linux-kernel - Re: [PATCH ] kvm: x86: Reduce unnecessary function call

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Message-ID: <Y0BcGnabCp9ukxDs@google.com>
Date:   Fri, 7 Oct 2022 17:04:26 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Hao Peng <flyingpenghao@...il.com>
Cc:     pbonzini@...hat.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH ] kvm: x86: Reduce unnecessary function call

On Fri, Oct 07, 2022, Hao Peng wrote:
> From: Peng Hao <flyingpeng@...cent.com>
> 
> kvm->lock is held very close to mutex_is_locked(kvm->lock).
> Do not need to call mutex_is_locked.
> 
> Signed-off-by: Peng Hao <flyingpeng@...cent.com>
> ---
>  arch/x86/kvm/pmu.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> index 02f9e4f245bd..8a7dbe2c469a 100644
> --- a/arch/x86/kvm/pmu.c
> +++ b/arch/x86/kvm/pmu.c
> @@ -601,8 +601,7 @@ int kvm_vm_ioctl_set_pmu_event_filter(struct kvm
> *kvm, void __user *argp)
>         sort(&filter->events, filter->nevents, sizeof(__u64), cmp_u64, NULL);
> 
>         mutex_lock(&kvm->lock);
> -       filter = rcu_replace_pointer(kvm->arch.pmu_event_filter, filter,
> -                                    mutex_is_locked(&kvm->lock));
> +       filter = rcu_replace_pointer(kvm->arch.pmu_event_filter, filter, 1);

I'd prefer to keep the mutex_is_locked() call, even though it's quite silly, as
it self-documents what is being used to protect writes to pmu_event_filter.
The third paramter is evaluated iff CONFIG_PROVE_RCU=y, which is the complete
oppositive of performance sensitive, so in practice there's no real downside to
the somewhat superfluous call.

>         mutex_unlock(&kvm->lock);
> 
>         synchronize_srcu_expedited(&kvm->srcu);
> --
> 2.27.0