linux-kernel - Re: [PATCH 5/6] KVM: x86: do not scan IRR twice on APICv vmentry

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Date:   Tue, 7 Feb 2017 21:19:55 +0100
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH 5/6] KVM: x86: do not scan IRR twice on APICv vmentry

2016-12-19 17:17+0100, Paolo Bonzini:
> Calls to apic_find_highest_irr are scanning IRR twice, once
> in vmx_sync_pir_from_irr and once in apic_search_irr.  Change
> sync_pir_from_irr to get the new maximum IRR from kvm_apic_update_irr;
> now that it does the computation, it can also do the RVI write.
> 
> In order to avoid complications in svm.c, make the callback optional.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> @@ -8734,20 +8736,24 @@ static void vmx_hwapic_irr_update(struct kvm_vcpu *vcpu, int max_irr)
>  	}
>  }
>  
> -static void vmx_sync_pir_to_irr(struct kvm_vcpu *vcpu)
> +static int vmx_sync_pir_to_irr(struct kvm_vcpu *vcpu)
>  {
>  	struct vcpu_vmx *vmx = to_vmx(vcpu);
> +	int max_irr;
>  
> -	if (!pi_test_on(&vmx->pi_desc))
> -		return;
> -
> -	pi_clear_on(&vmx->pi_desc);
> -	/*
> -	 * IOMMU can write to PIR.ON, so the barrier matters even on UP.
> -	 * But on x86 this is just a compiler barrier anyway.
> -	 */
> -	smp_mb__after_atomic();
> -	kvm_apic_update_irr(vcpu, vmx->pi_desc.pir);
> +	if (vcpu->arch.apicv_active && pi_test_on(&vmx->pi_desc)) {
> +		pi_clear_on(&vmx->pi_desc);
> +		/*
> +		 * IOMMU can write to PIR.ON, so the barrier matters even on UP.
> +		 * But on x86 this is just a compiler barrier anyway.
> +		 */
> +		smp_mb__after_atomic();
> +		max_irr = kvm_apic_update_irr(vcpu, vmx->pi_desc.pir);
> +	} else {
> +		max_irr = kvm_lapic_find_highest_irr(vcpu);
> +	}
> +	vmx_hwapic_irr_update(vcpu, max_irr);

Btw. a v1 discussion revolved about the need to have
vmx_hwapic_irr_update() here when the maximal IRR should always be in
RVI, and, uh, I didn't follow up (negligible attention span) ...

There is one place where that doesn't hold: we don't update RVI after a
EXTERNAL_INTERRUPT nested VM exit without VM_EXIT_ACK_INTR_ON_EXIT, but
IRR has likely changed.  Isn't that the problem?