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

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Date:   Wed, 8 Feb 2017 15:10:49 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Radim Krčmář <rkrcmar@...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



On 07/02/2017 21:19, Radim Krčmář wrote:
> 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?

I'm not sure... there shouldn't be any issue with missed RVI updates in
this series, since it does

	if (kvm_lapic_enabled(vcpu)) {
		/*
		 * This handles the case where a posted interrupt was
		 * notified with kvm_vcpu_kick.
		 */
		if (kvm_x86_ops->sync_pir_to_irr)
			kvm_x86_ops->sync_pir_to_irr(vcpu);
	}

on every VM entry (and kvm_lapic_find_highest_irr inside the callback).
That is not something I really like, but it's no worse than what was
there before

                if (vcpu->arch.apicv_active)
                        kvm_x86_ops->hwapic_irr_update(vcpu,
                                kvm_lapic_find_highest_irr(vcpu));
        }

and obviously better than going unnecessarily through KVM_REQ_EVENT
processing.

Paolo