Re: mdevctl: A shoestring mediated device management and persistence utility

[Cornelia Huck <cohuck@redhat.com>]

On Tue, 11 Jun 2019 14:28:22 -0600
Alex Williamson <alex.williamson@redhat.com> wrote:

> On Tue, 11 Jun 2019 21:45:08 +0200
> Cornelia Huck <cohuck@redhat.com> wrote:
> 
> > On Fri, 7 Jun 2019 18:06:30 +0200
> > Halil Pasic <pasic@linux.ibm.com> wrote:

> > > I guess for vfio-ccw one needs to make sure that the ccw device is bound
> > > to the vfio-ccw driver first, and only after that can one use  
> > > create-mdev to create the mdev on top of the subchannel.
> > > 
> > > So to make this work persistently (survive a reboot) one would need to
> > > take care of the subchannel getting bound to the right vfio_ccw driver
> > > before mdevctl is called. Right?
> > > 
> > > BTW how does this concurrence situation between the drivers io_subchannel
> > > and vfio_ccw work? Especially if both are build in?    
> > 
> > If you have two drivers that match to the same device type, you'll
> > always have the issue that the driver that is first matched with the
> > device will bind to it and you have to do the unbind/rebind dance to
> > get it bound to the correct device driver. (I guess that this was the
> > basic motivation behind the ap bus default driver infrastructure,
> > right?) I think that in our case the io_subchannel driver will be
> > called first (alphabetical order and the fact that vfio-ccw will often
> > be a module). I'm not sure if it is within the scope of mdevctl to
> > ensure that the device is bound to the correct driver, or if it rather
> > should work with devices already bound to the correct driver only.
> > Maybe a separate udev-rules generator?  
> 
> Getting a device bound to a specific driver is exactly the domain of
> driverctl.  Implement the sysfs interfaces driverctl uses and see if it
> works.  Driverctl defaults to PCI and knows some extra things about
> PCI, but appears to be written to be generally bus agnostic.  Thanks,
> 
> Alex

Ok, looked at driverctl. Extending this one for non-PCI seems like a
reasonable path. However, we would also need to extend any non-PCI
device type we want to support with a driver_override attribute like
you did for PCI in 782a985d7af26db39e86070d28f987cad2 -- so this is
only for newer kernels. Adding that attribute for subchannels looks
feasible at a glance, but I have not tried to actually do it :)

Halil, do you think that would make sense?

[This might also help with the lcs vs. ctc confusion on a certain 3088
cu model if this is added for ccw devices as well; but I'm not sure if
these are still out in the wild at all. Probably not worth the effort
for that.]