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

[Alex Williamson <alex.williamson@redhat.com>]

On Wed, 22 Sep 2021 22:34:42 +0000
"Tian, Kevin" <kevin.tian@intel.com> wrote:

> > From: Alex Williamson <alex.williamson@redhat.com>
> > Sent: Thursday, September 23, 2021 4:11 AM
> > 
> > On Wed, 22 Sep 2021 09:22:52 -0300
> > Jason Gunthorpe <jgg@nvidia.com> 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).

That's a rather important difference.  I don't really see how that's
comparable to the mutually exclusive nature of the legacy vs device
interface.  We're not going to present a vfio device interface for SW
mdevs that can't participate in iommufd, right?  Thanks,

Alex