Re: [RFC PATCH 40/41] KVM: x86/pmu: Separate passthrough PMU logic in set/get_msr() from non-passthrough vPMU

[Mingwei Zhang]

On Thu, Apr 11, 2024, Sean Christopherson wrote:
> On Fri, Jan 26, 2024, Xiong Zhang wrote:
> > From: Mingwei Zhang <mizhang@xxxxxxxxxx>
> > 
> > Separate passthrough PMU logic from non-passthrough vPMU code. There are
> > two places in passthrough vPMU when set/get_msr() may call into the
> > existing non-passthrough vPMU code: 1) set/get counters; 2) set global_ctrl
> > MSR.
> > 
> > In the former case, non-passthrough vPMU will call into
> > pmc_{read,write}_counter() which wires to the perf API. Update these
> > functions to avoid the perf API invocation.
> > 
> > The 2nd case is where global_ctrl MSR writes invokes reprogram_counters()
> > which will invokes the non-passthrough PMU logic. So use pmu->passthrough
> > flag to wrap out the call.
> > 
> > Signed-off-by: Mingwei Zhang <mizhang@xxxxxxxxxx>
> > ---
> >  arch/x86/kvm/pmu.c |  4 +++-
> >  arch/x86/kvm/pmu.h | 10 +++++++++-
> >  2 files changed, 12 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> > index 9e62e96fe48a..de653a67ba93 100644
> > --- a/arch/x86/kvm/pmu.c
> > +++ b/arch/x86/kvm/pmu.c
> > @@ -652,7 +652,9 @@ int kvm_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> >  		if (pmu->global_ctrl != data) {
> >  			diff = pmu->global_ctrl ^ data;
> >  			pmu->global_ctrl = data;
> > -			reprogram_counters(pmu, diff);
> > +			/* Passthrough vPMU never reprogram counters. */
> > +			if (!pmu->passthrough)
> 
> This should probably be handled in reprogram_counters(), otherwise we'll be
> playing whack-a-mole, e.g. this misses MSR_IA32_PEBS_ENABLE, which benign, but
> only because PEBS isn't yet supported.
> 
> > +				reprogram_counters(pmu, diff);
> >  		}
> >  		break;
> >  	case MSR_CORE_PERF_GLOBAL_OVF_CTRL:
> > diff --git a/arch/x86/kvm/pmu.h b/arch/x86/kvm/pmu.h
> > index 0fc37a06fe48..ab8d4a8e58a8 100644
> > --- a/arch/x86/kvm/pmu.h
> > +++ b/arch/x86/kvm/pmu.h
> > @@ -70,6 +70,9 @@ static inline u64 pmc_read_counter(struct kvm_pmc *pmc)
> >  	u64 counter, enabled, running;
> >  
> >  	counter = pmc->counter;
> > +	if (pmc_to_pmu(pmc)->passthrough)
> > +		return counter & pmc_bitmask(pmc);
> 
> Won't perf_event always be NULL for mediated counters?  I.e. this can be dropped,
> I think.

yeah. I double checked and seems when perf_event == NULL, the logic is
correct. If so, we can drop that.

Thanks.
-Mingwei