Re: [Xen-devel] [PATCH 1/4] x86/IRQ: don't keep EOI timer running without need

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 05.06.19 at 19:04, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 08/05/2019 13:46, Jan Beulich wrote:
>> The timer needs to remain active only until all pending IRQ instances
>> have seen EOIs from their respective domains. Stop it when the in-flight
>> count has reached zero in desc_guest_eoi(). Note that this is race free
>> (with __do_IRQ_guest()), as the IRQ descriptor lock is being held at
>> that point.
>>
>> Also pull up stopping of the timer in __do_IRQ_guest() itself: Instead
>> of stopping it immediately before re-setting, stop it as soon as we've
>> made it past any early returns from the function (and hence we're sure
>> it'll get set again).
> 
> Why this this a good thing?

For it to not fire when it doesn't need to. If we're about to set
a new timeout, we clearly don't want the previous one to have
any effect anymore.

>> --- a/xen/arch/x86/irq.c
>> +++ b/xen/arch/x86/irq.c
>> @@ -1115,6 +1115,9 @@ static void irq_guest_eoi_timer_fn(void
>>  
>>      action = (irq_guest_action_t *)desc->action;
>>  
> 
> /* Another instance of this timer already running? Skip everything to
> avoid forcing an EOI early. */

Fine with me, added.

Jan



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