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Re: [Xen-devel] fooey. no interrupts.

> >  4. The original value of vcpu_data[0].evtchn_upcall_pending must be zero.
> >  5. The value of vcpu_data[0].evtchn_upcall_mask must be zero.
> OK, this I don't totally see. From the code I posted before, it seems to 
> me only the first three conditions matter. 

The first three conditions cause us to decide whether or not to
schedule the target domain, sending a cross-cpu interrupt if
necessary. The final two are checks just before calling back to the
guest OS, just to check whether it is in a position to receive async
callbacks. The final two are only ever accessed on the CPU that is
running the guest, which is why we can access/update them using
non-atomic operations and compiler barriers (rather than atomic ops
and CPU barriers).

 -- Keir

> Thanks!
> ron
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