Re: [kvm-unit-tests PATCH v4 7/9] s390x: css: msch, enable test

[Cornelia Huck <cohuck@redhat.com>]

On Thu, 12 Dec 2019 15:21:21 +0100
Pierre Morel <pmorel@linux.ibm.com> wrote:

> On 2019-12-12 15:10, Cornelia Huck wrote:
> > On Thu, 12 Dec 2019 15:01:07 +0100
> > Pierre Morel <pmorel@linux.ibm.com> wrote:
> >   
> >> On 2019-12-12 13:01, Cornelia Huck wrote:  
> >>> On Wed, 11 Dec 2019 16:46:08 +0100
> >>> Pierre Morel <pmorel@linux.ibm.com> wrote:
> >>>      
> >>>> A second step when testing the channel subsystem is to prepare a channel
> >>>> for use.
> >>>> This includes:
> >>>> - Get the current SubCHannel Information Block (SCHIB) using STSCH
> >>>> - Update it in memory to set the ENABLE bit
> >>>> - Tell the CSS that the SCHIB has been modified using MSCH
> >>>> - Get the SCHIB from the CSS again to verify that the subchannel is
> >>>>     enabled.
> >>>>
> >>>> This tests the success of the MSCH instruction by enabling a channel.
> >>>>
> >>>> Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
> >>>> ---
> >>>>    s390x/css.c | 65 +++++++++++++++++++++++++++++++++++++++++++++++++++++
> >>>>    1 file changed, 65 insertions(+)  
> >   
> >>>> +	/* Read the SCHIB for this subchannel */
> >>>> +	cc = stsch(test_device_sid, &schib);
> >>>> +	if (cc) {
> >>>> +		report(0, "stsch cc=%d", cc);
> >>>> +		return;
> >>>> +	}
> >>>> +
> >>>> +	/* Update the SCHIB to enable the channel */
> >>>> +	pmcw->flags |= PMCW_ENABLE;
> >>>> +
> >>>> +	/* Tell the CSS we want to modify the subchannel */
> >>>> +	cc = msch(test_device_sid, &schib);
> >>>> +	if (cc) {
> >>>> +		/*
> >>>> +		 * If the subchannel is status pending or
> >>>> +		 * if a function is in progress,
> >>>> +		 * we consider both cases as errors.
> >>>> +		 */
> >>>> +		report(0, "msch cc=%d", cc);
> >>>> +		return;
> >>>> +	}
> >>>> +
> >>>> +	/*
> >>>> +	 * Read the SCHIB again to verify the enablement
> >>>> +	 * insert a little delay and try 5 times.
> >>>> +	 */
> >>>> +	do {
> >>>> +		cc = stsch(test_device_sid, &schib);
> >>>> +		if (cc) {
> >>>> +			report(0, "stsch cc=%d", cc);
> >>>> +			return;
> >>>> +		}
> >>>> +		delay(10);  
> >>>
> >>> That's just a short delay to avoid a busy loop, right? msch should be
> >>> immediate,  
> >>
> >> Thought you told to me that it may not be immediate in zVM did I
> >> misunderstand?  
> > 
> > Maybe I have been confusing... what I'm referring to is this
> > programming note for msch:
> > 
> > "It is recommended that the program inspect the
> > contents of the subchannel by subsequently
> > issuing STORE SUBCHANNEL when MODIFY
> > SUBCHANNEL sets condition code 0. Use of
> > STORE SUBCHANNEL is a method for deter-
> > mining if the designated subchannel was
> > changed or not. Failure to inspect the subchan-
> > nel following the setting of condition code 0 by
> > MODIFY SUBCHANNEL may result in conditions
> > that the program does not expect to occur."
> > 
> > That's exactly what we had to do under z/VM back then: do the msch,
> > check via stsch, redo the msch if needed, check again via stsch. It
> > usually worked with the second msch the latest.  
> 
> OK, I understand, then it is a bug in zVM that this test could enlighten.

Probably more a quirk than a bug... the explanation there is not
explicit about that :)

> 
> I think we should keep it so, it allows to recognize 3 cases (after I 
> change to test ENABLE in the loop as I said I will):
> - immediate ENABLE

This is the good case.

> - asynchrone ENABLE

This one I would consider an architecture violation.

> - failure to ENABLE

This is the quirk above.

But I'm not quite sure how you would be able to distinguish the last
two cases?

> >   
> >>  
> >>> and you probably should not delay on success?  
> >>
> >> yes, it is not optimized, I can test PMCW_ENABLE in the loop this way we
> >> can see if, in the zVM case we need to do retries or not.
> >>
> >>  
> >>>      
> >>>> +	} while (!(pmcw->flags & PMCW_ENABLE) && count++ < 5);  
> >>>
> >>> How is this supposed to work? Doesn't the stsch overwrite the control
> >>> block again, so you need to re-set the enable bit before you retry?  
> >>
> >> I do not think so, there is no msch() in the loop.
> >> Do I miss something?  
> > 
> > Well, _I_ missed that the msch() was missing :) You need it (see above);
> > just waiting and re-doing the stsch is useless, as msch is a
> > synchronous instruction which has finished its processing after the cc
> > has been set.
> >   
> 
> Since kvm-unit-test is a test system, not an OS so I think that here we 
> have one more point to leverage the enable function:
> - We need to test the enable (what I did (partially))

Maybe also log if you needed to retry? Not as an error, but as
additional information?

> - We need the enable to work (your proposition) to further test the I/O
> 
> OK, I rework this part with your comment in mind.
> 
> Thanks
> Pierre
> 
>