Re: [Xen-devel] Sched_op hypercall small questions

[Daniel Castro <evil.dani@xxxxxxxxx>]

On Thu, Sep 22, 2011 at 3:30 PM, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> 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 for the answer Ian, one question: If I do not manually change
the mask bit, then after writing to the ring and the response arrives
how can I be sure that xenstore is delivering an event? I am
suspecting that xenstore does not use event notification for simple
read and write operations. Could I be right?

>
>>
>> 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                         |
>> +-------------------------------------+
>>
>>
>>
>
>
>



-- 
+-=====---------------------------+
| +---------------------------------+ | This space intentionally blank
for notetaking.
| |   | Daniel Castro,                |
| |   | Consultant/Programmer.|
| |   | U Andes                         |
+-------------------------------------+

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