linux-kernel - Re: [PATCH 1/5] KVM: arm64: Divorce the perf code from oprofile helpers

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Date:   Thu, 15 Apr 2021 14:59:26 +0800
From:   Keqian Zhu <zhukeqian1@...wei.com>
To:     Marc Zyngier <maz@...nel.org>, <kvm@...r.kernel.org>,
        <kvmarm@...ts.cs.columbia.edu>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <linux-s390@...r.kernel.org>,
        <linux-sh@...r.kernel.org>
CC:     Rich Felker <dalias@...c.org>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Peter Zijlstra <peterz@...radead.org>,
        "Viresh Kumar" <viresh.kumar@...aro.org>,
        Heiko Carstens <hca@...ux.ibm.com>,
        "Arnaldo Carvalho de Melo" <acme@...nel.org>, <nathan@...nel.org>,
        "Christian Borntraeger" <borntraeger@...ibm.com>,
        <kernel-team@...roid.com>, Will Deacon <will@...nel.org>
Subject: Re: [PATCH 1/5] KVM: arm64: Divorce the perf code from oprofile
 helpers

Hi Marc,

On 2021/4/14 21:44, Marc Zyngier wrote:
> KVM/arm64 is the sole user of perf_num_counters(), and really
> could do without it. Stop using the obsolete API by relying on
> the existing probing code.
> 
> Signed-off-by: Marc Zyngier <maz@...nel.org>
> ---
>  arch/arm64/kvm/perf.c     | 7 +------
>  arch/arm64/kvm/pmu-emul.c | 2 +-
>  include/kvm/arm_pmu.h     | 4 ++++
>  3 files changed, 6 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/arm64/kvm/perf.c b/arch/arm64/kvm/perf.c
> index 739164324afe..b8b398670ef2 100644
> --- a/arch/arm64/kvm/perf.c
> +++ b/arch/arm64/kvm/perf.c
> @@ -50,12 +50,7 @@ static struct perf_guest_info_callbacks kvm_guest_cbs = {
>  
>  int kvm_perf_init(void)
>  {
> -	/*
> -	 * Check if HW_PERF_EVENTS are supported by checking the number of
> -	 * hardware performance counters. This could ensure the presence of
> -	 * a physical PMU and CONFIG_PERF_EVENT is selected.
> -	 */
> -	if (IS_ENABLED(CONFIG_ARM_PMU) && perf_num_counters() > 0)
> +	if (kvm_pmu_probe_pmuver() != 0xf)
The probe() function may be called many times (kvm_arm_pmu_v3_set_attr also calls it).
I don't know whether the first calling is enough. If so, can we use a static variable
in it, so the following calling can return the result right away?

Thanks,
Keqian