Re: [PATCH 07/49] KVM: x86: replace bitmap_weight with bitmap_empty where appropriate

From: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
To: "Yury Norov" <yury.norov@gmail.com>,
	"Andy Shevchenko" <andriy.shevchenko@linux.intel.com>,
	"Rasmus Villemoes" <linux@rasmusvillemoes.dk>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Michał Mirosław" <mirq-linux@rere.qmqm.pl>,
	"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
	"Peter Zijlstra" <peterz@infradead.org>,
	"David Laight" <David.Laight@aculab.com>,
	"Joe Perches" <joe@perches.com>,
	"Dennis Zhou" <dennis@kernel.org>,
	"Emil Renner Berthing" <kernel@esmil.dk>,
	"Nicholas Piggin" <npiggin@gmail.com>,
	"Matti Vaittinen" <matti.vaittinen@fi.rohmeurope.com>,
	"Alexey Klimov" <aklimov@redhat.com>,
	linux-kernel@vger.kernel.org,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Sean Christopherson" <seanjc@google.com>,
	"Vitaly Kuznetsov" <vkuznets@redhat.com>
Subject: Re: [PATCH 07/49] KVM: x86: replace bitmap_weight with bitmap_empty where appropriate
Date: Fri, 11 Feb 2022 18:13:03 +0100	[thread overview]
Message-ID: <ea4efad6-a15c-3749-f177-640396b52857@wanadoo.fr> (raw)
In-Reply-To: <20220210224933.379149-8-yury.norov@gmail.com>

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@gmail.com>
> Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.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);