Re: [PATCH v2 07/13] vfio/ccw: Convert to use vfio_register_group_dev()

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, Apr 29, 2021 at 01:58:55PM +0200, Cornelia Huck wrote:

> > This seems like one of these cases where using the mdev GUID API
> > was not a great fit. The ccs_driver should have just directly
> > created a vfio_device and not gone into the mdev guid lifecycle
> > world.
> 
> I don't remember much of the discussion back then, but I don't think
> the explicit generation of devices was the part we needed, but rather
> some other kind of mediation -- probably iommu related, as subchannels
> don't have that concept on their own. Anyway, too late to change now.

The mdev part does three significant things:
 - Provide a lifecycle model based on sysfs and the GUIDs
 - Hackily inject itself into the VFIO IOMMU code as a special case
 - Force the creation of a unique iommu group as the group FD is
   mandatory to get the device FD.

This is why PASID is such a mess for mdev because it requires even
more special hacky stuff to link up the dummy IOMMU but still operate
within the iommu group of the parent device.

I can see an alternative arrangement using the /dev/ioasid idea that
is a lot less hacky and does not force the mdev guid lifecycle on
everyone that wants to create vfio_device.

> > I almost did this, but couldn't figure out how the lifetime of the
> > ccs_driver callbacks are working relative to the lifetime of the mdev
> > device since they also reach into these structs. Maybe they can't be
> > called for some css related reason?
> 
> Moving allocations to the mdev driver probe makes sense, I guess. We
> should also move enabling the subchannel to that point in time (I don't
> remember why we enable it in the css probe function, and can't think of
> a good reason for that; obviously needs to be paired with quiescing and
> disabling the subchannel in the mdev driver remove function); that
> leaves the uevent dance (which can hopefully also be removed, if some
> discussed changes are implemented in the common I/O layer) and fencing
> QDIO.
> 
> Regarding the other callbacks,
> - vfio_ccw_sch_irq should not be invoked if the subchannel is not
>   enabled; maybe log a message before returning for !private.
> - vfio_ccw_sch_remove should be able to return 0 for !private (nothing
>   to quiesce, if the subchannel is not enabled).
> - vfio_ccw_sch_shutdown has nothing to do for !private (same reason.)
> - In vfio_ccw_sch_event, we should either skip the fsm_event and the
>   state change for !private, or return 0 in that case.
> - vfio_ccw_chp_event already checks for !private. Not sure whether we
>   should try to update some control blocks and return -ENODEV if the
>   subchannel is not operational, but it's probably not needed.

All the checks for !private need some kind of locking. The driver core
model is that the 'struct device_driver' callbacks are all called
under the device_lock (this prevents the driver unbinding during the
callback). I didn't check if ccs does this or not..

So if we NULL drvdata under the device_lock everything can be
quite simple here.

Jason