[Xen-devel] Question about pirq_guest_unmask

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

Hi, Keir,
        Though the logic to handle pirq has been enhanced recently, 
there's one point never changed which seems questionable to me. 
Why does pirq_guest_unmask need to scan all pirq_masks instead 
of the one that guest is exactly requesting? Does it aim to accelerate 
EOI when another vcpu is sitting between unmask_evtchn and 
pirq_unmask_notify?

        There seems to be a race condition by current logic:

__do_IRQ_guest:
        ...
        if ( (action->ack_type != ACKTYPE_NONE) &&
             !test_and_set_bit(irq, &d->pirq_mask) )
            action->in_flight++;
    send_guest_pirq(d, irq);
<- At this point, the pirq_mask is set, while the evtchn_mask is likely 
to be cleared. Evtchn_mask will be set later when target vcpu is 
scheduled and ack_pirq is issued.

While in pirq_guest_unmask:
        ...
        For all bits set in pirq_mask:
                if ( !test_bit(d->pirq_to_evtchn[pirq],
&s->evtchn_mask[0]) &&
             test_and_clear_bit(pirq, &d->pirq_mask) )
        ...

Above logic in pirq_guest_unmask tempts to consider clearing related 
event mask as an indicator for subsequent pirq unmask request. 
However it's possible for above checking happening right between 
send_guest_irq and ack_pirq for another pirq injection.

If that's the case, it's possible for one EOI issued early than
expected. 
Does I miss anything important? Is it OK to change it to service desired

pirq only?

Thanks,
Kevin

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