linux-kernel - [PATCH v1] KVM: x86: avoid redundant REQ

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Date:   Mon, 12 Dec 2016 17:02:23 +0300
From:   Denis Plotnikov <dplotnikov@...tuozzo.com>
To:     rkagan@...tuozzo.com, den@...tuozzo.com, pbonzini@...hat.com,
        rkrcmar@...hat.com
Cc:     svt-core@...ts.sw.ru, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH v1] KVM: x86: avoid redundant REQ_EVENT

When processing KVM_REQ_EVENT, apic_update_ppr is called which may set
KVM_REQ_EVENT again if the recalculated value of PPR becomes smaller
than the previous one. This results in cancelling the guest entry and
reiterating in vcpu_enter_guest.

However this is unnecessary because at this point KVM_REQ_EVENT is
already being processed and there are no other changes in the lapic
that may require full-fledged state recalculation.

This situation is often hit on systems with TPR shadow, where the
TPR can be updated by the guest without a vmexit, so that the first
apic_update_ppr to notice it is exactly the one called while
processing KVM_REQ_EVENT.

To avoid it, introduce a parameter in apic_update_ppr allowing to
suppress setting of KVM_REQ_EVENT, and use it on the paths called from
KVM_REQ_EVENT processing.

This microoptimization gives 10% performance increase on a synthetic
test doing a lot of IPC in Windows using window messages.

Reviewed-by: Roman Kagan <rkagan@...tuozzo.com>
Signed-off-by: Denis Plotnikov <dplotnikov@...tuozzo.com>
---
 arch/x86/kvm/lapic.c | 20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 6f69340..b3025d8 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -544,7 +544,7 @@ static void pv_eoi_clr_pending(struct kvm_vcpu *vcpu)
 	__clear_bit(KVM_APIC_PV_EOI_PENDING, &vcpu->arch.apic_attention);
 }
 
-static void apic_update_ppr(struct kvm_lapic *apic)
+static void apic_update_ppr(struct kvm_lapic *apic, bool make_req)
 {
 	u32 tpr, isrv, ppr, old_ppr;
 	int isr;
@@ -564,7 +564,7 @@ static void apic_update_ppr(struct kvm_lapic *apic)
 
 	if (old_ppr != ppr) {
 		kvm_lapic_set_reg(apic, APIC_PROCPRI, ppr);
-		if (ppr < old_ppr)
+		if (make_req && ppr < old_ppr)
 			kvm_make_request(KVM_REQ_EVENT, apic->vcpu);
 	}
 }
@@ -572,7 +572,7 @@ static void apic_update_ppr(struct kvm_lapic *apic)
 static void apic_set_tpr(struct kvm_lapic *apic, u32 tpr)
 {
 	kvm_lapic_set_reg(apic, APIC_TASKPRI, tpr);
-	apic_update_ppr(apic);
+	apic_update_ppr(apic, true);
 }
 
 static bool kvm_apic_broadcast(struct kvm_lapic *apic, u32 mda)
@@ -1032,7 +1032,7 @@ static int apic_set_eoi(struct kvm_lapic *apic)
 		return vector;
 
 	apic_clear_isr(vector, apic);
-	apic_update_ppr(apic);
+	apic_update_ppr(apic, true);
 
 	if (test_bit(vector, vcpu_to_synic(apic->vcpu)->vec_bitmap))
 		kvm_hv_synic_send_eoi(apic->vcpu, vector);
@@ -1147,7 +1147,7 @@ static u32 __apic_read(struct kvm_lapic *apic, unsigned int offset)
 		val = apic_get_tmcct(apic);
 		break;
 	case APIC_PROCPRI:
-		apic_update_ppr(apic);
+		apic_update_ppr(apic, true);
 		val = kvm_lapic_get_reg(apic, offset);
 		break;
 	case APIC_TASKPRI:
@@ -1841,7 +1841,7 @@ void kvm_lapic_reset(struct kvm_vcpu *vcpu, bool init_event)
 		kvm_lapic_set_base(vcpu,
 				vcpu->arch.apic_base | MSR_IA32_APICBASE_BSP);
 	vcpu->arch.pv_eoi.msr_val = 0;
-	apic_update_ppr(apic);
+	apic_update_ppr(apic, true);
 
 	vcpu->arch.apic_arb_prio = 0;
 	vcpu->arch.apic_attention = 0;
@@ -1964,7 +1964,7 @@ int kvm_apic_has_interrupt(struct kvm_vcpu *vcpu)
 	if (!apic_enabled(apic))
 		return -1;
 
-	apic_update_ppr(apic);
+	apic_update_ppr(apic, false);
 	highest_irr = apic_find_highest_irr(apic);
 	if ((highest_irr == -1) ||
 	    ((highest_irr & 0xF0) <= kvm_lapic_get_reg(apic, APIC_PROCPRI)))
@@ -2013,12 +2013,12 @@ int kvm_get_apic_interrupt(struct kvm_vcpu *vcpu)
 	 */
 
 	apic_set_isr(vector, apic);
-	apic_update_ppr(apic);
+	apic_update_ppr(apic, true);
 	apic_clear_irr(vector, apic);
 
 	if (test_bit(vector, vcpu_to_synic(vcpu)->auto_eoi_bitmap)) {
 		apic_clear_isr(vector, apic);
-		apic_update_ppr(apic);
+		apic_update_ppr(apic, true);
 	}
 
 	return vector;
@@ -2068,7 +2068,7 @@ int kvm_apic_set_state(struct kvm_vcpu *vcpu, struct kvm_lapic_state *s)
 	recalculate_apic_map(vcpu->kvm);
 	kvm_apic_set_version(vcpu);
 
-	apic_update_ppr(apic);
+	apic_update_ppr(apic, true);
 	hrtimer_cancel(&apic->lapic_timer.timer);
 	apic_update_lvtt(apic);
 	apic_manage_nmi_watchdog(apic, kvm_lapic_get_reg(apic, APIC_LVT0));
-- 
2.10.1.352.g0cf3611