linux-kernel - Re: [RFC 02/20] vfio: Add device class for /dev/vfio/devices

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Date:   Wed, 29 Sep 2021 13:05:21 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     David Gibson <david@...son.dropbear.id.au>
Cc:     Liu Yi L <yi.l.liu@...el.com>, jgg@...dia.com, hch@....de,
        jasowang@...hat.com, joro@...tes.org, jean-philippe@...aro.org,
        kevin.tian@...el.com, parav@...lanox.com, lkml@...ux.net,
        pbonzini@...hat.com, lushenming@...wei.com, eric.auger@...hat.com,
        corbet@....net, ashok.raj@...el.com, yi.l.liu@...ux.intel.com,
        jun.j.tian@...el.com, hao.wu@...el.com, dave.jiang@...el.com,
        jacob.jun.pan@...ux.intel.com, kwankhede@...dia.com,
        robin.murphy@....com, kvm@...r.kernel.org,
        iommu@...ts.linux-foundation.org, dwmw2@...radead.org,
        linux-kernel@...r.kernel.org, baolu.lu@...ux.intel.com,
        nicolinc@...dia.com
Subject: Re: [RFC 02/20] vfio: Add device class for /dev/vfio/devices

On Wed, 29 Sep 2021 12:08:59 +1000
David Gibson <david@...son.dropbear.id.au> wrote:

> On Sun, Sep 19, 2021 at 02:38:30PM +0800, Liu Yi L wrote:
> > This patch introduces a new interface (/dev/vfio/devices/$DEVICE) for
> > userspace to directly open a vfio device w/o relying on container/group
> > (/dev/vfio/$GROUP). Anything related to group is now hidden behind
> > iommufd (more specifically in iommu core by this RFC) in a device-centric
> > manner.
> > 
> > In case a device is exposed in both legacy and new interfaces (see next
> > patch for how to decide it), this patch also ensures that when the device
> > is already opened via one interface then the other one must be blocked.
> > 
> > Signed-off-by: Liu Yi L <yi.l.liu@...el.com>  
> [snip]
> 
> > +static bool vfio_device_in_container(struct vfio_device *device)
> > +{
> > +	return !!(device->group && device->group->container);  
> 
> You don't need !! here.  && is already a logical operation, so returns
> a valid bool.
> 
> > +}
> > +
> >  static int vfio_device_fops_release(struct inode *inode, struct file *filep)
> >  {
> >  	struct vfio_device *device = filep->private_data;
> > @@ -1560,7 +1691,16 @@ static int vfio_device_fops_release(struct inode *inode, struct file *filep)
> >  
> >  	module_put(device->dev->driver->owner);
> >  
> > -	vfio_group_try_dissolve_container(device->group);
> > +	if (vfio_device_in_container(device)) {
> > +		vfio_group_try_dissolve_container(device->group);
> > +	} else {
> > +		atomic_dec(&device->opened);
> > +		if (device->group) {
> > +			mutex_lock(&device->group->opened_lock);
> > +			device->group->opened--;
> > +			mutex_unlock(&device->group->opened_lock);
> > +		}
> > +	}
> >  
> >  	vfio_device_put(device);
> >  
> > @@ -1613,6 +1753,7 @@ static int vfio_device_fops_mmap(struct file *filep, struct vm_area_struct *vma)
> >  
> >  static const struct file_operations vfio_device_fops = {
> >  	.owner		= THIS_MODULE,
> > +	.open		= vfio_device_fops_open,
> >  	.release	= vfio_device_fops_release,
> >  	.read		= vfio_device_fops_read,
> >  	.write		= vfio_device_fops_write,
> > @@ -2295,6 +2436,52 @@ static struct miscdevice vfio_dev = {
> >  	.mode = S_IRUGO | S_IWUGO,
> >  };
> >  
> > +static char *vfio_device_devnode(struct device *dev, umode_t *mode)
> > +{
> > +	return kasprintf(GFP_KERNEL, "vfio/devices/%s", dev_name(dev));  
> 
> Others have pointed out some problems with the use of dev_name()
> here.  I'll add that I think you'll make things much easier if instead
> of using one huge "devices" subdir, you use a separate subdir for each
> vfio sub-driver (so, one for PCI, one for each type of mdev, one for
> platform, etc.).  That should make avoiding name conflicts a lot simpler.

It seems like this is unnecessary if we use the vfioX naming approach.
Conflicts are trivial to ignore if we don't involve dev_name() and
looking for the correct major:minor chardev in the correct subdirectory
seems like a hassle for userspace.  Thanks,

Alex