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

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Date:   Tue, 24 Apr 2018 13:49:14 +0200
From:   Pierre Morel <pmorel@...ux.vnet.ibm.com>
To:     Cornelia Huck <cohuck@...hat.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 24/04/2018 11:59, 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)

We agree.

>
> 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.

Yes, agreed too.

>
> (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.)

This patch does not change the current functionalities, only 
consolidates the FSM.
The current way to handle unsolicited interrupts is to report them to 
the guest
along with the deferred code AFAIU.

>
> 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.

I agree too, it would just make things unnecessary complicated.


-- 

Pierre Morel
Linux/KVM/QEMU in Böblingen - Germany