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>

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 3b32d3b094b5b27ef8b8432fd07303e8af5eb325..51ed4f0b13905f780645e628b37a8a1241b36105 100644 (file)
--- 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 c91acabf18d026f73b77332e8ba08095b2799b12..a74426214a6d5dd19b70f0fc63eb537db55acc9b 100644 (file)
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -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

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index d4dfb7a23a0cd176532f926d0337b0cc48e1ae43..f4cfc18366deff0b7955ab0db4d62ff501b71068 100644 (file)
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -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