linux-kernel - Re: [RFC 03/20] vfio: Add vfio_[un]register

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Date:   Tue, 21 Sep 2021 21:53:37 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     "Liu, Yi L" <yi.l.liu@...el.com>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
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        "yi.l.liu@...ux.intel.com" <yi.l.liu@...ux.intel.com>,
        "Tian, Jun J" <jun.j.tian@...el.com>, "Wu, Hao" <hao.wu@...el.com>,
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Subject: Re: [RFC 03/20] vfio: Add vfio_[un]register_device()

On Tue, Sep 21, 2021 at 11:10:15PM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe <jgg@...dia.com>
> > Sent: Wednesday, September 22, 2021 12:01 AM
> > 
> > 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?
> 
> a vfio-pci device can be opened via the existing group interface. if no b) it 
> means legacy vfio userspace can never use vfio-pci device any more
> once the latter is moved to iommufd.

Sorry, I think I ment a, which I guess you will say is SW mdev devices

But even so, I think the way forward here is to still always expose
the device /dev/vfio/devices/X and some devices may not allow iommufd
usage initially.

Providing an ioctl to bind to a normal VFIO container or group might
allow a reasonable fallback in userspace..

Jason