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

[Pierre Morel]

On 24/04/2018 13:55, Cornelia Huck wrote:
> On Tue, 24 Apr 2018 13:49:14 +0200
> Pierre Morel <pmorel@xxxxxxxxxxxxxxxxxx> wrote:
>
> > On 24/04/2018 11:59, Cornelia Huck wrote:
> > > On Tue, 24 Apr 2018 10:40:56 +0200
> > > Pierre Morel <pmorel@xxxxxxxxxxxxxxxxxx> wrote:
> > >   
> > > > On 24/04/2018 08:54, Dong Jia Shi wrote:
> > > > > * Pierre Morel <pmorel@xxxxxxxxxxxxxxxxxx> [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.
> My question was more along the line of "do we actually want to
> _generate_ a deferred cc1 or unsolicited interrupt, based upon what we
> do in our state machine". My guess is no, regardless of the changes you
> do in this series.
>
> > > 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.
> I'm a big fan of just throwing everything at the hardware and let it
> sort out any races etc. We just need to be sure we don't mix up
> interrupts :)
OK, I understand, I can do somthing in the interrupt handler to make 
sure we do not loose interrupt IRQs.

I make a proposition in V2.

Thanks,

Pierre


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