iommu: Turn fault_data to iommufd private pointer

A "fault_data" was added exclusively for the iommufd_fault_iopf_handler()
used by IOPF/PRI use cases, along with the attach_handle. Now, the iommufd
version of the sw_msi function will reuse the attach_handle and fault_data
for a non-fault case.

Rename "fault_data" to "iommufd_hwpt" so as not to confine it to a "fault"
case. Move it into a union to be the iommufd private pointer. A following
patch will move the iova_cookie to the union for dma-iommu too after the
iommufd_sw_msi implementation is added.

Since we have two unions now, add some simple comments for readability.

Link: https://patch.msgid.link/r/ee5039503f28a16590916e9eef28b917e2d1607a.1740014950.git.nicolinc@nvidia.com
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/iommu/iommufd/fault.c

@@ -329,7 +329,7 @@ int iommufd_fault_iopf_handler(struct iopf_group *group)
 	struct iommufd_hw_pagetable *hwpt;
 	struct iommufd_fault *fault;
 
-	hwpt = group->attach_handle->domain->fault_data;
+	hwpt = group->attach_handle->domain->iommufd_hwpt;
 	fault = hwpt->fault;
 
 	spin_lock(&fault->lock);

drivers/iommu/iommufd/hw_pagetable.c

@@ -406,10 +406,10 @@ int iommufd_hwpt_alloc(struct iommufd_ucmd *ucmd)
 		}
 		hwpt->fault = fault;
 		hwpt->domain->iopf_handler = iommufd_fault_iopf_handler;
-		hwpt->domain->fault_data = hwpt;
 		refcount_inc(&fault->obj.users);
 		iommufd_put_object(ucmd->ictx, &fault->obj);
 	}
+	hwpt->domain->iommufd_hwpt = hwpt;
 
 	cmd->out_hwpt_id = hwpt->obj.id;
 	rc = iommufd_ucmd_respond(ucmd, sizeof(*cmd));

include/linux/iommu.h

@@ -224,8 +224,10 @@ struct iommu_domain {
 		      phys_addr_t msi_addr);
 #endif
 
-	void *fault_data;
-	union {
+	union { /* Pointer usable by owner of the domain */
+		struct iommufd_hw_pagetable *iommufd_hwpt; /* iommufd */
+	};
+	union { /* Fault handler */
 		struct {
 			iommu_fault_handler_t handler;
 			void *handler_token;