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

[Cornelia Huck <cohuck@redhat.com>]

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?

>
>>> 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?

>
>>> 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().