iommufd: Disallow allocating nested parent domain with fault ID

Allocating a domain with a fault ID indicates that the domain is faultable.
However, there is a gap for the nested parent domain to support PRI. Some
hardware lacks the capability to distinguish whether PRI occurs at stage 1
or stage 2. This limitation may require software-based page table walking
to resolve. Since no in-tree IOMMU driver currently supports this
functionality, it is disallowed. For more details, refer to the related
discussion at [1].

[1] https://lore.kernel.org/linux-iommu/bd1655c6-8b2f-4cfa-adb1-badc00d01811@intel.com/

Link: https://patch.msgid.link/r/20250226104012.82079-1-yi.l.liu@intel.com
Suggested-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Yi Liu <yi.l.liu@intel.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/iommu/iommufd/hw_pagetable.c

@@ -126,6 +126,9 @@ iommufd_hwpt_paging_alloc(struct iommufd_ctx *ictx, struct iommufd_ioas *ioas,
 	if ((flags & IOMMU_HWPT_ALLOC_DIRTY_TRACKING) &&
 	    !device_iommu_capable(idev->dev, IOMMU_CAP_DIRTY_TRACKING))
 		return ERR_PTR(-EOPNOTSUPP);
+	if ((flags & IOMMU_HWPT_FAULT_ID_VALID) &&
+	    (flags & IOMMU_HWPT_ALLOC_NEST_PARENT))
+		return ERR_PTR(-EOPNOTSUPP);
 
 	hwpt_paging = __iommufd_object_alloc(
 		ictx, hwpt_paging, IOMMUFD_OBJ_HWPT_PAGING, common.obj);

tools/testing/selftests/iommu/iommufd.c

@@ -439,6 +439,10 @@ TEST_F(iommufd_ioas, alloc_hwpt_nested)
 				    &test_hwpt_id);
 		test_err_hwpt_alloc(EINVAL, self->device_id, self->device_id, 0,
 				    &test_hwpt_id);
+		test_err_hwpt_alloc(EOPNOTSUPP, self->device_id, self->ioas_id,
+				    IOMMU_HWPT_ALLOC_NEST_PARENT |
+						IOMMU_HWPT_FAULT_ID_VALID,
+				    &test_hwpt_id);
 
 		test_cmd_hwpt_alloc(self->device_id, self->ioas_id,
 				    IOMMU_HWPT_ALLOC_NEST_PARENT,