linux-kernel - Re: [PATCH v7 06/12] KVM: arm64: PMU: Add a helper to read the number of counters

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Date:   Fri, 13 Oct 2023 05:43:01 +0000
From:   Oliver Upton <oliver.upton@...ux.dev>
To:     Raghavendra Rao Ananta <rananta@...gle.com>
Cc:     Marc Zyngier <maz@...nel.org>,
        Alexandru Elisei <alexandru.elisei@....com>,
        James Morse <james.morse@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Zenghui Yu <yuzenghui@...wei.com>,
        Shaoqin Huang <shahuang@...hat.com>,
        Jing Zhang <jingzhangos@...gle.com>,
        Reiji Watanabe <reijiw@...gle.com>,
        Colton Lewis <coltonlewis@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v7 06/12] KVM: arm64: PMU: Add a helper to read the
 number of counters

On Tue, Oct 10, 2023 at 10:30:31PM +0000, Oliver Upton wrote:
> On Mon, Oct 09, 2023 at 11:08:52PM +0000, Raghavendra Rao Ananta wrote:
> > Add a helper, kvm_arm_get_num_counters(), to read the number
> > of counters from the arm_pmu associated to the VM. Make the
> > function global as upcoming patches will be interested to
> > know the value while setting the PMCR.N of the guest from
> > userspace.
> > 
> > Signed-off-by: Raghavendra Rao Ananta <rananta@...gle.com>
> > ---
> >  arch/arm64/kvm/pmu-emul.c | 17 +++++++++++++++++
> >  include/kvm/arm_pmu.h     |  6 ++++++
> >  2 files changed, 23 insertions(+)
> > 
> > diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
> > index a161d6266a5c..84aa8efd9163 100644
> > --- a/arch/arm64/kvm/pmu-emul.c
> > +++ b/arch/arm64/kvm/pmu-emul.c
> > @@ -873,6 +873,23 @@ static bool pmu_irq_is_valid(struct kvm *kvm, int irq)
> >  	return true;
> >  }
> >  
> > +/**
> > + * kvm_arm_get_num_counters - Get the number of general-purpose PMU counters.
> > + * @kvm: The kvm pointer
> > + */
> > +int kvm_arm_get_num_counters(struct kvm *kvm)
> 
> nit: the naming suggests this returns the configured number of PMCs, not
> the limit.
> 
> Maybe kvm_arm_pmu_get_max_counters()?

Following up on the matter -- please try to avoid sending patches that
add helpers without any users. Lifting *existing* logic into a helper
and updating the callsites is itself worthy of a separate patch. But
adding a new function called by nobody doesn't do much, and can easily
be squashed into the patch that consumes the new logic.

-- 
Thanks,
Oliver