Re: [Xen-devel] xen/evtchn and forced threaded irq

[Oleksandr Andrushchenko]

On 2/25/19 3:55 PM, Julien Grall wrote:
> Hi Oleksandr,
>
> On 25/02/2019 13:24, Oleksandr Andrushchenko wrote:
> > On 2/22/19 3:33 PM, Julien Grall wrote:
> > > Hi,
> > >
> > > On 22/02/2019 12:38, Oleksandr Andrushchenko wrote:
> > > > On 2/20/19 10:46 PM, Julien Grall wrote:
> > > > > Discussing with my team, a solution that came up would be to 
> > > > > introduce one atomic field per event to record the number of event 
> > > > > received. I will explore that solution tomorrow.
> > > > How will this help if events have some payload?
> > >
> > > What payload? The event channel does not carry any payload. It only 
> > > notify you that something happen. Then this is up to the user to 
> > > decide what to you with it.
> > Sorry, I was probably not precise enough. I mean that an event might 
> > have
> > associated payload in the ring buffer, for example [1]. So, counting 
> > events
> > may help somehow, but the ring's data may still be lost
>
> From my understanding of event channels are edge interrupts. By 
> definition, they can be merged so you can get a signal notification to 
> the guest for multiple "events". So if you rely on the event to have 
> an associated payload, then you probably have done something wrong in 
> your driver.
>
> I haven't implemented PV drivers myself, but I would expect either 
> side to block if there were no space in the ring.
>
> What do you do in the displif driver when the ring is full?
It is handled by the originator, the display backend in our case: it 
doesn't send
events if it sees that the ring will overflow. But I was worried about
such a generic change with counting number of events received and if this
really helps to recover in general case
> Cheers,
>
> > [1] 
> > https://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=xen/include/public/io/displif.h;h=cc5de9cb1f35dedc99c866d73d086b19e496852a;hb=HEAD#l756