KVM: X86: Reduce the overhead when lapic_timer_advance is disabled

When I run ebizzy in a 32 vCPUs guest on a 32 pCPUs Xeon box, I can observe
~8000 kvm_wait_lapic_expire CurAvg/s through kvm_stat tool even if the advance
tscdeadline hrtimer expiration is disabled. Each call to wait_lapic_expire()
will consume ~70 cycles when a timer fires since apic_timer_expire() will
set expired_tscdeadline and then wait_lapic_expire() will do some caculation
before bailing out. So total ~175us per second is lost on this 3.2Ghz machine.
This patch reduces the overhead by skipping the function wait_lapic_expire()
when lapic_timer_advance is disabled.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -7018,7 +7018,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 	}
 
 	trace_kvm_entry(vcpu->vcpu_id);
-	wait_lapic_expire(vcpu);
+	if (lapic_timer_advance_ns)
+		wait_lapic_expire(vcpu);
 	guest_enter_irqoff();
 
 	if (unlikely(vcpu->arch.switch_db_regs)) {