vfio-ccw: add force unlimited prefetch property

vfio-ccw: add force unlimited prefetch property

There is at least one guest (OS) such that although it does not rely on
the guarantees provided by ORB 1 word 9 bit (aka unlimited prefetch, aka
P bit) not being set, it fails to tell this to the machine.

Usually this ain't a big deal, as the original purpose of the P bit is to
allow for performance optimizations. vfio-ccw however can not provide the
guarantees required if the bit is not set.

It is not possible to implement support for the P bit not set without
transitioning to lower level protocols for vfio-ccw.  So let's give the
user the opportunity to force setting the P bit, if the user knows this
is safe.  For self modifying channel programs forcing the P bit is not
safe.  If the P bit is forced for a self modifying channel program things
are expected to break in strange ways.

Let's also avoid warning multiple about P bit not set in the ORB in case
P bit is not told to be forced, and designate the affected vfio-ccw
device.

Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
Suggested-by: Dong Jia Shi <bjsdjshi@linux.ibm.com>
Acked-by: Jason J. Herne <jjherne@linux.ibm.com>
Tested-by: Jason J. Herne <jjherne@linux.ibm.com>
Message-Id: <20180524175828.3143-2-pasic@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>