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

Paolo Bonzini
2017-02-08T14:18:14Z

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@redhat.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