Re: [Xen-devel] [question] what's the purpose of smp_send_event_check_mask?

[Kai Huang <mail.kai.huang@xxxxxxxxx>]

On Thu, Jan 19, 2012 at 5:26 PM, Kai Huang <mail.kai.huang@xxxxxxxxx> wrote:
> On Thu, Jan 19, 2012 at 5:14 PM, Keir Fraser <keir.xen@xxxxxxxxx> wrote:
>>
>> On 19/01/2012 09:02, "Kai Huang" <mail.kai.huang@xxxxxxxxx> wrote:
>>
>>> Hi,
>>>
>>> I see cpu_raise_softirq calls smp_send_event_check_mask after setting
>>> softirq bit for target CPU if target CPU is not the current CPU. I
>>> thought smp_send_event_check_mask will send IPI to target CPU and then
>>> trigger target CPU to run pending irq, but seems
>>> smp_send_event_check_mask does nothing but jast ACK the IPI. So what's
>>> the purpose of this function? Why do we need this function?
>>
>> Xen always checks softirqs on return from interrupt context to guest
>> context, and also in its idle loopon wakeup. Hence the event_check interrupt
>> handler itself doesn't need to do anything.
>
> Does this mean if raising softirq for remote CPU, we don't need to
> call do_softirq explicitly on target CPU?
>

I think I understand now after looking at the code. When interrupt
returns, if there's any pending softirqs, the do_softirq will be
called. do_softirq will run handler for all pending bits and
additional call to do_softirq will have no harm as it just do nothing
if no bits are pending.

Thanks.

-cody

> -cody
>
>>
>>> BTW, would someone gives me some knowledge when will the pending irqs
>>> be triggered to run after setting up the pending bits? Seems normally
>>> they are called asynchronously.
>>
>> Yes, of course it's asynchronous, the smp_send_event_check_mask interrupt is
>> just to make sure it happens "soon".
>>
>>  -- Keir
>>
>>> Thanks in advance!
>>>
>>> -cody
>>>
>>> _______________________________________________
>>> Xen-devel mailing list
>>> Xen-devel@xxxxxxxxxxxxxxxxxxx
>>> http://lists.xensource.com/xen-devel
>>
>>

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