Re: [PATCH v3 3/4] KVM: arm64: pmu: Set the CHAINED attribute before creating the in-kernel event

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

On Fri, Oct 11, 2019 at 01:39:53PM +0100, Marc Zyngier wrote:
> The current convention for KVM to request a chained event from the
> host PMU is to set bit[0] in attr.config1 (PERF_ATTR_CFG1_KVM_PMU_CHAINED).
> 
> But as it turns out, this bit gets set *after* we create the kernel
> event that backs our virtual counter, meaning that we never get
> a 64bit counter.
> 
> Moving the setting to an earlier point solves the problem.
> 
> Fixes: 80f393a23be6 ("KVM: arm/arm64: Support chained PMU counters")
> Signed-off-by: Marc Zyngier <maz@kernel.org>

Reviewed-by: Andrew Murray <andrew.murray@arm.com>

> ---
>  virt/kvm/arm/pmu.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/virt/kvm/arm/pmu.c b/virt/kvm/arm/pmu.c
> index c30c3a74fc7f..f291d4ac3519 100644
> --- a/virt/kvm/arm/pmu.c
> +++ b/virt/kvm/arm/pmu.c
> @@ -569,12 +569,12 @@ static void kvm_pmu_create_perf_event(struct kvm_vcpu *vcpu, u64 select_idx)
>  		 * high counter.
>  		 */
>  		attr.sample_period = (-counter) & GENMASK(63, 0);
> +		if (kvm_pmu_counter_is_enabled(vcpu, pmc->idx + 1))
> +			attr.config1 |= PERF_ATTR_CFG1_KVM_PMU_CHAINED;
> +
>  		event = perf_event_create_kernel_counter(&attr, -1, current,
>  							 kvm_pmu_perf_overflow,
>  							 pmc + 1);
> -
> -		if (kvm_pmu_counter_is_enabled(vcpu, pmc->idx + 1))
> -			attr.config1 |= PERF_ATTR_CFG1_KVM_PMU_CHAINED;
>  	} else {
>  		/* The initial sample period (overflow count) of an event. */
>  		if (kvm_pmu_idx_is_64bit(vcpu, pmc->idx))
> -- 
> 2.20.1
> 
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