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

[Julien Grall]

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?

Cheers,

> [1] 
> https://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=xen/include/public/io/displif.h;h=cc5de9cb1f35dedc99c866d73d086b19e496852a;hb=HEAD#l756 

--
Julien Grall