KVM: LAPIC: Optimize timer latency further

Advance lapic timer tries to hidden the hypervisor overhead between the
host emulated timer fires and the guest awares the timer is fired. However,
it just hidden the time between apic_timer_fn/handle_preemption_timer ->
wait_lapic_expire, instead of the real position of vmentry which is
mentioned in the orignial commit d0659d946be0 ("KVM: x86: add option to
advance tscdeadline hrtimer expiration"). There is 700+ cpu cycles between
the end of wait_lapic_expire and before world switch on my haswell desktop.

This patch tries to narrow the last gap(wait_lapic_expire -> world switch),
it takes the real overhead time between apic_timer_fn/handle_preemption_timer
and before world switch into consideration when adaptively tuning timer
advancement. The patch can reduce 40% latency (~1600+ cycles to ~1000+ cycles
on a haswell desktop) for kvm-unit-tests/tscdeadline_latency when testing
busy waits.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Liran Alon <liran.alon@oracle.com>
Reviewed-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index f8615872ae64fc86169c12c58ed420be8e9b3489..fcf42a34079027dd2a68a566d3f804f27417ee08 100644 (file)
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -1531,7 +1531,7 @@ static inline void adjust_lapic_timer_advance(struct kvm_vcpu *vcpu,
        apic->lapic_timer.timer_advance_ns = timer_advance_ns;
 }
 
-void wait_lapic_expire(struct kvm_vcpu *vcpu)
+void kvm_wait_lapic_expire(struct kvm_vcpu *vcpu)
 {
        struct kvm_lapic *apic = vcpu->arch.apic;
        u64 guest_tsc, tsc_deadline;
@@ -1553,6 +1553,7 @@ void wait_lapic_expire(struct kvm_vcpu *vcpu)
        if (unlikely(!apic->lapic_timer.timer_advance_adjust_done))
                adjust_lapic_timer_advance(vcpu, apic->lapic_timer.advance_expire_delta);
 }
+EXPORT_SYMBOL_GPL(kvm_wait_lapic_expire);
 
 static void start_sw_tscdeadline(struct kvm_lapic *apic)
 {

diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
index 3e72a255543d7b8f8db1acb28f1a3ee13c96d607..f974a3d5a44d6710ccb9308c2f5fcfe5202e2e30 100644 (file)
--- a/arch/x86/kvm/lapic.h
+++ b/arch/x86/kvm/lapic.h
@@ -220,7 +220,7 @@ static inline int kvm_lapic_latched_init(struct kvm_vcpu *vcpu)
 
 bool kvm_apic_pending_eoi(struct kvm_vcpu *vcpu, int vector);
 
-void wait_lapic_expire(struct kvm_vcpu *vcpu);
+void kvm_wait_lapic_expire(struct kvm_vcpu *vcpu);
 
 bool kvm_intr_is_single_vcpu_fast(struct kvm *kvm, struct kvm_lapic_irq *irq,
                        struct kvm_vcpu **dest_vcpu);

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 5beca1030c9ab993e264ac9bd1ac33768a41ef60..a7ea34bed3fed07b90f36ee1d3d6a2fbed214e40 100644 (file)
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -5643,6 +5643,10 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
        clgi();
        kvm_load_guest_xcr0(vcpu);
 
+       if (lapic_in_kernel(vcpu) &&
+               vcpu->arch.apic->lapic_timer.timer_advance_ns)
+               kvm_wait_lapic_expire(vcpu);
+
        /*
         * If this vCPU has touched SPEC_CTRL, restore the guest's value if
         * it's non-zero. Since vmentry is serialising on affected CPUs, there

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index b93e36ddee5e470b88609b2523189653472300d8..b35b3800a3c0a4f7fb9c553ddbd4e462f3e6422f 100644 (file)
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6445,6 +6445,10 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
 
        vmx_update_hv_timer(vcpu);
 
+       if (lapic_in_kernel(vcpu) &&
+               vcpu->arch.apic->lapic_timer.timer_advance_ns)
+               kvm_wait_lapic_expire(vcpu);
+
        /*
         * If this vCPU has touched SPEC_CTRL, restore the guest's value if
         * it's non-zero. Since vmentry is serialising on affected CPUs, there

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 7a26aa5b08614a02003bac77ab97a6b5d45697f2..2a713a74ca2e9d4838e8742c003a047c8e1533c5 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7922,9 +7922,6 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
        }
 
        trace_kvm_entry(vcpu->vcpu_id);
-       if (lapic_in_kernel(vcpu) &&
-           vcpu->arch.apic->lapic_timer.timer_advance_ns)
-               wait_lapic_expire(vcpu);
        guest_enter_irqoff();
 
        fpregs_assert_state_consistent();