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Date: Wed, 24 Sep 2008 14:35:52 +0200
From: Sebastien Dugue <sebastien.dugue@bull.net>
To: benh@kernel.crashing.org
Subject: Re: [PATCH HACK] powerpc: quick hack to get a functional eHEA with
	hardirq preemption
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Cc: linux-kernel@vger.kernel.org, Milton Miller <miltonm@bga.com>,
	linuxppc-dev@ozlabs.org, Christoph Raisch <RAISCH@de.ibm.com>,
	Paul Mackerras <paulus@samba.org>, Jan-Bernd Themann <ossthema@de.ibm.com>
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  Hi Ben,

On Wed, 24 Sep 2008 20:17:47 +1000 Benjamin Herrenschmidt <benh@kernel.crashing.org> wrote:

> On Wed, 2008-09-24 at 04:58 -0500, Milton Miller wrote:
> > The per-interrupt mask and unmask calls have to go through RTAS, a
> > single-threaded global context, which in addition to increasing
> > path length will really limit scalability.  The interrupt controller
> > poll and reject facilities are accessed through hypervisor calls
> > which are comparable to a fast syscall, and parallel to all cpus.
> 
> Note also that the XICS code thus assumes, iirc, as does the cell IIC
> code, that eoi is called on the -same- cpu that fetched the interrupt
> initially. That assumption can be broken with IRQ threads no ?

  No, the fetch and the eoi are both done in interrupt context before
the hardirq thread is woken up.

  On the other hand, the mask+eoi and the unmask may well happen
on different cpus as there's only one hardirq thread per irq on
the system. Don't know if this is a problem with the XICS though.

  Thanks,

  Sebastien.

> 
> Ben.
> 
> 
>