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

From: Tiwei Bie <[email-masked]>
Date: Thu, 20 Feb 2020 10:42:21 +0800

On Wed, Feb 19, 2020 at 09:11:02AM -0400, Jason Gunthorpe wrote:
> 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.

I see. Thanks! I will embed the cdev in each vhost_vdpa
structure directly.

Regards,
Tiwei

> Jason