[Xen-devel] Sched_op hypercall small questions

[Daniel Castro <evil.dani@xxxxxxxxx>]

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? would that also explain why poll hypercall returns
immediately? 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. How
can I change the flag, I tried changing the value but it does not work
like that apparently.

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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