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

[Tian, Kevin]

> From: Alex Williamson <alex.williamson@xxxxxxxxxx>
> Sent: Thursday, September 23, 2021 4:11 AM
> 
> On Wed, 22 Sep 2021 09:22:52 -0300
> Jason Gunthorpe <jgg@xxxxxxxxxx> wrote:
> 
> > On Wed, Sep 22, 2021 at 09:23:34AM +0000, Tian, Kevin wrote:
> >
> > > > Providing an ioctl to bind to a normal VFIO container or group might
> > > > allow a reasonable fallback in userspace..
> > >
> > > I didn't get this point though. An error in binding already allows the
> > > user to fall back to the group path. Why do we need introduce another
> > > ioctl to explicitly bind to container via the nongroup interface?
> >
> > New userspace still needs a fallback path if it hits the 'try and
> > fail'. Keeping the device FD open and just using a different ioctl to
> > bind to a container/group FD, which new userspace can then obtain as a
> > fallback, might be OK.
> >
> > Hard to see without going through the qemu parts, so maybe just keep
> > it in mind
> 
> If we assume that the container/group/device interface is essentially
> deprecated once we have iommufd, it doesn't make a lot of sense to me
> to tack on a container/device interface just so userspace can avoid
> reverting to the fully legacy interface.
> 
> But why would we create vfio device interface files at all if they
> can't work?  I'm not really on board with creating a try-and-fail
> interface for a mechanism that cannot work for a given device.  The
> existence of the device interface should indicate that it's supported.
> Thanks,
> 

Now it's a try-and-fail model even for devices which support iommufd.
Per Jason's suggestion, a device is always opened with a parked fops
which supports only bind. Binding serves as the contract for handling
exclusive ownership on a device and switching to normal fops if
succeed. So the user has to try-and-fail in case multiple threads attempt 
to open a same device. Device which doesn't support iommufd is not
different, except binding request 100% fails (due to missing .bind_iommufd
in kernel driver).

Thanks
Kevin