Re: [Xen-devel] RE: event delay issue on SMP machine when xen0 is SMPenabled

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 9 Dec 2005, at 08:59, Tian, Kevin wrote:

> > I have a question on the following function, why l1 is updated only 
> > one
> > time, while l2 is updated in each loop? I think they should handle in
> > the same way. Anyway, in current code, l2 &= ~(1UL << l2i); is not
> > needed.
> Does that mean in the middle of processing l2, some bits previously 
> active may become inactive due to unmasked by other vcpus? So 
> conservative check in each loop may be necessary. ;-)
You can reenter evtchn_do_upcall() on the same CPU from within an IRQ 
handler that does not specify SA_INTERRUPT. It will then handle all the 
pending l2's and the saved l2 in your outer stack activation is now 
stale. You then end up double-executing IRQs occasionally which I think 
we found to be a bad thing in a few cases.
 -- Keir


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