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

Farhan Ali
2018-11-28T15:00:59Z

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.

>
>