x86/kvm/hyper-v: direct mode for synthetic timers

Turns out Hyper-V on KVM (as of 2016) will only use synthetic timers
if direct mode is available. With direct mode we notify the guest by
asserting APIC irq instead of sending a SynIC message.

The implementation uses existing vec_bitmap for letting lapic code
know that we're interested in the particular IRQ's EOI request. We assume
that the same APIC irq won't be used by the guest for both direct mode
stimer and as sint source (especially with AutoEOI semantics). It is
unclear how things should be handled if that's not true.

Direct mode is also somewhat less expensive; in my testing
stimer_send_msg() takes not less than 1500 cpu cycles and
stimer_notify_direct() can usually be done in 300-400. WS2016 without
Hyper-V, however, always sticks to non-direct version.
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Roman Kagan <rkagan@virtuozzo.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/hyperv.c

@@ -38,6 +38,9 @@
 
 #define KVM_HV_MAX_SPARSE_VCPU_SET_BITS DIV_ROUND_UP(KVM_MAX_VCPUS, 64)
 
+static void stimer_mark_pending(struct kvm_vcpu_hv_stimer *stimer,
+				bool vcpu_kick);
+
 static inline u64 synic_read_sint(struct kvm_vcpu_hv_synic *synic, int sint)
 {
 	return atomic64_read(&synic->sint[sint]);
@@ -53,8 +56,21 @@ static inline int synic_get_sint_vector(u64 sint_value)
 static bool synic_has_vector_connected(struct kvm_vcpu_hv_synic *synic,
 				      int vector)
 {
+	struct kvm_vcpu *vcpu = synic_to_vcpu(synic);
+	struct kvm_vcpu_hv *hv_vcpu = vcpu_to_hv_vcpu(vcpu);
+	struct kvm_vcpu_hv_stimer *stimer;
 	int i;
 
+	for (i = 0; i < ARRAY_SIZE(hv_vcpu->stimer); i++) {
+		stimer = &hv_vcpu->stimer[i];
+		if (stimer->config.enable && stimer->config.direct_mode &&
+		    stimer->config.apic_vector == vector)
+			return true;
+	}
+
+	if (vector < HV_SYNIC_FIRST_VALID_VECTOR)
+		return false;
+
 	for (i = 0; i < ARRAY_SIZE(synic->sint); i++) {
 		if (synic_get_sint_vector(synic_read_sint(synic, i)) == vector)
 			return true;
@@ -80,14 +96,14 @@ static bool synic_has_vector_auto_eoi(struct kvm_vcpu_hv_synic *synic,
 static void synic_update_vector(struct kvm_vcpu_hv_synic *synic,
 				int vector)
 {
-	if (vector < HV_SYNIC_FIRST_VALID_VECTOR)
-		return;
-
 	if (synic_has_vector_connected(synic, vector))
 		__set_bit(vector, synic->vec_bitmap);
 	else
 		__clear_bit(vector, synic->vec_bitmap);
 
+	if (vector < HV_SYNIC_FIRST_VALID_VECTOR)
+		return;
+
 	if (synic_has_vector_auto_eoi(synic, vector))
 		__set_bit(vector, synic->auto_eoi_bitmap);
 	else
@@ -173,6 +189,7 @@ static void kvm_hv_notify_acked_sint(struct kvm_vcpu *vcpu, u32 sint)
 	for (idx = 0; idx < ARRAY_SIZE(hv_vcpu->stimer); idx++) {
 		stimer = &hv_vcpu->stimer[idx];
 		if (stimer->msg_pending && stimer->config.enable &&
+		    !stimer->config.direct_mode &&
 		    stimer->config.sintx == sint) {
 			set_bit(stimer->index,
 				hv_vcpu->stimer_pending_bitmap);
@@ -342,7 +359,9 @@ int kvm_hv_synic_set_irq(struct kvm *kvm, u32 vpidx, u32 sint)
 
