Re: [PATCH] KVM: PPC: e500mc: Relax tlb invalidation condition on vcpu schedule

[Scott Wood <scottwood@freescale.com>]

On Fri, 2014-06-13 at 16:55 +0200, Alexander Graf wrote:
> On 13.06.14 16:43, mihai.caraman@freescale.com wrote:
> >> -----Original Message-----
> >> From: Alexander Graf [mailto:agraf@suse.de]
> >> Sent: Thursday, June 12, 2014 8:05 PM
> >> To: Caraman Mihai Claudiu-B02008
> >> Cc: kvm-ppc@vger.kernel.org; kvm@vger.kernel.org; linuxppc-
> >> dev@lists.ozlabs.org; Wood Scott-B07421
> >> Subject: Re: [PATCH] KVM: PPC: e500mc: Relax tlb invalidation condition
> >> on vcpu schedule
> >>
> >> On 06/12/2014 04:00 PM, Mihai Caraman wrote:
> >>> On vcpu schedule, the condition checked for tlb pollution is too tight.
> >>> The tlb entries of one vcpu are polluted when a different vcpu from the
> >>> same partition runs in-between. Relax the current tlb invalidation
> >>> condition taking into account the lpid.

Can you quantify the performance improvement from this?  We've had bugs
in this area before, so let's make sure it's worth it before making this
more complicated.

> >>> Signed-off-by: Mihai Caraman <mihai.caraman <at> freescale.com>
> >> Your mailer is broken? :)
> >> This really should be an @.
> >>
> >> I think this should work. Scott, please ack.
> > Alex, you were right. I screwed up the patch description by inverting relax
> > and tight terms :) It should have been more like this:
> >
> > KVM: PPC: e500mc: Enhance tlb invalidation condition on vcpu schedule
> >
> > On vcpu schedule, the condition checked for tlb pollution is too loose.
> > The tlb entries of a vcpu are polluted (vs stale) only when a different vcpu
> > within the same logical partition runs in-between. Optimize the tlb invalidation
> > condition taking into account the lpid.
> 
> Can't we give every vcpu its own lpid? Or don't we trap on global 
> invalidates?

That would significantly increase the odds of exhausting LPIDs,
especially on large chips like t4240 with similarly large VMs.  If we
were to do that, the LPIDs would need to be dynamically assigned (like
PIDs), and should probably be a separate numberspace per physical core.

-Scott