[PATCH 4/4] kvmppc: convert wrteei to wrtee as kvm guest optimization

[Josh Boyer]

On Wed, 2008-08-20 at 13:30 -0500, Hollis Blanchard wrote:
> On Wed, 2008-08-20 at 14:53 +0200, Christian Ehrhardt wrote:
> > 
> > Arnd Bergmann wrote:
> > > On Tuesday 19 August 2008, ehrhardt at linux.vnet.ibm.com wrote:
> > >   
> > >> Dependent on the already existing CONFIG_KVM_GUEST config option
> > this patch
> > >> changes wrteei to wrtee allowing the hypervisor to rewrite those to
> > nontrapping
> > >> instructions. Maybe we should split the kvm guest otpimizations in
> > two parts
> > >> one for the overhead free optimizations and on for the rest that
> > might add
> > >> some complexity for non virtualized execution (like this one).
> > >>
> > >> Signed-off-by: Christian Ehrhardt <ehrhardt at linux.vnet.ibm.com>
> > >>     
> > >
> > > How significant is the performance impact of this change for
> > non-virtualized
> > > systems? If it's very low, maybe you should not bother with the
> > #ifdef, and
> > > if it's noticable, you might be better off using dynamic patching
> > for this.
> > >
> > >       Arnd <><
> > >   
> > To be honest I unfortunately don't know how big the impact for 
> > non-virtualized systems is. I would like to test it, but without 
> > hardware performance counters on the core I have I'm not sure (yet)
> > how 
> > to measure that in a good way - any suggestion welcome.
> 
> I don't see why we need performance counters. Can't we just compare any
> bare metal benchmark results with the patch both applied and not?

Do you know of one that causes a large amount of
local_irq_{disable,enable}s to be called?

josh