Re: [Xen-devel] [PATCH 2/4] x86/IRQ: bail early from irq_guest_eoi_timer_fn() when nothing is in flight

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 16.05.19 at 13:37, <roger.pau@xxxxxxxxxx> wrote:
> On Wed, May 08, 2019 at 06:46:51AM -0600, Jan Beulich wrote:
>> There's no point entering the loop in the function in this case. Instead
>> there still being something in flight _after_ the loop would be an
>> actual problem: No timer would be running anymore for issuing the EOI
>> eventually, and hence this IRQ (and possibly lower priority ones) would
>> be blocked, perhaps indefinitely.
>> 
>> Issue a warning instead and prefer breaking some (presumably
>> misbehaving) guest over stalling perhaps the entire system.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> 
>> --- a/xen/arch/x86/irq.c
>> +++ b/xen/arch/x86/irq.c
>> @@ -1115,7 +1115,7 @@ static void irq_guest_eoi_timer_fn(void
>>  
>>      action = (irq_guest_action_t *)desc->action;
>>  
>> -    if ( timer_is_active(&action->eoi_timer) )
>> +    if ( !action->in_flight || timer_is_active(&action->eoi_timer) )
>>          goto out;
>>  
>>      if ( action->ack_type != ACKTYPE_NONE )
>> @@ -1130,8 +1130,10 @@ static void irq_guest_eoi_timer_fn(void
>>          }
>>      }
>>  
>> -    if ( action->in_flight != 0 )
>> -        goto out;
>> +    if ( action->in_flight )
>> +        printk(XENLOG_G_WARNING
>> +               "IRQ%d: %d handlers still in flight at forced EOI\n",
>> +               desc->irq, action->in_flight);
> 
> AFAICT action->in_flight should contain the number of guests pirqs
> that have the pirq masked (pirq->masked == true), because in_flight is
> only increased by __do_IRQ_guest when the pirq is not already masked.
> At guest EOI (desc_guest_eoi) the in_flight count is also only
> decreased if the pirq is unmasked.
> 
> Hence I think this condition could be turned into an ASSERT, but I'm
> likely missing something.

I don't think you are. Going from if() straight to ASSERT() simply
seemed too harsh to me, the more in a subsystem where I could
easily have overlooked some corner case, due to how convoluted
some of the implementation is.

Jan



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