Re: [PATCH 3/3] vfio-ccw: add handling for asnyc channel instructions

[Farhan Ali <alifm@linux.ibm.com>]

On 11/28/2018 09:52 AM, Cornelia Huck wrote:
> On Wed, 28 Nov 2018 09:31:51 -0500
> Farhan Ali <alifm@linux.ibm.com> wrote:
> 
>> On 11/28/2018 04:02 AM, Cornelia Huck wrote:
>>> On Tue, 27 Nov 2018 14:09:49 -0500
>>> Farhan Ali <alifm@linux.ibm.com> wrote:
>>>    
>>>> On 11/22/2018 11:54 AM, Cornelia Huck wrote:
>>>>> Add a region to the vfio-ccw device that can be used to submit
>>>>> asynchronous I/O instructions. ssch continues to be handled by the
>>>>> existing I/O region; the new region handles hsch and csch.
>>>>>
>>>>> Interrupt status continues to be reported through the same channels
>>>>> as for ssch.
>>>>>
>>>>> Signed-off-by: Cornelia Huck <cohuck@redhat.com>
>>>>> ---
>>>>>     drivers/s390/cio/Makefile           |   3 +-
>>>>>     drivers/s390/cio/vfio_ccw_async.c   |  88 ++++++++++++++++
>>>>>     drivers/s390/cio/vfio_ccw_drv.c     |  48 ++++++---
>>>>>     drivers/s390/cio/vfio_ccw_fsm.c     | 158 +++++++++++++++++++++++++++-
>>>>>     drivers/s390/cio/vfio_ccw_ops.c     |  13 ++-
>>>>>     drivers/s390/cio/vfio_ccw_private.h |   6 ++
>>>>>     include/uapi/linux/vfio.h           |   4 +
>>>>>     include/uapi/linux/vfio_ccw.h       |  12 +++
>>>>>     8 files changed, 313 insertions(+), 19 deletions(-)
>>>>>     create mode 100644 drivers/s390/cio/vfio_ccw_async.c
>>>>>   
>>>    
>>>>> @@ -76,7 +79,8 @@ static void vfio_ccw_sch_io_todo(struct work_struct *work)
>>>>>     	private = container_of(work, struct vfio_ccw_private, io_work);
>>>>>     	irb = &private->irb;
>>>>>     
>>>>> -	if (scsw_is_solicited(&irb->scsw)) {
>>>>> +	if (scsw_is_solicited(&irb->scsw) &&
>>>>> +	    (scsw_fctl(&irb->scsw) & SCSW_FCTL_START_FUNC)) {
>>>>>     		cp_update_scsw(&private->cp, &irb->scsw);
>>>>>     		cp_free(&private->cp);
>>>>>     	}
>>>>
>>>> I am a little confused about this. Why do we need to update the scsw.cpa
>>>> if we have the start function function control bit set? Is it an
>>>> optimization?
>>>
>>> No, it's not an optimization. This is the work function that is
>>> scheduled if we get an interrupt for the device. Previously, we only
>>> got an interrupt if either the device presented us an unsolicited
>>> status or if we got an interrupt as a response to the channel program
>>> we submitted. Now, we can get an interrupt for halt/clear subchannel as
>>> well, and in that case, we don't necessarily have a cp.
>>>
>>> [Thinking some more about it, we need to verify if the start function
>>> actually remains set if we try to terminate a running channel program
>>> with halt/clear. A clear might scrub too much. If that's the case, we
>>> also need to free the cp if the start function is not set.]
>>>
>>>    
>>
>> According to PoPs (Chapter 16: I/O interruptions, under function control):
>>
>> The start-function indication is also cleared at
>> the subchannel during the execution of CLEAR SUB-
>> CHANNEL.
>>
>> So maybe we do need to free the cp.
> 
> Hm... so we need to make sure that cp_update_scsw() and cp_free() only
> do something when there's actually a valid cp around and call them
> unconditionally. 

Yes, I agree.

Maybe add a ->valid flag to struct channel_program?

We could do that. So we would set the flag once we have copied the 
channel program to kernel memory? since that's when we should care about 
freeing it.

> 
>