Re: Event Channels

[Brendan Cully <brendan@cs.ubc.ca>]

On Friday, 12 January 2007 at 19:33, Julian Stecklina wrote:
> Hello,
> 
> in an application I develop, event channel semantics seem a bit odd to 
> me. Consider the following scenario: A Mini-OS binds an unbound port for
> domain 0 and issues a notification every second:
> 
> 	evtchn_alloc_unbound(0, msg_handler, NULL,
> 			     &port);
> 
> 	printk("%u %u %u\n", domid, port);
> 
> 	while (1) {
> 		sleep(1000);
> 		notify_remote_via_evtchn(port);
> 	}
> 
> A Linux program running on Domain 0 calls bind_interdomain using domid
> and port to obtain local_port and watches for pending notifications:
> 
> 	evtchn_port_t local_port = xc_evtchn_bind_interdomain(ev, domid, 
> 	port);
> 
> 	while (1) {
> 		evtchn_port_t port;
> 
> 		port = xc_evtchn_pending(ev);
> 		printf("%u\n", port);
> 		xc_evtchn_unmask(ev, port);
> 	}
> 
> Now what I do not understand is, why the Linux program sees exactly one
> notification and then indefinitely blocks. Any help on what I may be 
> doing wrong is greatly appreciated. This is with xen-3.0.4-testing.

According to the interface guide

http://www.cl.cam.ac.uk/research/srg/netos/xen/readmes/interface/interface.html#SECTION00600000000000000000

the event is masked until the receiver clears the notification. I
think you do this by sending a notify back along the same port:

while (1) {
	evtchn_port_t port;
 
	port = xc_evtchn_pending(ev);
	printf("%u\n", port);
	xc_evtchn_notify(ev, port);
	xc_evtchn_unmask(ev, port);
}