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

Radim Krčmář
2017-02-07T21:49:56Z

2017-02-07 21:19+0100, Radim Krčmář:
> 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?

The following patch on top of the whole series survives kvm-unit-tests
and very mimimal testing: (Not sure if I have missed something.)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index a1e9cab7d01f..8b98c1681803 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -577,10 +577,10 @@ static void pv_eoi_clr_pending(struct kvm_vcpu *vcpu)

static int apic_has_interrupt_for_ppr(struct kvm_lapic *apic, u32 ppr)
{
- int highest_irr;
+ int highest_irr = -1;
if (kvm_x86_ops->sync_pir_to_irr)
highest_irr = kvm_x86_ops->sync_pir_to_irr(apic->vcpu);
- else
+ if (highest_irr == -1)
highest_irr = apic_find_highest_irr(apic);
if (highest_irr == -1 || (highest_irr & 0xF0) <= ppr)
return -1;
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 9c8a16edf88d..637c7bd2f3ab 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -8728,18 +8728,18 @@ static int vmx_sync_pir_to_irr(struct kvm_vcpu *vcpu)
struct vcpu_vmx *vmx = to_vmx(vcpu);
int max_irr;

- 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);
- }
+ if (!vcpu->arch.apicv_active || !pi_test_on(&vmx->pi_desc))
+ return -1;
+
+ 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);
vmx_hwapic_irr_update(vcpu, max_irr);
+
return max_irr;
}

@@ -11145,6 +11145,7 @@ static void nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 exit_reason,

/* in case we halted in L2 */
vcpu->arch.mp_state = KVM_MP_STATE_RUNNABLE;
+ kvm_x86_ops->hwapic_irr_update(vcpu, kvm_lapic_find_highest_irr(vcpu));
}

/*