vfio: Support IO page table replacement

Now both the physical path and the emulated path can support an IO page
table replacement. Call iommufd_device_replace/iommufd_access_replace(),
when vdev->iommufd_attached is true.

Also update the VFIO_DEVICE_ATTACH_IOMMUFD_PT kdoc in the uAPI header.

Link: https://lore.kernel.org/r/b5f01956ff161f76aa52c95b0fa1ad6eaca95c4a.1690523699.git.nicolinc@nvidia.com
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/vfio/iommufd.c

@@ -146,9 +146,9 @@ int vfio_iommufd_physical_attach_ioas(struct vfio_device *vdev, u32 *pt_id)
 		return -EINVAL;
 
 	if (vdev->iommufd_attached)
-		return -EBUSY;
-
-	rc = iommufd_device_attach(vdev->iommufd_device, pt_id);
+		rc = iommufd_device_replace(vdev->iommufd_device, pt_id);
+	else
+		rc = iommufd_device_attach(vdev->iommufd_device, pt_id);
 	if (rc)
 		return rc;
 	vdev->iommufd_attached = true;
@@ -223,8 +223,9 @@ int vfio_iommufd_emulated_attach_ioas(struct vfio_device *vdev, u32 *pt_id)
 	lockdep_assert_held(&vdev->dev_set->lock);
 
 	if (vdev->iommufd_attached)
-		return -EBUSY;
-	rc = iommufd_access_attach(vdev->iommufd_access, *pt_id);
+		rc = iommufd_access_replace(vdev->iommufd_access, *pt_id);
+	else
+		rc = iommufd_access_attach(vdev->iommufd_access, *pt_id);
 	if (rc)
 		return rc;
 	vdev->iommufd_attached = true;

include/uapi/linux/vfio.h

@@ -939,6 +939,12 @@ struct vfio_device_bind_iommufd {
  * Undo by VFIO_DEVICE_DETACH_IOMMUFD_PT or device fd close.  This is only
  * allowed on cdev fds.
  *
+ * If a vfio device is currently attached to a valid hw_pagetable, without doing
+ * a VFIO_DEVICE_DETACH_IOMMUFD_PT, a second VFIO_DEVICE_ATTACH_IOMMUFD_PT ioctl
+ * passing in another hw_pagetable (hwpt) id is allowed. This action, also known
+ * as a hw_pagetable replacement, will replace the device's currently attached
+ * hw_pagetable with a new hw_pagetable corresponding to the given pt_id.
+ *
  * Return: 0 on success, -errno on failure.
  */
 struct vfio_device_attach_iommufd_pt {