[PATCH 1/4] powerpc/book3e: introduce external_input_edge exception handler for 64bit kernel

[Kevin Hao]

On Mon, May 13, 2013 at 10:47:17AM -0500, Scott Wood wrote:
> On 05/11/2013 06:26:21 PM, Kevin Hao wrote:
> >In the external proxy facility mode, the interrupt is automatically
> >acknowledged with the same effect as reading the IACK register. So
> >this makes external input interrupt more like edge sensitive. That
> >means we can leave the irq hard enabled when it occurs with irq soft
> >disabled just like the dec and doorbell interrupt. But the External
> >Proxy Register(EPR) is only considered valid from the time that the
> >external interrupt occurs until MSR[EE] is set to 1. So we have to
> >save the EPR before irq hard enabled.
> 
> Is it really worth it?

Maybe. :-)
Compare with the current kernel:
  * The overhead is that we need additional load & store the contents of
    the EPR from/to PACA.
  * The bonus is we keep the irq hard enabled when a external interrupt occurs
    with irq soft-disabled. As I know we should leave the irq hard enabled as
    much as possible. This is also the primary reason that we introduce the
    Lazy EE.

>  Are you having a real-world problem with
> profilability as things stand, that this resolves?

No. 

>  We should
> already be no worse off than non-external-proxy hardware...

Yes, I agree. But we can leverage the advantage of external proxy and
make it even more better.

Thanks,
Kevin

> 
> -Scott
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