linux-kernel - Re: [PATCH] svm/avic: Do not send AVIC doorbell to self

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Message-ID: <ca64dd06-df77-050a-92ff-5d5448382390@amazon.com>
Date:   Tue, 7 May 2019 15:16:15 +0200
From:   "Graf, Alexander" <graf@...zon.com>
To:     "Suthikulpanit, Suravee" <Suravee.Suthikulpanit@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
CC:     "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "joro@...tes.org" <joro@...tes.org>
Subject: Re: [PATCH] svm/avic: Do not send AVIC doorbell to self

On 03.05.19 15:38, Suthikulpanit, Suravee wrote:
> AVIC doorbell is used to notify a running vCPU that interrupts
> has been injected into the vCPU AVIC backing page. Current logic
> checks only if a VCPU is running before sending a doorbell.
> However, the doorbell is not necessary if the destination
> CPU is itself.
> 
> Add logic to check currently running CPU before sending doorbell.
> 
> Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@....com>

Reviewed-by: Alexander Graf <graf@...zon.com>

> ---
>   arch/x86/kvm/svm.c | 11 +++++++----
>   1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 122788f..4bbf6fc 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -5283,10 +5283,13 @@ static void svm_deliver_avic_intr(struct kvm_vcpu *vcpu, int vec)
>   	kvm_lapic_set_irr(vec, vcpu->arch.apic);
>   	smp_mb__after_atomic();
>   
> -	if (avic_vcpu_is_running(vcpu))
> -		wrmsrl(SVM_AVIC_DOORBELL,
> -		       kvm_cpu_get_apicid(vcpu->cpu));
> -	else
> +	if (avic_vcpu_is_running(vcpu)) { > +		int cpuid = vcpu->cpu;
> +
> +		if (cpuid != get_cpu())

Tiny nitpick: What would keep you from checking vcpu->cpu directly here?

Alex

> +			wrmsrl(SVM_AVIC_DOORBELL, kvm_cpu_get_apicid(cpuid));
> +		put_cpu();
> +	} else
>   		kvm_vcpu_wake_up(vcpu);
>   }
>   
>