Commit 010fd37f authored by Wanpeng Li's avatar Wanpeng Li Committed by Paolo Bonzini
Browse files

KVM: LAPIC: Reduce world switch latency caused by timer_advance_ns 



All the checks in lapic_timer_int_injected(), __kvm_wait_lapic_expire(), and
these function calls waste cpu cycles when the timer mode is not tscdeadline.
We can observe ~1.3% world switch time overhead by kvm-unit-tests/vmexit.flat
vmcall testing on AMD server. This patch reduces the world switch latency
caused by timer_advance_ns feature when the timer mode is not tscdeadline by
simpling move the check against apic->lapic_timer.expired_tscdeadline much
earlier.

Signed-off-by: default avatarWanpeng Li <wanpengli@tencent.com>
Message-Id: <1599731444-3525-7-git-send-email-wanpengli@tencent.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 68ca7663

arch/x86/kvm/lapic.c

+5 −6
Original line number Diff line number Diff line
@@ -1582,9 +1582,6 @@ static void __kvm_wait_lapic_expire(struct kvm_vcpu *vcpu) 
	struct kvm_lapic *apic = vcpu->arch.apic;

	u64 guest_tsc, tsc_deadline;



	if (apic->lapic_timer.expired_tscdeadline == 0)

		return;



	tsc_deadline = apic->lapic_timer.expired_tscdeadline;

	apic->lapic_timer.expired_tscdeadline = 0;

	guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc());
@@ -1599,7 +1596,10 @@ static void __kvm_wait_lapic_expire(struct kvm_vcpu *vcpu) 


void kvm_wait_lapic_expire(struct kvm_vcpu *vcpu)

{

	if (lapic_timer_int_injected(vcpu))

	if (lapic_in_kernel(vcpu) &&

	    vcpu->arch.apic->lapic_timer.expired_tscdeadline &&

	    vcpu->arch.apic->lapic_timer.timer_advance_ns &&

	    lapic_timer_int_injected(vcpu))

		__kvm_wait_lapic_expire(vcpu);

}

EXPORT_SYMBOL_GPL(kvm_wait_lapic_expire);
@@ -1635,8 +1635,7 @@ static void apic_timer_expired(struct kvm_lapic *apic, bool from_timer_fn) 
	}



	if (kvm_use_posted_timer_interrupt(apic->vcpu)) {

		if (apic->lapic_timer.timer_advance_ns)

			__kvm_wait_lapic_expire(vcpu);

		kvm_wait_lapic_expire(vcpu);

		kvm_apic_inject_pending_timer_irqs(apic);

		return;

	}

arch/x86/kvm/svm/svm.c

+1 −3
Original line number Diff line number Diff line
@@ -3454,8 +3454,6 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu) 
	clgi();

	kvm_load_guest_xsave_state(vcpu);



	if (lapic_in_kernel(vcpu) &&

		vcpu->arch.apic->lapic_timer.timer_advance_ns)

	kvm_wait_lapic_expire(vcpu);



	/*

arch/x86/kvm/vmx/vmx.c

+1 −3
Original line number Diff line number Diff line
@@ -6800,8 +6800,6 @@ reenter_guest: 
	if (enable_preemption_timer)

		vmx_update_hv_timer(vcpu);



	if (lapic_in_kernel(vcpu) &&

		vcpu->arch.apic->lapic_timer.timer_advance_ns)

	kvm_wait_lapic_expire(vcpu);



	/*

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