KVM: VMX: Handle preemption timer fastpath

This patch implements a fastpath for the preemption timer vmexit.  The vmexit
can be handled quickly so it can be performed with interrupts off and going
back directly to the guest.

Testing on SKX Server.

cyclictest in guest(w/o mwait exposed, adaptive advance lapic timer is default -1):

5540.5ns -> 4602ns       17%

kvm-unit-test/vmexit.flat:

w/o avanced timer:
tscdeadline_immed: 3028.5  -> 2494.75  17.6%
tscdeadline:       5765.7  -> 5285      8.3%

w/ adaptive advance timer default -1:
tscdeadline_immed: 3123.75 -> 2583     17.3%
tscdeadline:       4663.75 -> 4537      2.7%
Tested-by: Haiwei Li <lihaiwei@tencent.com>
Cc: Haiwei Li <lihaiwei@tencent.com>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
Message-Id: <1588055009-12677-8-git-send-email-wanpengli@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/vmx.c

@@ -5601,14 +5601,22 @@ static int handle_pml_full(struct kvm_vcpu *vcpu)
 	return 1;
 }
 
-static int handle_preemption_timer(struct kvm_vcpu *vcpu)
+static fastpath_t handle_fastpath_preemption_timer(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
 
 	if (!vmx->req_immediate_exit &&
-	    !unlikely(vmx->loaded_vmcs->hv_timer_soft_disabled))
+	    !unlikely(vmx->loaded_vmcs->hv_timer_soft_disabled)) {
 		kvm_lapic_expired_hv_timer(vcpu);
+		return EXIT_FASTPATH_REENTER_GUEST;
+	}
+
+	return EXIT_FASTPATH_NONE;
+}
 
+static int handle_preemption_timer(struct kvm_vcpu *vcpu)
+{
+	handle_fastpath_preemption_timer(vcpu);
 	return 1;
 }
 
@@ -6631,6 +6639,8 @@ static fastpath_t vmx_exit_handlers_fastpath(struct kvm_vcpu *vcpu)
 	switch (to_vmx(vcpu)->exit_reason) {
 	case EXIT_REASON_MSR_WRITE:
 		return handle_fastpath_set_msr_irqoff(vcpu);
+	case EXIT_REASON_PREEMPTION_TIMER:
+		return handle_fastpath_preemption_timer(vcpu);
 	default:
 		return EXIT_FASTPATH_NONE;
 	}