linux-kernel - Re: [PATCH 07/49] KVM: x86: replace bitmap_weight with bitmap

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Date:   Fri, 11 Feb 2022 18:13:03 +0100
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     Yury Norov <yury.norov@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michał Mirosław <mirq-linux@...e.qmqm.pl>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        David Laight <David.Laight@...lab.com>,
        Joe Perches <joe@...ches.com>, Dennis Zhou <dennis@...nel.org>,
        Emil Renner Berthing <kernel@...il.dk>,
        Nicholas Piggin <npiggin@...il.com>,
        Matti Vaittinen <matti.vaittinen@...rohmeurope.com>,
        Alexey Klimov <aklimov@...hat.com>,
        linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH 07/49] KVM: x86: replace bitmap_weight with bitmap_empty
 where appropriate

Le 10/02/2022 à 23:48, Yury Norov a écrit :
> In some places kvm/hyperv.c code calls bitmap_weight() to check if any bit
> of a given bitmap is set. It's better to use bitmap_empty() in that case
> because bitmap_empty() stops traversing the bitmap as soon as it finds
> first set bit, while bitmap_weight() counts all bits unconditionally.
> 
> Signed-off-by: Yury Norov <yury.norov@...il.com>
> Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
>   arch/x86/kvm/hyperv.c | 8 ++++----
>   1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> index 6e38a7d22e97..06c2a5603123 100644
> --- a/arch/x86/kvm/hyperv.c
> +++ b/arch/x86/kvm/hyperv.c
> @@ -90,7 +90,7 @@ static void synic_update_vector(struct kvm_vcpu_hv_synic *synic,
>   {
>   	struct kvm_vcpu *vcpu = hv_synic_to_vcpu(synic);
>   	struct kvm_hv *hv = to_kvm_hv(vcpu->kvm);
> -	int auto_eoi_old, auto_eoi_new;
> +	bool auto_eoi_old, auto_eoi_new;
>   
>   	if (vector < HV_SYNIC_FIRST_VALID_VECTOR)
>   		return;
> @@ -100,16 +100,16 @@ static void synic_update_vector(struct kvm_vcpu_hv_synic *synic,
>   	else
>   		__clear_bit(vector, synic->vec_bitmap);
>   
> -	auto_eoi_old = bitmap_weight(synic->auto_eoi_bitmap, 256);
> +	auto_eoi_old = !bitmap_empty(synic->auto_eoi_bitmap, 256);

I think that you can also remove the "!" here, ...

>   
>   	if (synic_has_vector_auto_eoi(synic, vector))
>   		__set_bit(vector, synic->auto_eoi_bitmap);
>   	else
>   		__clear_bit(vector, synic->auto_eoi_bitmap);
>   
> -	auto_eoi_new = bitmap_weight(synic->auto_eoi_bitmap, 256);
> +	auto_eoi_new = !bitmap_empty(synic->auto_eoi_bitmap, 256);

... and there...

>   
> -	if (!!auto_eoi_old == !!auto_eoi_new)
> +	if (auto_eoi_old == auto_eoi_new)

... because this test would still give the same result.

Just my 2c,
CJ

>   		return;
>   
>   	down_write(&vcpu->kvm->arch.apicv_update_lock);