[PATCH v2 0/6] Implement split core for POWER8

Michael Neuling mikey at neuling.org
Fri May 23 20:00:24 EST 2014

On Fri, 2014-05-23 at 11:53 +0200, Alexander Graf wrote:
> On 23.05.14 10:15, Michael Neuling wrote:
> > This patch series implements split core mode on POWER8.  This enables up to 4
> > subcores per core which can each independently run guests (per guest SPRs like
> > SDR1, LPIDR etc are replicated per subcore).  Lots more documentation on this
> > feature in the code and commit messages.
> >
> > Most of this code is in the powernv platform but there's a couple of KVM
> > specific patches too.
> >
> > Patch series authored by mpe and me with a few bug fixes from others.
> >
> > v2:
> >    There are some minor updates based on comments and I've added the Acks by
> >    Paulus and Alex for the KVM code.
> I don't see changelogs inside the individual patches. Please make sure 
> to always mention what changed from one version to the next in a 
> particular patch, so that I have the chance to check whether that change 
> was good :).

Sure, that was a bit sloppy

Only the last patch was the only one that changed.  I changed the sysfs
file from 600 permissions to 644 so that users can read it more easily
as requested by Joel.

The other change was to fix the possibility of a race when coming out of
nap and checking if we need to split.  This fix was form paulus' (worked

> Also, is there any performance penalty associated with split core mode? 
> If not, could we just always default to split-by-4 on POWER8 bare metal?

Yeah, there is a performance hit .  When you are split (ie
subcores_per_core = 2 or 4), the core is stuck in SMT8 mode.  So if you
only have 1 thread active (others napped), you won't get the benefit of
ST mode in the core (more register renames per HW thread, more FXUs,
more FPUs etc).


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