
> > going to return back from the impl_ops->vsmmu_alloc with -EOPNOTSUPP.
> 
> That's not necessarily true. An impl_ops->vsmmu_alloc can support
> IOMMU_VIOMMU_TYPE_ARM_SMMUV3 potentially, e.g. an impl could just
> toggle a few special bits in a register and return a valid vsmmu
> pointer.
> 
> It doesn't work like this with VCMDQ as it supports its own type,
> but for the long run I think we should pass in the standard type
> to impl_ops->vsmmu_alloc too.
> 
> > Then we'll again check if the retval was -EOPNOTSUPP and re-check the
> > viommu_type requested.. which seems a little counter intuitive.
> 
> It's just prioritizing the impl_ops->vsmmu_alloc. Similar to the
> probe, if VCMDQ is missing or encountering some initialization
> problem, give it a chance to fallback to the standard SMMU.

I changed to this and it should be clear now:

+       /* Prioritize the impl that may support IOMMU_VIOMMU_TYPE_ARM_SMMUV3 */
+       if (master->smmu->impl_ops && master->smmu->impl_ops->vsmmu_alloc)
+               vsmmu = master->smmu->impl_ops->vsmmu_alloc(
+                       master->smmu, s2_parent, ictx, viommu_type, user_data);
+       if (PTR_ERR(vsmmu) == -EOPNOTSUPP) {
+               /* Otherwise, allocate an IOMMU_VIOMMU_TYPE_ARM_SMMUV3 here */
+               if (viommu_type == IOMMU_VIOMMU_TYPE_ARM_SMMUV3)
+                       vsmmu = iommufd_viommu_alloc(ictx, struct arm_vsmmu,
+                                                    core, &arm_vsmmu_ops);

Thanks
Nicolin