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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: Wanpeng Li <wanpengli@tencent.com>
Message-Id: <1599731444-3525-7-git-send-email-wanpengli@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Wanpeng Li 2020-09-10 17:50:41 +08:00 committed by Paolo Bonzini
parent 68ca7663c7
commit 010fd37fdd
3 changed files with 7 additions and 12 deletions
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11
arch/x86/kvm/lapic.c
View File

@ -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;
}

4
arch/x86/kvm/svm/svm.c
View File

@ -3454,9 +3454,7 @@ 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

4
arch/x86/kvm/vmx/vmx.c
View File

@ -6800,9 +6800,7 @@ 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);
kvm_wait_lapic_expire(vcpu);
/*
* If this vCPU has touched SPEC_CTRL, restore the guest's value if