Subject: Re: [PATCH] powerpc/booke: Eliminate rfi from exception entry path.

From: Benjamin Herrenschmidt <[email-masked]>
Date: Wed, 11 Jul 2012 10:54:17 +1000

On Tue, 2012-07-10 at 19:47 -0500, Scott Wood wrote:
> On 07/10/2012 07:44 PM, Alexander Graf wrote:
> > 
> > On 11.07.2012, at 02:34, Scott Wood wrote:
> >> +#ifdef CONFIG_BOOKE
> >> +	/*
> >> +	 * We're not changing address space on Book E, and the extra rfi
> >> +	 * can hurt when virtualized without hardware support -- whereas
> >> +	 * mtmsr can be paravirtualized.
> > 
> > We can always paravirtualize RFI as well if it makes sense.
> 
> I'm not sure that's possible.  We thought about it a while back, but
> IIRC the difficulty was not leaving a register clobbered.

Besides mtmsr is slow on real HW as well. Also paravirt as done today
for complex instructions like mtmsr is racy :-) (I already had a chat
about that with Alex a while back, we might want to re-consider what
kind of fix can be done at some point).

Cheers,
Ben.