linux-kernel - Re: [PATCH 04/10] vfio: ccw: replace IO_REQ event with SSCH

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Message-Id: <86a4be7f-6c4c-19db-f743-ed4af0173125@linux.ibm.com>
Date:   Wed, 23 May 2018 09:50:00 +0200
From:   Pierre Morel <pmorel@...ux.ibm.com>
To:     Cornelia Huck <cohuck@...hat.com>,
        Pierre Morel <pmorel@...ux.vnet.ibm.com>
Cc:     Dong Jia Shi <bjsdjshi@...ux.ibm.com>,
        Dong Jia Shi <bjsdjshi@...ux.vnet.ibm.com>,
        pasic@...ux.vnet.ibm.com, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH 04/10] vfio: ccw: replace IO_REQ event with SSCH_REQ event

On 22/05/2018 17:41, Cornelia Huck wrote:
> On Fri, 4 May 2018 13:02:36 +0200
> Pierre Morel <pmorel@...ux.vnet.ibm.com> wrote:
>
>> On 04/05/2018 03:19, Dong Jia Shi wrote:
>>> * Pierre Morel <pmorel@...ux.vnet.ibm.com> [2018-05-03 16:26:29 +0200]:
>>>   
>>>> On 02/05/2018 09:46, Dong Jia Shi wrote:
>>>>> * Cornelia Huck <cohuck@...hat.com> [2018-04-30 17:33:05 +0200]:
>>>>>   
>>>>>> On Thu, 26 Apr 2018 15:48:06 +0800
>>>>>> Dong Jia Shi <bjsdjshi@...ux.vnet.ibm.com> wrote:
>>>>>>   
>>>>>>> * Dong Jia Shi <bjsdjshi@...ux.vnet.ibm.com> [2018-04-26 15:30:54 +0800]:
>>>>>>>
>>>>>>> [...]
>>>>>>>   
>>>>>>>>> @@ -179,7 +160,7 @@ static int fsm_irq(struct vfio_ccw_private *private,
>>>>>>>>>    	if (private->io_trigger)
>>>>>>>>>    		eventfd_signal(private->io_trigger, 1);
>>>>>>>>>
>>>>>>>>> -	return private->state;
>>>>>>>>> +	return VFIO_CCW_STATE_IDLE;
>>>>>>>> This is not right. For example, if we are in STANDBY state (subch driver
>>>>>>>> is probed, but mdev device is not created), we can not jump to IDLE
>>>>>>>> state.
>>>>>>> I see my problem, for STANDBY state, we should introduce another event
>>>>>>> callback for VFIO_CCW_EVENT_INTERRUPT. It doesn't make sense to call
>>>>>>> fsm_irq() which tries to signal userspace with interrupt notification
>>>>>>> when mdev is not created yet... So we'd need a separated fix for this
>>>>>>> issue too.
>>>>>> But how do we even get into that situation when we don't have an mdev
>>>>>> yet?
>>>>>>   
>>>>> We cann't... So let's assign fsm_nop() as the interrupt callback for
>>>>> STANDBY state?
>>>>>   
>>>> :) Isn't it exactly what my patch series handle?
>>> As far as I see, that's not true. ;)
>>>
>>> After this series applied,
>>> vfio_ccw_jumptable[VFIO_CCW_STATE_STANDBY][VFIO_CCW_EVENT_INTERRUPT] is
>>> still fsm_irq().
>>>   
>>
>> What I mean is, this code tries to handle design problems
>> without changing too much of the original code at first.
>>
>> The problem here is not that the fsm_irq function is called on interrupt,
>> if we have an interrupt it must be signaled to user land.
>> The problem is that this state is entered at the wrong moment.
>>
>> STANDBY should be entered, during the mdev_open when we realize the QEMU
>> device,
>> and not during the probe, in which we should stay in NOT_OPER until we
>> get the QEMU device.
>>
>> The probe() and mdev_open() function should be modified, not the state
>> table.
> So, the takeaway is that we should handle starting via the init
> callbacks and not via the state machine?
>
hum, sorry, I think that my previous answer was not completely right,
and did not really answer to Dong Jia comment, yes fsm_irq was not
at its place, thinking again about the comments of both of you
I think that we can suppress the INIT event.

I would like to rebase the patch to include the comments you both did.


-- 
Pierre Morel
Linux/KVM/QEMU in Böblingen - Germany