s390/vfio-ap: prepare for dynamic update of guest's APCB on queue probe/remove

s390/vfio-ap: prepare for dynamic update of guest's APCB on queue probe/remove

The callback functions for probing and removing a queue device must take
and release the locks required to perform a dynamic update of a guest's
APCB in the proper order.

The proper order for taking the locks is:

        matrix_dev->guests_lock => kvm->lock => matrix_dev->mdevs_lock

The proper order for releasing the locks is:

        matrix_dev->mdevs_lock => kvm->lock => matrix_dev->guests_lock

A new helper function is introduced to be used by the probe callback to
acquire the required locks. Since the probe callback only has
access to a queue device when it is called, the helper function will find
the ap_matrix_mdev object to which the queue device's APQN is assigned and
return it so the KVM guest to which the mdev is attached can be dynamically
updated.

Note that in order to find the ap_matrix_mdev (matrix_mdev) object, it is
necessary to search the matrix_dev->mdev_list. This presents a
locking order dilemma because the matrix_dev->mdevs_lock can't be taken to
protect against changes to the list while searching for the matrix_mdev to
which a queue device's APQN is assigned. This is due to the fact that the
proper locking order requires that the matrix_dev->mdevs_lock be taken
after both the matrix_mdev->kvm->lock and the matrix_dev->mdevs_lock.
Consequently, the matrix_dev->guests_lock will be used to protect against
removal of a matrix_mdev object from the list while a queue device is
being probed. This necessitates changes to the mdev probe/remove
callback functions to take the matrix_dev->guests_lock prior to removing
a matrix_mdev object from the list.

A new macro is also introduced to acquire the locks required to dynamically
update the guest's APCB in the proper order when a queue device is
removed.

Signed-off-by: Tony Krowiak <akrowiak@linux.ibm.com>
Reviewed-by: Jason J. Herne <jjherne@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>