linux-kernel - Re: [PATCH 01/10] vfio: ccw: Moving state change out of IRQ context

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Message-Id: <245ee547-75bc-95f5-5acb-bfa96f91afd2@linux.ibm.com>
Date:   Tue, 24 Apr 2018 18:42:38 +0200
From:   Halil Pasic <pasic@...ux.ibm.com>
To:     Cornelia Huck <cohuck@...hat.com>,
        Pierre Morel <pmorel@...ux.vnet.ibm.com>
Cc:     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 01/10] vfio: ccw: Moving state change out of IRQ context



On 04/24/2018 11:59 AM, Cornelia Huck wrote:
> On Tue, 24 Apr 2018 10:40:56 +0200
> Pierre Morel <pmorel@...ux.vnet.ibm.com> wrote:
> 
>> On 24/04/2018 08:54, Dong Jia Shi wrote:
>>> * Pierre Morel <pmorel@...ux.vnet.ibm.com> [2018-04-19 16:48:04 +0200]:
>>>
>>> [...]
>>>   
>>>> @@ -94,9 +83,15 @@ static void vfio_ccw_sch_io_todo(struct work_struct *work)
>>>>    static void vfio_ccw_sch_irq(struct subchannel *sch)
>>>>    {
>>>>    	struct vfio_ccw_private *private = dev_get_drvdata(&sch->dev);
>>>> +	struct irb *irb = this_cpu_ptr(&cio_irb);
>>>>
>>>>    	inc_irq_stat(IRQIO_CIO);
>>>> -	vfio_ccw_fsm_event(private, VFIO_CCW_EVENT_INTERRUPT);
>>>> +	memcpy(&private->irb, irb, sizeof(*irb));
>>>> +
>>>> +	WARN_ON(work_pending(&private->io_work));
>>> Hmm, why do we need this?
>>
>> The current design insure that we have not two concurrent SSCH requests.
>> How ever I want here to track spurious interrupt.
>> If we implement cancel, halt or clear requests, we also may trigger (AFAIU)
>> a second interrupts depending on races between instructions, controller
>> and device.
> 
> You won't get an interrupt for a successful cancel. If you do a
> halt/clear, you will make the subchannel halt/clear pending in addition
> to start pending and you'll only get one interrupt (if the I/O has
> progressed far enough, you won't be able to issue a hsch). The
> interesting case is:
> - guest does a ssch, we do a ssch on the device
> - the guest does a csch before it got the interrupt for the ssch
> - before we do the csch on the device, the subchannel is already status
>    pending with completion of the ssch
> - after we issue the csch, we get a second interrupt (for the csch)
> 
> I think we should present two interrupts to the guest in that case.
> Races between issuing ssch/hsch/csch and the subchannel becoming status
> pending happen on real hardware as well, we're just more likely to see
> them with the vfio layer in between.
> 

AFAIU this will be the problem of the person implementing the clear
and the halt for vfio-ccw. I.e. it's a non-problem right now.

> (I'm currently trying to recall what we're doing with unsolicited
> interrupts. These are fun wrt deferred cc 1; I'm not sure if there are
> cases where we want to present a deferred cc to the guest.)

What are 'fun wrt deferred cc 1' again? The deferred cc I understand
but wrt does not click at all.

> 
> Also, doing a second ssch before we got final state for the first one
> is perfectly valid. Linux just does not do it, so I'm not sure if we
> should invest too much time there.
> 
>>
>> We do not need it strongly.
>>
>>>   
>>>> +	queue_work(vfio_ccw_work_q, &private->io_work);
>>>> +	if (private->completion)
>>>> +		complete(private->completion);
>>>>    }
>>>>
>>>>    static int vfio_ccw_sch_probe(struct subchannel *sch)
>>> [...]
>>>   
>>
>