linux-kernel - Re: [PATCH v2 4/6] powerpc/smp: Disable MC domain for shared processor

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Message-ID: <20231019075046.GN33217@noisy.programming.kicks-ass.net>
Date:   Thu, 19 Oct 2023 09:50:46 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Nicholas Piggin <npiggin@...il.com>,
        Rohan McLure <rmclure@...ux.ibm.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/6] powerpc/smp: Disable MC domain for shared
 processor

On Thu, Oct 19, 2023 at 03:48:48PM +1100, Michael Ellerman wrote:
> Srikar Dronamraju <srikar@...ux.vnet.ibm.com> writes:
> > Like L2-cache info, coregroup information which is used to determine MC
> > sched domains is only present on dedicated LPARs. i.e PowerVM doesn't
> > export coregroup information for shared processor LPARs. Hence disable
> > creating MC domains on shared LPAR Systems.
> >
> > Signed-off-by: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
> > ---
> >  arch/powerpc/kernel/smp.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> >
> > diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> > index 498c2d51fc20..29da9262cb17 100644
> > --- a/arch/powerpc/kernel/smp.c
> > +++ b/arch/powerpc/kernel/smp.c
> > @@ -1046,6 +1046,10 @@ static struct cpumask *cpu_coregroup_mask(int cpu)
> >  
> >  static bool has_coregroup_support(void)
> >  {
> > +	/* Coregroup identification not available on shared systems */
> > +	if (is_shared_processor())
> > +		return 0;
> 
> That will catch guests running under KVM too right? Do we want that?

Some KVM people use vcpu pinning and pass-through topology things,
slice-of-hardware or something like that. In that scenario you actively
do want this.

I'm fairly clueless on when this is_shared_processor() gets to be true,
so that might already be dealt with..