Re: [PATCH v2 06/14] KVM: arm64: Remap PMUv3 events onto hardware

[Oliver Upton <oliver.upton@linux.dev>]

On Wed, Feb 19, 2025 at 04:45:27PM +0000, Marc Zyngier wrote:
> On Mon, 03 Feb 2025 18:31:03 +0000,
> Oliver Upton <oliver.upton@linux.dev> wrote:
> > 
> > Use the provided helper to map PMUv3 event IDs onto hardware, if the
> > driver exposes such a helper. This is expected to be quite rare, and
> > only useful for non-PMUv3 hardware.
> > 
> > Tested-by: Janne Grunau <j@jannau.net>
> > Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
> > ---
> >  arch/arm64/kvm/pmu-emul.c | 14 +++++++++++++-
> >  1 file changed, 13 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
> > index 62349b670cf9..60cf973e2af9 100644
> > --- a/arch/arm64/kvm/pmu-emul.c
> > +++ b/arch/arm64/kvm/pmu-emul.c
> > @@ -673,6 +673,18 @@ static bool kvm_pmc_counts_at_el2(struct kvm_pmc *pmc)
> >  	return kvm_pmc_read_evtreg(pmc) & ARMV8_PMU_INCLUDE_EL2;
> >  }
> >  
> > +static u64 kvm_map_pmu_event(struct kvm *kvm, u64 eventsel)
> > +{
> > +	struct arm_pmu *pmu = kvm->arch.arm_pmu;
> > +	int hw_event;
> > +
> > +	if (!pmu->map_pmuv3_event)
> > +		return eventsel;
> > +
> > +	hw_event = pmu->map_pmuv3_event(eventsel);
> > +	return (hw_event < 0) ? eventsel : hw_event;
> 
> I find this a bit odd. If we can translate events, but failed to do
> so, we still install the originally requested event, and we have no
> idea what this maps to on the HW.
> 
> I'd rather we just don't install that event at all rather than
> counting something random.

Heh, this was a leftover party trick that I was using to try "raw"
events from inside a VM. Happy to limit things to the PMUv3 event space
though.

Thanks,
Oliver