Re: [PATCH 13/23] KVM: PPC: Book3S HV: Accumulate timing information for real-mode code

[Paul Mackerras <paulus@samba.org>]

On Fri, Mar 20, 2015 at 12:35:43PM +0100, Alexander Graf wrote:
> 
> 
> On 20.03.15 12:25, Paul Mackerras wrote:
> > On Fri, Mar 20, 2015 at 12:15:15PM +0100, Alexander Graf wrote:
> >> Have you measure the additional overhead this brings?
> > 
> > I haven't - in fact I did this patch so I could measure the overhead
> > or improvement from other changes I did, but it doesn't measure its
> > own overhead, of course.  I guess I need a workload that does a
> > defined number of guest entries and exits and measure how fast it runs
> > with and without the patch (maybe something like H_SET_MODE in a
> > loop).  I'll figure something out and post the results.  
> 
> Yeah, just measure the number of exits you can handle for a simple
> hcall. If there is measurable overhead, it's probably a good idea to
> move the statistics gathering into #ifdef paths for DEBUGFS or maybe
> even a separate EXIT_TIMING config option as we have it for booke.

For 1-vcpu guest on POWER8, it adds 29ns to the time for an hcall that
is handled in real mode (H_SET_DABR), which is 25%.  It adds 43ns to
the time for an hcall that is handled in the host kernel in virtual
mode (H_PROD), which is 1.2%.

I'll add a config option.

Paul.