Re: [PATCH v1] s390/virtio_ccw: Also suppress -EINVAL on device detach

[William Bezenah <wbezenah@linux.ibm.com>]

On 6/16/2026 10:53 AM, Peter Oberparleiter wrote:

> On 16.06.2026 11:16, Cornelia Huck wrote:
>> On Tue, Jun 16 2026, Peter Oberparleiter <oberpar@linux.ibm.com> wrote:
>>
>>> On 15.06.2026 23:42, Halil Pasic wrote:
>>>> On Mon, 15 Jun 2026 16:01:55 -0400
>>>> William Bezenah <wbezenah@linux.ibm.com> wrote:
>>>>
>>>>> On 6/15/2026 10:58 AM, Cornelia Huck wrote:
>>>>>> On Mon, Jun 15 2026, Halil Pasic <pasic@linux.ibm.com> wrote:
>>>>>>  
>>>>>>> On Fri, 12 Jun 2026 17:54:07 +0200
>>>>>>> William Bezenah <wbezenah@linux.ibm.com> wrote:
>>>>>>>  
>>>>>>>> Since commit 8c58a229688c ("s390/cio: Do not unregister the
>>>>>>>> subchannel based on DNV"), subchannel behavior following a device
>>>>>>>> detach has been updated and results in -EINVAL being propagated
>>>>>>>> rather than -ENODEV, originating from ccw_device_start_timeout_key()
>>>>>>>> in cio/device_ops. In the end, the virtio driver has no ability to
>>>>>>>> react to the difference between device and subchannel states here,
>>>>>>>> and during detach, both -ENODEV and -EINVAL indicate the device
>>>>>>>> cannot be used and should not be treated as errors requiring
>>>>>>>> attention. Update error handling in virtio_ccw_del_vq() and
>>>>>>>> virtio_ccw_drop_indicator() to suppress -EINVAL in addition to
>>>>>>>> -ENODEV.  
>>>>>>> Hi William!
>>>>>>>
>>>>>>> Are you saying that ccw_device_start() started returning -EINVAL
>>>>>>> since 8c58a229688c ("s390/cio: Do not unregister the subchannel based on
>>>>>>> DNV")? Or did I somehow read the paragraph wrong?
>>>>>>>
>>>>>>> The funcition ccw_device_start is documented to return:
>>>>>>>  * Returns:                                                                     
>>>>>>>  *  %0, if the operation was successful;                                        
>>>>>>>  *  -%EBUSY, if the device is busy, or status pending;                          
>>>>>>>  *  -%EACCES, if no path specified in @lpm is operational;                      
>>>>>>>  *  -%ENODEV, if the device is not operational. 
>>>>>>> and the commit message does not say a thing about introducing -EINVAL to
>>>>>>> the mix.  
>>>>>> The function may return -EINVAL for non-enabled subchannels
>>>>>> (i.e. pmcw.ena == 0), maybe we get an all-zeroes schib with dnv == 0?
>>>>>> I'd expect it not to be enabled in that case anyway.  
>>>>> Yep, that's at least how I've come to understand what changed. The
>>>>> function ccw_device_start_timeout_key() has always returned -EINVAL
>>>>> for non-enabled subchannels (pmcw.ena == 0), though it's not
>>>>> documented in the header.
>>>> Wasn't his -EINVAL actually introduced by commit:
>>>> 823d494ac111 ("[S390] pm: ccw bus power management callbacks")?
>>> In the context of virtio-ccw added in 2012, an EINVAL return code
>>> introduced in 2009 might be considered "always" :)
>> :)
>>
>> I'm wondering whether we should still expect to hit the "ssch with
>> ena==0" situation, given that pm support has been removed again in the
>> meanwhile. (Well, other than in situations like this, where it is a
>> follow-up to other problems.) IOW, can callers expect not to see
>> -EINVAL, unless they are doing something really stupid?
> As Halil also pointed out, this would be a programming error, either on
> the side of the driver that starts I/O without setting the device
> properly online, or in the common I/O layer (hopefully not, but you
> never know). Having a dedicated return code to identify this situation
> is definitely useful, and we'll also consider documenting it accordingly
> in the function comment.
>
>>>>> What changed with commit 8c58a229688c is that cio_update_schib() now
>>>>> updates the schib even when DNV=0, rather than returning early as it
>>>>> did previously. Somehow this update results in pmcw.ena == 0 in
>>>>> ccw_device_start_timeout_key(). Previously, it saw pmcw.ena == 1 and
>>>>> moved to the condition (cdev->private->state == DEV_STATE_NOT_OPER)
>>>>> where it returned -ENODEV.
>>>> Sounds fishy to me. As far as I understand the DNV takes precedence over
>>>> all other pieces of PMCW.
>>> And you're right about that! The Principles of Operation states (p. 15-4
>>> in SA22-7832-14 [1]) that the contents of all other fields in the PMCW
>>> are unpredictable when DNV is 0, therefore 8c58a229688c is in error.
>>>
>>> I'll work with Vineeth to determine how to fix this issue, potentially
>>> via manually clearing some relevant SCHIB fields instead of copying the
>>> unpredictable results of the STSCH instruction.
>> Can't you zero the whole SCHIB, or do you still need some of the
>> measurement block things for cleanup?
> I faintly remember that there WAS a reason to use the remainder of the
> SCHIB contents because of some unwanted effect that occurred if we
> didn't, but I don't recall the details. We'll need to dig up the
> associated bug report to understand it and determine if we can simply
> clear all of the SCHIB, or need to keep some of the information intact.
>
>>>>> So the commit didn't introduce -EINVAL as a new return value, rather,
>>>>> it changed the subchannel lifecycle such that existing paths now
>>>>> propagate -EINVAL rather than -ENODEV during the device detach
>>>>> scenario.
>>>> I'm not convinced returning -EINVAL in the given situation is the
>>>> right thing to do. Peter, would you mind to chime in?
>>> I tend to agree that an attempt to start I/O for a subchannel that has
>>> DNV 0 should result in ENODEV rather than EINVAL, though the latter is
>>> still valid when a driver tries to start I/O on a subchannel that is not
>>> enabled for I/O.
>>>
>>> We'll make sure to design the fix for 8c58a229688c in away that ENODEV
>>> will be returned when DNV is 0. Assuming that this is the only situation
>>> where virtio-ccw's ccw_io_helper() receives -EINVAL from
>>> ccw_device_start__timeout_key() during detach, the subject patch should
>>> no longer be necessary.
>> I agree, I'd not expect to get -EINVAL in ccw_io_helper().
> Yeah, this was definitely an unexpected side effect of the DNV commit.
>
FWIW, during my testing, that was indeed the only situation in which
I saw -EINVAL during the detach process. So assuming the right fix
would correctly return ENODEV, I agree this patch is likely
unnecessary.

Thanks all.