Subject: Re: [PATCH] vhost: introduce vDPA based backend

From: Jason Gunthorpe <[email-masked]>
Date: Wed, 19 Feb 2020 09:11:02 -0400

On Wed, Feb 19, 2020 at 10:52:38AM +0800, Tiwei Bie wrote:
> > > +static int __init vhost_vdpa_init(void)
> > > +{
> > > +	int r;
> > > +
> > > +	idr_init(&vhost_vdpa.idr);
> > > +	mutex_init(&vhost_vdpa.mutex);
> > > +	init_waitqueue_head(&vhost_vdpa.release_q);
> > > +
> > > +	/* /dev/vhost-vdpa/$vdpa_device_index */
> > > +	vhost_vdpa.class = class_create(THIS_MODULE, "vhost-vdpa");
> > > +	if (IS_ERR(vhost_vdpa.class)) {
> > > +		r = PTR_ERR(vhost_vdpa.class);
> > > +		goto err_class;
> > > +	}
> > > +
> > > +	vhost_vdpa.class->devnode = vhost_vdpa_devnode;
> > > +
> > > +	r = alloc_chrdev_region(&vhost_vdpa.devt, 0, MINORMASK + 1,
> > > +				"vhost-vdpa");
> > > +	if (r)
> > > +		goto err_alloc_chrdev;
> > > +
> > > +	cdev_init(&vhost_vdpa.cdev, &vhost_vdpa_fops);
> > > +	r = cdev_add(&vhost_vdpa.cdev, vhost_vdpa.devt, MINORMASK + 1);
> > > +	if (r)
> > > +		goto err_cdev_add;
> > 
> > It is very strange, is the intention to create a single global char
> > dev?
> 
> No. It's to create a per-vdpa char dev named
> vhost-vdpa/$vdpa_device_index in dev.
> 
> I followed the code in VFIO which creates char dev
> vfio/$GROUP dynamically, e.g.:
> 
> https://github.com/torvalds/linux/blob/b1da3acc781c/drivers/vfio/vfio.c#L2164-L2180
> https://github.com/torvalds/linux/blob/b1da3acc781c/drivers/vfio/vfio.c#L373-L387
> https://github.com/torvalds/linux/blob/b1da3acc781c/drivers/vfio/vfio.c#L1553
> 
> Is it something unwanted?

Yes it is unwanted. This is some special pattern for vfio's unique
needs. 

Since this has a struct device for each char dev instance please use
the normal cdev_device_add() driven pattern here, or justify why it
needs to be special like this.

Jason