linux-kernel - Re: [PATCH v3 3/8] KVM: x86/pmu: Rewrite reprogram

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Date:   Fri, 20 Jan 2023 01:09:53 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/8] KVM: x86/pmu: Rewrite reprogram_counters() to
 improve performance

On Fri, Nov 11, 2022, Like Xu wrote:
> From: Like Xu <likexu@...cent.com>
> 
> A valid pmc is always tested before using pmu->reprogram_pmi. Eliminate
> this part of the redundancy by setting the counter's bitmask directly,
> and in addition, trigger KVM_REQ_PMU only once to save more cpu cycles.
> 
> Signed-off-by: Like Xu <likexu@...cent.com>
> ---
>  arch/x86/kvm/pmu.h           | 11 +++++++++++
>  arch/x86/kvm/vmx/pmu_intel.c | 12 ------------
>  2 files changed, 11 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/x86/kvm/pmu.h b/arch/x86/kvm/pmu.h
> index 2b5376ba66ea..be552c8217a0 100644
> --- a/arch/x86/kvm/pmu.h
> +++ b/arch/x86/kvm/pmu.h
> @@ -189,6 +189,17 @@ static inline void kvm_pmu_request_counter_reprogam(struct kvm_pmc *pmc)
>  	kvm_make_request(KVM_REQ_PMU, pmc->vcpu);
>  }
>  
> +static inline void reprogram_counters(struct kvm_pmu *pmu, u64 diff)
> +{
> +	int bit;
> +
> +	if (diff) {

Prefer this, it's slightly cleaner:

	if (!diff)
		return;

> +		for_each_set_bit(bit, (unsigned long *)&diff, X86_PMC_IDX_MAX)
> +			__set_bit(bit, pmu->reprogram_pmi);

Needs to use the atomic set_bit().