Re: [Xen-devel] Sched_op hypercall small questions

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Thu, 2011-09-22 at 06:41 +0100, Daniel Castro wrote:
> On Wed, Sep 21, 2011 at 10:28 AM, Jeremy Fitzhardinge <jeremy@xxxxxxxx> wrote:
> > On 09/19/2011 11:17 PM, Daniel Castro wrote:
> >> On Tue, Sep 20, 2011 at 2:41 PM, Keir Fraser <keir.xen@xxxxxxxxx> wrote:
> >>> On 19/09/2011 22:21, "Daniel Castro" <evil.dani@xxxxxxxxx> wrote:
> >>>
> >>>> Greetings all.
> >>>>
> >>>> Some small question regarding schedule poll operation hypercall.
> >>>>
> >>>> 1. struct sched_poll poll.timeout is measured in what unit of time?
> >>>> Secs, ms? ns?
> >>> It is an absolute system time (rather than a duration), in nanoseconds.
> >> really an absolute system time?
> >>
> >> When the timeout is set and the timeout is reached, the system behaves
> >> like if the event had been received? i.e the bit is changed?
> >
> > You specify the timeout in the the form "wake up by time t".  If t is in
> > the past, it times out immediately.
> >
> >>>> 2. After issuing the hypercall_sched_op(SCHEDOP_poll, &poll); if no
> >>>> timeout is used in poll struct how long will I yield the CPU?
> >>> Until one of the specified event channel ports is pending.
> >> If the channel port never changes (the event never arrives) then I
> >> would yield for ever?
> >
> > If you have events unmasked and you get an unmasked event, then it will
> > go into the event handler.
> 
> My vcpu[0].evntchn_upcall_mask is 0, does this prevents the guest from
> receiving events?

IIRC it's vice versa: setting it to 1 prevents the guest from receiving
events.

> would that also explain why poll hypercall returns
> immediately?

I don't see any reference to the masks in
xen/common/schedule.c:do_poll() so I don't think so.

>  According to Xen's Definitive Guide evntchn_upcall_mask
> is unset at boot (my case exactly) so if it is 0 from the start and
> the guest (me) has to change it to 1 in order to receive events.

I think that's the wrong way round.

> How can I change the flag, I tried changing the value but it does not work
> like that apparently.

evtchn_upcall_mask is the global mask but you may also need to unmask
the specific evtchn, which means clearing the relevant bit in
evtchn_mask.

> 
> Thanks
> >
> >>>> 3. If I issue the hypercall and the event never comes is it possible
> >>>> to to yield the CPU for ever?
> >>> Yes, if you do not specify a timeout.
> >> Keir thanks for the answer.
> >>
> >> I am trying to read from xenstore, so I have the following:
> >> I write on my xenstore ring the query I want, then,
> >> hypercall_event_channel_op(EVTCHNOP_send ...
> >> If I read the ring inmediatly the answer is not ready so I issue a
> >> hypercall_sched_op(SCHEDOP_poll, &poll);
> >> But while I am entering the yield state the answer comes, so the event
> >> is never seen because it has already been delivered.
> >
> > It generally only makes sense to poll on masked events.
> >
> >>
> >> If I use some way to wait (just for very brief instant) after the
> >> event_channel_op send then, when I check the ring the answer is there;
> >> And I do not need to yield the CPU.
> >>
> >> Should I issue the wait after the send, rather than when I am about to
> >> read the answer?
> >
> > What environment is this in?
> >
> >    J
> >
> 
> 
> 
> --
> +-=====---------------------------+
> | +---------------------------------+ | This space intentionally blank
> for notetaking.
> | |   | Daniel Castro,                |
> | |   | Consultant/Programmer.|
> | |   | U Andes                         |
> +-------------------------------------+
> 
> 
> 



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