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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

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.

Thanks, Roger.

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