Re: [PATCH v8 5/6] KVM: arm/arm64: remove pmc->bitmask

[Andrew Murray <andrew.murray@arm.com>]

On Wed, May 22, 2019 at 05:07:31PM +0100, Marc Zyngier wrote:
> On 22/05/2019 16:30, Andrew Murray wrote:
> > We currently use pmc->bitmask to determine the width of the pmc - however
> > it's superfluous as the pmc index already describes if the pmc is a cycle
> > counter or event counter. The architecture clearly describes the widths of
> > these counters.
> > 
> > Let's remove the bitmask to simplify the code.
> > 
> > Signed-off-by: Andrew Murray <andrew.murray@arm.com>
> > ---
> >  include/kvm/arm_pmu.h |  1 -
> >  virt/kvm/arm/pmu.c    | 15 +++++----------
> >  2 files changed, 5 insertions(+), 11 deletions(-)
> > 
> > diff --git a/include/kvm/arm_pmu.h b/include/kvm/arm_pmu.h
> > index b73f31baca52..2f0e28dc5a9e 100644
> > --- a/include/kvm/arm_pmu.h
> > +++ b/include/kvm/arm_pmu.h
> > @@ -28,7 +28,6 @@
> >  struct kvm_pmc {
> >  	u8 idx;	/* index into the pmu->pmc array */
> >  	struct perf_event *perf_event;
> > -	u64 bitmask;
> >  };
> >  
> >  struct kvm_pmu {
> > diff --git a/virt/kvm/arm/pmu.c b/virt/kvm/arm/pmu.c
> > index ae1e886d4a1a..c4e2bc213617 100644
> > --- a/virt/kvm/arm/pmu.c
> > +++ b/virt/kvm/arm/pmu.c
> > @@ -47,7 +47,10 @@ u64 kvm_pmu_get_counter_value(struct kvm_vcpu *vcpu, u64 select_idx)
> >  		counter += perf_event_read_value(pmc->perf_event, &enabled,
> >  						 &running);
> >  
> > -	return counter & pmc->bitmask;
> > +	if (select_idx != ARMV8_PMU_CYCLE_IDX)
> > +		counter = lower_32_bits(counter);
> > +
> > +	return counter;
> >  }
> >  
> >  /**
> > @@ -113,7 +116,6 @@ void kvm_pmu_vcpu_reset(struct kvm_vcpu *vcpu)
> >  	for (i = 0; i < ARMV8_PMU_MAX_COUNTERS; i++) {
> >  		kvm_pmu_stop_counter(vcpu, &pmu->pmc[i]);
> >  		pmu->pmc[i].idx = i;
> > -		pmu->pmc[i].bitmask = 0xffffffffUL;
> >  	}
> >  }
> >  
> > @@ -348,8 +350,6 @@ void kvm_pmu_software_increment(struct kvm_vcpu *vcpu, u64 val)
> >   */
> >  void kvm_pmu_handle_pmcr(struct kvm_vcpu *vcpu, u64 val)
> >  {
> > -	struct kvm_pmu *pmu = &vcpu->arch.pmu;
> > -	struct kvm_pmc *pmc;
> >  	u64 mask;
> >  	int i;
> >  
> > @@ -368,11 +368,6 @@ void kvm_pmu_handle_pmcr(struct kvm_vcpu *vcpu, u64 val)
> >  		for (i = 0; i < ARMV8_PMU_CYCLE_IDX; i++)
> >  			kvm_pmu_set_counter_value(vcpu, i, 0);
> >  	}
> > -
> > -	if (val & ARMV8_PMU_PMCR_LC) {
> > -		pmc = &pmu->pmc[ARMV8_PMU_CYCLE_IDX];
> > -		pmc->bitmask = 0xffffffffffffffffUL;
> > -	}
> >  }
> >  
> >  static bool kvm_pmu_counter_is_enabled(struct kvm_vcpu *vcpu, u64 select_idx)
> > @@ -420,7 +415,7 @@ static void kvm_pmu_create_perf_event(struct kvm_vcpu *vcpu, u64 select_idx)
> >  
> >  	counter = kvm_pmu_get_counter_value(vcpu, select_idx);
> >  	/* The initial sample period (overflow count) of an event. */
> > -	attr.sample_period = (-counter) & pmc->bitmask;
> > +	attr.sample_period = (-counter) & GENMASK(31, 0);
> 
> Isn't this the one case where the bitmask actually matters? If we're
> dealing with the cycle counter, it shouldn't be truncated, right?

Ah yes, that should be conditional on idx as well. 

Thanks,

Andrew Murray

> 
> >  
> >  	event = perf_event_create_kernel_counter(&attr, -1, current,
> >  						 kvm_pmu_perf_overflow, pmc);
> > 
> 
> Thanks,
> 
> 	M.
> -- 
> Jazz is not dead. It just smells funny...
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