linux-kernel - Re: [PATCH] KVM: avoid unnecessary bitmap clear ops

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Date:   Fri, 25 Oct 2019 11:42:47 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Miaohe Lin <linmiaohe@...wei.com>, rkrcmar@...hat.com,
        sean.j.christopherson@...el.com, vkuznets@...hat.com,
        wanpengli@...cent.com, jmattson@...gle.com, joro@...tes.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com
Cc:     x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: avoid unnecessary bitmap clear ops

On 25/10/19 04:24, Miaohe Lin wrote:
> When set one bit in bitmap, there is no need to
> clear it before.

Hi,

in general the Linux coding style prefers:

	a = x;
	if (...);
		a = y;

to

	if (...)
		a = y;
	else
		a = x;

which is why these lines were written this way.

Thanks,

Paolo

> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> ---
>  arch/x86/kvm/svm.c | 3 ++-
>  arch/x86/kvm/x86.c | 3 ++-
>  2 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index ca200b50cde4..d997d011a942 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -2044,9 +2044,10 @@ static void avic_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>  	entry &= ~AVIC_PHYSICAL_ID_ENTRY_HOST_PHYSICAL_ID_MASK;
>  	entry |= (h_physical_id & AVIC_PHYSICAL_ID_ENTRY_HOST_PHYSICAL_ID_MASK);
>  
> -	entry &= ~AVIC_PHYSICAL_ID_ENTRY_IS_RUNNING_MASK;
>  	if (svm->avic_is_running)
>  		entry |= AVIC_PHYSICAL_ID_ENTRY_IS_RUNNING_MASK;
> +	else
> +		entry &= ~AVIC_PHYSICAL_ID_ENTRY_IS_RUNNING_MASK;
>  
>  	WRITE_ONCE(*(svm->avic_physical_id_cache), entry);
>  	avic_update_iommu_vcpu_affinity(vcpu, h_physical_id,
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index ff395f812719..9b535888ea90 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1036,9 +1036,10 @@ static void kvm_update_dr7(struct kvm_vcpu *vcpu)
>  	else
>  		dr7 = vcpu->arch.dr7;
>  	kvm_x86_ops->set_dr7(vcpu, dr7);
> -	vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_BP_ENABLED;
>  	if (dr7 & DR7_BP_EN_MASK)
>  		vcpu->arch.switch_db_regs |= KVM_DEBUGREG_BP_ENABLED;
> +	else
> +		vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_BP_ENABLED;
>  }
>  
>  static u64 kvm_dr6_fixed(struct kvm_vcpu *vcpu)
>