Re: [RFC 03/20] vfio: Add vfio_[un]register_device()

[Jason Gunthorpe]

On Sun, Sep 19, 2021 at 02:38:31PM +0800, Liu Yi L wrote:
> With /dev/vfio/devices introduced, now a vfio device driver has three
> options to expose its device to userspace:
> 
> a)  only legacy group interface, for devices which haven't been moved to
>     iommufd (e.g. platform devices, sw mdev, etc.);
> 
> b)  both legacy group interface and new device-centric interface, for
>     devices which supports iommufd but also wants to keep backward
>     compatibility (e.g. pci devices in this RFC);
> 
> c)  only new device-centric interface, for new devices which don't carry
>     backward compatibility burden (e.g. hw mdev/subdev with pasid);

We shouldn't have 'b'? Where does it come from?

> This patch introduces vfio_[un]register_device() helpers for the device
> drivers to specify the device exposure policy to vfio core. Hence the
> existing vfio_[un]register_group_dev() become the wrapper of the new
> helper functions. The new device-centric interface is described as
> 'nongroup' to differentiate from existing 'group' stuff.

Detect what the driver supports based on the ops it declares. There
should be a function provided through the ops for the driver to bind
to the iommufd.

>  One open about how to organize the device nodes under /dev/vfio/devices/.
> This RFC adopts a simple policy by keeping a flat layout with mixed devname
> from all kinds of devices. The prerequisite of this model is that devnames
> from different bus types are unique formats:

This isn't reliable, the devname should just be vfio0, vfio1, etc

The userspace can learn the correct major/minor by inspecting the
sysfs.

This whole concept should disappear into the prior patch that adds the
struct device in the first place, and I think most of the code here
can be deleted once the struct device is used properly.

Jason