[PATCH] Don't access HID registers if running on a Hypervisor.

[Olof Johansson]

On Wed, Jun 21, 2006 at 10:35:28PM -0400, Jimi Xenidis wrote:
> 
> On Jun 21, 2006, at 7:58 PM, Paul Mackerras wrote:
> 
> > Jimi Xenidis writes:
> >
> >> The following patch avoids accessing Hypervisor privilege HID
> >> registers when running on a Hypervisor (MSR[HV]=0).
> >
> > Why?  Aren't the writes defined to be no-ops when not in hypervisor
> > mode?
> 
> The write are no-ops but look at the instructions required to be  
> around the write for some of them.
> I believe read may have performance issues as well.

Are you aware that this code runs only once per processor during
boot (and/or during cpu hotplug)? A few extra cycles there won't kill
anyone.

That being said, not touching the registers makes sense, but I don't buy
the performance motivation. :-)

> I juts think its good practice and as Xell and newer processes come  
> along we may want to start using the PACA(?) rather then peeking at  
> the msr all the time.

PACA means it's a mfspr + dependent load + test instead of mfmsr + mask,
right? It'd need benchmarking on several processors to show if it's
better or not.


-Olof