Re: [PATCH v4 04/16] KVM: arm64: PMU: Distinguish between 64bit counter and 64bit overflow

[Ricardo Koller <ricarkol@google.com>]

On Thu, Dec 01, 2022 at 08:47:47AM -0800, Ricardo Koller wrote:
> On Sun, Nov 13, 2022 at 04:38:20PM +0000, Marc Zyngier wrote:
> > The PMU architecture makes a subtle difference between a 64bit
> > counter and a counter that has a 64bit overflow. This is for example
> > the case of the cycle counter, which can generate an overflow on
> > a 32bit boundary if PMCR_EL0.LC==0 despite the accumulation being
> > done on 64 bits.
> > 
> > Use this distinction in the few cases where it matters in the code,
> > as we will reuse this with PMUv3p5 long counters.
> > 
> > Signed-off-by: Marc Zyngier <maz@kernel.org>
> > ---
> >  arch/arm64/kvm/pmu-emul.c | 43 ++++++++++++++++++++++++++++-----------
> >  1 file changed, 31 insertions(+), 12 deletions(-)
> > 
> > diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
> > index 69b67ab3c4bf..d050143326b5 100644
> > --- a/arch/arm64/kvm/pmu-emul.c
> > +++ b/arch/arm64/kvm/pmu-emul.c
> > @@ -50,6 +50,11 @@ static u32 kvm_pmu_event_mask(struct kvm *kvm)
> >   * @select_idx: The counter index
> >   */
> >  static bool kvm_pmu_idx_is_64bit(struct kvm_vcpu *vcpu, u64 select_idx)
> > +{
> > +	return (select_idx == ARMV8_PMU_CYCLE_IDX);
> > +}
> > +
> > +static bool kvm_pmu_idx_has_64bit_overflow(struct kvm_vcpu *vcpu, u64 select_idx)
> >  {
> >  	return (select_idx == ARMV8_PMU_CYCLE_IDX &&
> >  		__vcpu_sys_reg(vcpu, PMCR_EL0) & ARMV8_PMU_PMCR_LC);
> > @@ -57,7 +62,8 @@ static bool kvm_pmu_idx_is_64bit(struct kvm_vcpu *vcpu, u64 select_idx)
> >  
> >  static bool kvm_pmu_counter_can_chain(struct kvm_vcpu *vcpu, u64 idx)
> >  {
> > -	return (!(idx & 1) && (idx + 1) < ARMV8_PMU_CYCLE_IDX);
> > +	return (!(idx & 1) && (idx + 1) < ARMV8_PMU_CYCLE_IDX &&
> > +		!kvm_pmu_idx_has_64bit_overflow(vcpu, idx));
> >  }
> >  
> >  static struct kvm_vcpu *kvm_pmc_to_vcpu(struct kvm_pmc *pmc)
> > @@ -97,7 +103,7 @@ u64 kvm_pmu_get_counter_value(struct kvm_vcpu *vcpu, u64 select_idx)
> >  		counter += perf_event_read_value(pmc->perf_event, &enabled,
> >  						 &running);
> >  
> > -	if (select_idx != ARMV8_PMU_CYCLE_IDX)
> > +	if (!kvm_pmu_idx_is_64bit(vcpu, select_idx))
> >  		counter = lower_32_bits(counter);
> >  
> >  	return counter;
> > @@ -423,6 +429,23 @@ static void kvm_pmu_counter_increment(struct kvm_vcpu *vcpu,
> >  	}
> >  }
> >  
> > +/* Compute the sample period for a given counter value */
> > +static u64 compute_period(struct kvm_vcpu *vcpu, u64 select_idx, u64 counter)
> > +{
> > +	u64 val;
> > +
> > +	if (kvm_pmu_idx_is_64bit(vcpu, select_idx)) {
> > +		if (!kvm_pmu_idx_has_64bit_overflow(vcpu, select_idx))
> > +			val = -(counter & GENMASK(31, 0));
> 
> If I understand things correctly, this might be missing another mask:
> 
> +		if (!kvm_pmu_idx_has_64bit_overflow(vcpu, select_idx)) {
> +			val = -(counter & GENMASK(31, 0));
> +			val &= GENMASK(31, 0);
> +		} else {
> 
> For example, if the counter is 64-bits wide, it overflows at 32-bits,
> and it is _one_ sample away from overflowing at 32-bits:
> 
> 	0x01010101_ffffffff
> 
> Then "val = (-counter) & GENMASK(63, 0)" would return 0xffffffff_00000001.

Sorry, this should be:

	Then "val = -(counter & GENMASK(31, 0))" would return 0xffffffff_00000001.

> But the right period is 0x00000000_00000001 (it's one sample away from
> overflowing).
> 
> > +		else
> > +			val = (-counter) & GENMASK(63, 0);
> > +	} else {
> > +		val = (-counter) & GENMASK(31, 0);
> > +	}
> > +
> > +	return val;
> > +}
> > +
> >  /**
> >   * When the perf event overflows, set the overflow status and inform the vcpu.
> >   */
> > @@ -442,10 +465,7 @@ static void kvm_pmu_perf_overflow(struct perf_event *perf_event,
> >  	 * Reset the sample period to the architectural limit,
> >  	 * i.e. the point where the counter overflows.
> >  	 */
> > -	period = -(local64_read(&perf_event->count));
> > -
> > -	if (!kvm_pmu_idx_is_64bit(vcpu, pmc->idx))
> > -		period &= GENMASK(31, 0);
> > +	period = compute_period(vcpu, idx, local64_read(&perf_event->count));
> >  
> >  	local64_set(&perf_event->hw.period_left, 0);
> >  	perf_event->attr.sample_period = period;
> > @@ -571,14 +591,13 @@ static void kvm_pmu_create_perf_event(struct kvm_vcpu *vcpu, u64 select_idx)
> >  
> >  	/*
> >  	 * If counting with a 64bit counter, advertise it to the perf
> > -	 * code, carefully dealing with the initial sample period.
> > +	 * code, carefully dealing with the initial sample period
> > +	 * which also depends on the overflow.
> >  	 */
> > -	if (kvm_pmu_idx_is_64bit(vcpu, select_idx)) {
> > +	if (kvm_pmu_idx_is_64bit(vcpu, select_idx))
> >  		attr.config1 |= PERF_ATTR_CFG1_COUNTER_64BIT;
> > -		attr.sample_period = (-counter) & GENMASK(63, 0);
> > -	} else {
> > -		attr.sample_period = (-counter) & GENMASK(31, 0);
> > -	}
> > +
> > +	attr.sample_period = compute_period(vcpu, select_idx, counter);
> >  
> >  	event = perf_event_create_kernel_counter(&attr, -1, current,
> >  						 kvm_pmu_perf_overflow, pmc);
> > -- 
> > 2.34.1
> > 
> > 

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