Re: [PATCH] vfio: replace vfio->device_class with a const struct class

[Alex Williamson <alex@shazbot.org>]

On Fri,  6 Mar 2026 21:40:31 +0100
Jori Koolstra <jkoolstra@xs4all.nl> wrote:

> The class_create() call has been deprecated in favor of class_register()
> as the driver core now allows for a struct class to be in read-only
> memory. Replace vfio->device_class with a const struct class and drop
> the class_create() call.
> 
> Compile tested with both CONFIG_VFIO_DEVICE_CDEV on and off (and
> CONFIG_VFIO on); found no errors/warns in dmesg.
> 
> Link: https://lore.kernel.org/all/2023040244-duffel-pushpin-f738@gregkh/
> 
> Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Signed-off-by: Jori Koolstra <jkoolstra@xs4all.nl>
> ---
>  drivers/vfio/device_cdev.c |  8 +-------
>  drivers/vfio/vfio.h        |  4 ++--
>  drivers/vfio/vfio_main.c   | 27 ++++++++++++++++-----------
>  3 files changed, 19 insertions(+), 20 deletions(-)
> 
> diff --git a/drivers/vfio/device_cdev.c b/drivers/vfio/device_cdev.c
> index 8ceca24ac136..f926685a371d 100644
> --- a/drivers/vfio/device_cdev.c
> +++ b/drivers/vfio/device_cdev.c
> @@ -293,14 +293,8 @@ int vfio_df_ioctl_detach_pt(struct vfio_device_file *df,
>  	return 0;
>  }
>  
> -static char *vfio_device_devnode(const struct device *dev, umode_t *mode)
> +int vfio_cdev_init(const struct class *device_class)
>  {
> -	return kasprintf(GFP_KERNEL, "vfio/devices/%s", dev_name(dev));
> -}
> -
> -int vfio_cdev_init(struct class *device_class)
> -{
> -	device_class->devnode = vfio_device_devnode;

device_class is no longer used in the function, we should drop the
parameter rather than change it to a const.  Thanks,

Alex

>  	return alloc_chrdev_region(&device_devt, 0,
>  				   MINORMASK + 1, "vfio-dev");
>  }
> diff --git a/drivers/vfio/vfio.h b/drivers/vfio/vfio.h
> index 50128da18bca..fdc918771b05 100644
> --- a/drivers/vfio/vfio.h
> +++ b/drivers/vfio/vfio.h
> @@ -378,7 +378,7 @@ int vfio_device_fops_cdev_open(struct inode *inode, struct file *filep);
>  long vfio_df_ioctl_bind_iommufd(struct vfio_device_file *df,
>  				struct vfio_device_bind_iommufd __user *arg);
>  void vfio_df_unbind_iommufd(struct vfio_device_file *df);
> -int vfio_cdev_init(struct class *device_class);
> +int vfio_cdev_init(const struct class *device_class);
>  void vfio_cdev_cleanup(void);
>  #else
>  static inline void vfio_init_device_cdev(struct vfio_device *device)
> @@ -411,7 +411,7 @@ static inline void vfio_df_unbind_iommufd(struct vfio_device_file *df)
>  {
>  }
>  
> -static inline int vfio_cdev_init(struct class *device_class)
> +static inline int vfio_cdev_init(const struct class *device_class)
>  {
>  	return 0;
>  }
> diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
> index 742477546b15..eecf8a1a5237 100644
> --- a/drivers/vfio/vfio_main.c
> +++ b/drivers/vfio/vfio_main.c
> @@ -49,7 +49,6 @@
>  #define VFIO_MAGIC 0x5646494f /* "VFIO" */
>  
>  static struct vfio {
> -	struct class			*device_class;
>  	struct ida			device_ida;
>  	struct vfsmount			*vfs_mount;
>  	int				fs_count;
> @@ -64,6 +63,16 @@ MODULE_PARM_DESC(enable_unsafe_noiommu_mode, "Enable UNSAFE, no-IOMMU mode.  Thi
>  
>  static DEFINE_XARRAY(vfio_device_set_xa);
>  
> +static char *vfio_device_devnode(const struct device *dev, umode_t *mode)
> +{
> +	return kasprintf(GFP_KERNEL, "vfio/devices/%s", dev_name(dev));
> +}
> +
> +static const struct class vfio_device_class = {
> +	.name		= "vfio-dev",
> +	.devnode	= vfio_device_devnode
> +};
> +
>  int vfio_assign_device_set(struct vfio_device *device, void *set_id)
>  {
>  	unsigned long idx = (unsigned long)set_id;
> @@ -299,7 +308,7 @@ static int vfio_init_device(struct vfio_device *device, struct device *dev,
>  
>  	device_initialize(&device->device);
>  	device->device.release = vfio_device_release;
> -	device->device.class = vfio.device_class;
> +	device->device.class = &vfio_device_class;
>  	device->device.parent = device->dev;
>  	return 0;
>  
> @@ -1783,13 +1792,11 @@ static int __init vfio_init(void)
>  		goto err_virqfd;
>  
>  	/* /sys/class/vfio-dev/vfioX */
> -	vfio.device_class = class_create("vfio-dev");
> -	if (IS_ERR(vfio.device_class)) {
> -		ret = PTR_ERR(vfio.device_class);
> +	ret = class_register(&vfio_device_class);
> +	if (ret)
>  		goto err_dev_class;
> -	}
>  
> -	ret = vfio_cdev_init(vfio.device_class);
> +	ret = vfio_cdev_init(&vfio_device_class);
>  	if (ret)
>  		goto err_alloc_dev_chrdev;
>  
> @@ -1798,8 +1805,7 @@ static int __init vfio_init(void)
>  	return 0;
>  
>  err_alloc_dev_chrdev:
> -	class_destroy(vfio.device_class);
> -	vfio.device_class = NULL;
> +	class_unregister(&vfio_device_class);
>  err_dev_class:
>  	vfio_virqfd_exit();
>  err_virqfd:
> @@ -1812,8 +1818,7 @@ static void __exit vfio_cleanup(void)
>  	vfio_debugfs_remove_root();
>  	ida_destroy(&vfio.device_ida);
>  	vfio_cdev_cleanup();
> -	class_destroy(vfio.device_class);
> -	vfio.device_class = NULL;
> +	class_unregister(&vfio_device_class);
>  	vfio_virqfd_exit();
>  	vfio_group_cleanup();
>  	xa_destroy(&vfio_device_set_xa);
> 
> base-commit: d466c332e106fe666d1e2f5a24d08e308bebbfa1