libxc/evtchn: Missed events when using several handles.

libxc/evtchn: Missed events when using several handles.

[Vincent Bernardoff]

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Using the attached test programs, the select fails when using h2, and 
works with using h1 (line 41).

It seems that using several evtchn handles does not work correctly. What 
is the expected behaviour when using multiple event channel handles ?

Cheers,

Vincent

[-- Attachment #2: test_evtchn.c --]
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#include <stdio.h>
#include <stdlib.h>
#include <sys/select.h>
#include <xenctrl.h>

int main(int argc, char** argv)
{
  // This test is to check that you can use several handles to evtchn.

  xc_evtchn *h1, *h2;
  evtchn_port_or_error_t dest, src;
  int fd, ret;

  fd_set fdset;

  FD_ZERO(&fdset);

  h1 = xc_evtchn_open(NULL, 0);
  h2 = xc_evtchn_open(NULL, 0);

  if (h1 == NULL || h2 == NULL)
    {
      perror("xc_evtchn_open");
      exit(EXIT_FAILURE);
    }

  dest = xc_evtchn_bind_unbound_port(h1, 0);
  if (dest == -1)
    {
      perror("xc_evtchn_bind_unbound_port");
      exit(EXIT_FAILURE);
    }

  src = xc_evtchn_bind_interdomain(h1, 0, dest);
  if (src == -1)
    {
      perror("xc_evtchn_bind_interdomain");
      exit(EXIT_FAILURE);
    }

  fd = xc_evtchn_fd(h2);
  if (fd < 0)
    {
      perror("xc_evtchn_fd");
      exit(EXIT_FAILURE);
    }

  FD_SET(fd, &fdset);

  ret = xc_evtchn_notify(h1, src);
  if (ret == -1)
    {
      perror("xc_evtchn_notify");
      exit(EXIT_FAILURE);
    }

  ret = select(fd+1, &fdset, NULL, NULL, NULL);
  if (ret == -1)
    {
      perror("select");
      exit(EXIT_FAILURE);
    }

  printf("select says there is %d bit set in the bitmask.\n", ret);
  exit(EXIT_SUCCESS);
}

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Re: libxc/evtchn: Missed events when using several handles.

[Ian Campbell]

On Tue, 2013-08-06 at 11:15 +0100, Vincent Bernardoff wrote:
> Using the attached test programs, the select fails when using h2, and 
> works with using h1 (line 41).

What do yo mean by "fails", does it return an error or does it just
block waiting for an event which never happens?

> It seems that using several evtchn handles does not work correctly. What 
> is the expected behaviour when using multiple event channel handles ?

You only appear to bind an evtchn to h1, so I would expect that you
would only see evtchns on h1 and selecting on h2 will just block waiting
for something to happen, or maybe h2 will return an error because there
are no evtchns to wait on. If you bind a different evtchn to h2 then you
should be able to wait on both h1 and h2.

Ian.

Re: libxc/evtchn: Missed events when using several handles.

[Vincent Bernardoff]

On 06/08/2013 11:29, Ian Campbell wrote:
> On Tue, 2013-08-06 at 11:15 +0100, Vincent Bernardoff wrote:
>> >Using the attached test programs, the select fails when using h2, and
>> >works with using h1 (line 41).
> What do yo mean by "fails", does it return an error or does it just
> block waiting for an event which never happens?

It does block waiting for an event which never happens.

>> >It seems that using several evtchn handles does not work correctly. What
>> >is the expected behaviour when using multiple event channel handles ?
> You only appear to bind an evtchn to h1, so I would expect that you
> would only see evtchns on h1 and selecting on h2 will just block waiting
> for something to happen,

It seems to be the case.

  or maybe h2 will return an error because there
> are no evtchns to wait on.

No, this doesn't happen.

If you bind a different evtchn to h2 then you
> should be able to wait on both h1 and h2.

I thought that an handle was an interface to the eventchn, and that 
binding events was completely independant of handles, that is, all 
handles receive all events. I was obviously wrong, but it was not clear 
to guess.

Thanks,

Vincent.

Re: libxc/evtchn: Missed events when using several handles.

[Ian Campbell]

On Tue, 2013-08-06 at 11:48 +0100, Vincent Bernardoff wrote:
> I thought that an handle was an interface to the eventchn, and that 
> binding events was completely independant of handles, that is, all 
> handles receive all events. I was obviously wrong, but it was not clear 
> to guess.

Each handle is an independent entity with its own set of bound event
channels.

Ian.