linux-kernel - Re: [PATCH 4/4] KVM: VMX: Register posted interrupt wakeup handler iff APICv is enabled

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Message-ID: <4b81af0a-559e-b8f5-ab8a-167c88fec2d5@redhat.com>
Date:   Thu, 21 Oct 2021 21:43:31 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org
Cc:     "H. Peter Anvin" <hpa@...or.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH 4/4] KVM: VMX: Register posted interrupt wakeup handler
 iff APICv is enabled

On 09/10/21 02:11, Sean Christopherson wrote:
> Don't bother registering a posted interrupt wakeup handler if APICv is
> disabled, KVM utilizes the wakeup vector if and only if APICv is enabled.
> Practically speaking, there's no meaningful functional change as KVM's
> wakeup handler is a glorified nop if there are no vCPUs using posted
> interrupts, not to mention that nothing in the system should be sending
> wakeup interrupts when APICv is disabled.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>   arch/x86/kvm/vmx/vmx.c | 6 ++++--
>   1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 9164f1870d49..df9ad4675215 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -7553,7 +7553,8 @@ static void vmx_migrate_timers(struct kvm_vcpu *vcpu)
>   
>   static void hardware_unsetup(void)
>   {
> -	kvm_unregister_posted_intr_wakeup_handler(pi_wakeup_handler);
> +	if (enable_apicv)
> +		kvm_unregister_posted_intr_wakeup_handler(pi_wakeup_handler);
>   
>   	if (nested)
>   		nested_vmx_hardware_unsetup();
> @@ -7907,7 +7908,8 @@ static __init int hardware_setup(void)
>   	if (r)
>   		nested_vmx_hardware_unsetup();
>   
> -	kvm_register_posted_intr_wakeup_handler(pi_wakeup_handler);
> +	if (enable_apicv)
> +		kvm_register_posted_intr_wakeup_handler(pi_wakeup_handler);
>   
>   	return r;
>   }

Also adds unnecessary complexity (you have to ensure the condition 
cannot change from one call to the other), so I am only queuing patches 
1 and 2.

Paolo