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

[Pierre Morel <pmorel@linux.ibm.com>]

On 2019-12-12 15:33, Cornelia Huck wrote:
> 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?

This is where the delay can help.
But yes, not sure that we can differentiate this without to know how 
long we should delay.


> 
>>>    
>>>>   
>>>>> 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?

Yes.

Regards,
Pierre

-- 
Pierre Morel
IBM Lab Boeblingen