linux-kernel - Re: [PATCH 06/54] x86/kvm: replace bitmap_weight with bitmap

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Date:   Mon, 24 Jan 2022 10:11:12 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Yury Norov <yury.norov@...il.com>
Cc:     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>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH 06/54] x86/kvm: replace bitmap_weight with bitmap_empty
 where appropriate

Yury Norov <yury.norov@...il.com> writes:

> 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>
> ---
>  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..2c3400dea4b3 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 would've preferred this written as 

	auto_eoi_old = !bitmap_empty(synic->auto_eoi_bitmap, 256);

so the variable would indicate wether AutoEOI was previosly enabled, not
disabled.

>  
>  	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);

Same here, of course. "auto_eoi_new = true" sounds like "AutoEOI is now
enabled".

>  
> -	if (!!auto_eoi_old == !!auto_eoi_new)
> +	if (auto_eoi_old == auto_eoi_new)
>  		return;
>  
>  	down_write(&vcpu->kvm->arch.apicv_update_lock);

The change look good to me otherwise, feel free to add

Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

-- 
Vitaly