Re: [PATCH 7/7] s390/cio: Remove vfio-ccw checks of command codes

[Cornelia Huck <cohuck@redhat.com>]

On Wed, 8 May 2019 11:22:07 +0200
Pierre Morel <pmorel@linux.ibm.com> wrote:

> The TEST command is used to retrieve the status of the I/O-device 
> __path__ and do not go up to the device.
> I did not find clearly that it does not start a data transfer but I 
> really do not think it does.
> May be we should ask people from hardware.
> I only found that test I/O (a specific test command) do not initiate an 
> operation.

FWIW, I'm not sure about what we should do with the test command in any
case.

Currently, I see it defined as a proper command in the rather ancient
"Common I/O Device Commands" (I don't know of any newer public
version), which states that it retrieves the status on the parallel
interface _only_ (and generates a command reject on the serial
interface). IIRC, the parallel interface has been phased out quite some
time ago.

The current PoP, in contrast, defines this as an _invalid_ command
(generating a channel program check).

So, while the test command originally was designed to never initiate a
data transfer, we now have an invalid command in its place, and we
don't know if something else might change in the future (for transfer
mode, a test-like command is already defined in the PoP).

So, the safest course would probably be to handle the ->cda portion and
send the command down. We'll get a check condition on current hardware,
but it should be safe if something changes in the future.

Of course, asking some hardware folks is not a bad idea, either :)