Re: [PATCH v8 3/3] xen/events: do some cleanups in evtchn_fifo_set_pending()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 25.11.2020 11:51, Juergen Gross wrote:
> --- a/xen/common/event_fifo.c
> +++ b/xen/common/event_fifo.c
> @@ -175,6 +175,18 @@ static void evtchn_fifo_set_pending(struct vcpu *v, 
> struct evtchn *evtchn)
>          return;
>      }
>  
> +    /*
> +     * Control block not mapped.  The guest must not unmask an
> +     * event until the control block is initialized, so we can
> +     * just drop the event.
> +     */
> +    if ( unlikely(!v->evtchn_fifo->control_block) )
> +    {
> +        printk(XENLOG_G_WARNING
> +               "%pv has no FIFO event channel control block\n", v);
> +        return;
> +    }

This results in bypassing the setting of PENDING and the possible
call to evtchn_check_pollers(). It may in particular be the case
that a very special purpose guest uses event channels just for
waking up pollers, which - afaict - then doesn't require setting
up a control block. To give an example, I could easily see an XTF
test avoid that step if indeed it's unnecessary.

Jan