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Subject[PATCH v2 6/9] KVM: LAPIC: Reduce world switch latency caused by timer_advance_ns
Date
From: Wanpeng Li <wanpengli@tencent.com>

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: Wanpeng Li <wanpengli@tencent.com>
---
arch/x86/kvm/lapic.c | 11 +++++------
arch/x86/kvm/svm/svm.c | 4 +---
arch/x86/kvm/vmx/vmx.c | 4 +---
3 files changed, 7 insertions(+), 12 deletions(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 3b32d3b..51ed4f0 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -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;
}
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 0194336..19e622a 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3456,9 +3456,7 @@ 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);
+ kvm_wait_lapic_expire(vcpu);

/*
* If this vCPU has touched SPEC_CTRL, restore the guest's value if
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index a544351..d6e1656 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6800,9 +6800,7 @@ static fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu)
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);
+ kvm_wait_lapic_expire(vcpu);

/*
* If this vCPU has touched SPEC_CTRL, restore the guest's value if
--
2.7.4
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