[PATCH HACK] powerpc: quick hack to get a functional eHEA with hardirq preemption

[Benjamin Herrenschmidt]

> There may be some implicit assumption in that we expect the cpu 
> priority to be returned to normal by the EOI, but there is nothing in 
> the hardware that requires the EOI to come from the same cpu as 
> accepted the interrupt for processing, with the exception of the IPI 
> which is per-cpu (and the only interrupt that is per-cpu).

Well, there is one fundamental one: The XIRR register we access is
per-CPU, so if we are to return the right processor priority, we must
make sure we write the right XIRR.

Same with Cell, MPIC, actually and a few others. In general I'd say most
fast_eoi type PICs have this requirement.

> It would probably mean adding the concept of the current cpu priority 
> vs interrupts and making sure we write it to hardware at irq_exit() 
> time when deferring the actual irq handlers.

I think we need something like a special -rt variant of the fast_eoi
handler that masks & eoi's in ack() before the thread is spun off, and
unmasks instead of eoi() when the irq processing is complete.

Ben.