Re: [PATCH v9 4/5] KVM: arm/arm64: remove pmc->bitmask

[Julien Thierry <julien.thierry@arm.com>]

Hi Andrew,

On 12/06/2019 20:04, 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    | 19 +++++++++----------
>  2 files changed, 9 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..88ce24ae0b45 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);

Shouldn't this depend on PMCR.LC as well? If PMCR.LC is clear we only
want the lower 32bits of the cycle counter.

Cheers,

-- 
Julien Thierry

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