linux-kernel - Re: [PATCH] KVM: apic: reuse smp_wmb() in kvm_make

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Date:   Thu, 06 Feb 2020 11:47:02 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     linmiaohe <linmiaohe@...wei.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org, x86@...nel.org,
        pbonzini@...hat.com, rkrcmar@...hat.com,
        sean.j.christopherson@...el.com, wanpengli@...cent.com,
        jmattson@...gle.com, joro@...tes.org, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, hpa@...or.com
Subject: Re: [PATCH] KVM: apic: reuse smp_wmb() in kvm_make_request()

linmiaohe <linmiaohe@...wei.com> writes:

> From: Miaohe Lin <linmiaohe@...wei.com>
>
> There is already an smp_mb() barrier in kvm_make_request(). We reuse it
> here.
>
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> ---
>  arch/x86/kvm/lapic.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index eafc631d305c..ea871206a370 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1080,9 +1080,12 @@ static int __apic_accept_irq(struct kvm_lapic *apic, int delivery_mode,
>  			result = 1;
>  			/* assumes that there are only KVM_APIC_INIT/SIPI */
>  			apic->pending_events = (1UL << KVM_APIC_INIT);
> -			/* make sure pending_events is visible before sending
> -			 * the request */
> -			smp_wmb();
> +			/*
> +			 * Make sure pending_events is visible before sending
> +			 * the request.
> +			 * There is already an smp_wmb() in kvm_make_request(),
> +			 * we reuse that barrier here.
> +			 */

Let me suggest an alternative wording,

"kvm_make_request() provides smp_wmb() so pending_events changes are
guaranteed to be visible"

But there is nothing wrong with yours, it's just longer than it could be
:-)

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

>  			kvm_make_request(KVM_REQ_EVENT, vcpu);
>  			kvm_vcpu_kick(vcpu);
>  		}

-- 
Vitaly