 void kvm_hv_synic_send_eoi(struct kvm_vcpu *vcpu, int vector)
 {
+	struct kvm_vcpu_hv *hv_vcpu = vcpu_to_hv_vcpu(vcpu);
 	struct kvm_vcpu_hv_synic *synic = vcpu_to_synic(vcpu);
+	struct kvm_vcpu_hv_stimer *stimer;
 	int i;
 
 	trace_kvm_hv_synic_send_eoi(vcpu->vcpu_id, vector);
@@ -350,6 +369,14 @@ void kvm_hv_synic_send_eoi(struct kvm_vcpu *vcpu, int vector)
 	for (i = 0; i < ARRAY_SIZE(synic->sint); i++)
 		if (synic_get_sint_vector(synic_read_sint(synic, i)) == vector)
 			kvm_hv_notify_acked_sint(vcpu, i);
+
+	for (i = 0; i < ARRAY_SIZE(hv_vcpu->stimer); i++) {
+		stimer = &hv_vcpu->stimer[i];
+		if (stimer->msg_pending && stimer->config.enable &&
+		    stimer->config.direct_mode &&
+		    stimer->config.apic_vector == vector)
+			stimer_mark_pending(stimer, false);
+	}
 }
 
 static int kvm_hv_set_sint_gsi(struct kvm *kvm, u32 vpidx, u32 sint, int gsi)
@@ -516,15 +543,25 @@ static int stimer_start(struct kvm_vcpu_hv_stimer *stimer)
 static int stimer_set_config(struct kvm_vcpu_hv_stimer *stimer, u64 config,
 			     bool host)
 {
-	union hv_stimer_config new_config = {.as_uint64 = config};
+	struct kvm_vcpu *vcpu = stimer_to_vcpu(stimer);
+	struct kvm_vcpu_hv *hv_vcpu = vcpu_to_hv_vcpu(vcpu);
+	union hv_stimer_config new_config = {.as_uint64 = config},
+		old_config = {.as_uint64 = stimer->config.as_uint64};
 
 	trace_kvm_hv_stimer_set_config(stimer_to_vcpu(stimer)->vcpu_id,
 				       stimer->index, config, host);
 
 	stimer_cleanup(stimer);
-	if (stimer->config.enable && new_config.sintx == 0)
+	if (old_config.enable &&
+	    !new_config.direct_mode && new_config.sintx == 0)
 		new_config.enable = 0;
 	stimer->config.as_uint64 = new_config.as_uint64;
+
+	if (old_config.direct_mode)
+		synic_update_vector(&hv_vcpu->synic, old_config.apic_vector);
+	if (new_config.direct_mode)
+		synic_update_vector(&hv_vcpu->synic, new_config.apic_vector);
+
 	stimer_mark_pending(stimer, false);
 	return 0;
 }
@@ -634,14 +671,28 @@ static int stimer_send_msg(struct kvm_vcpu_hv_stimer *stimer)
 				 no_retry);
 }
 
+static int stimer_notify_direct(struct kvm_vcpu_hv_stimer *stimer)
+{
+	struct kvm_vcpu *vcpu = stimer_to_vcpu(stimer);
+	struct kvm_lapic_irq irq = {
+		.delivery_mode = APIC_DM_FIXED,
+		.vector = stimer->config.apic_vector
+	};
+
+	return !kvm_apic_set_irq(vcpu, &irq, NULL);
+}
+
 static void stimer_expiration(struct kvm_vcpu_hv_stimer *stimer)
 {
-	int r;
+	int r, direct = stimer->config.direct_mode;
 
 	stimer->msg_pending = true;
+	if (!direct)
 		r = stimer_send_msg(stimer);
+	else
+		r = stimer_notify_direct(stimer);
 	trace_kvm_hv_stimer_expiration(stimer_to_vcpu(stimer)->vcpu_id,
-				       stimer->index, r);
+				       stimer->index, direct, r);
 	if (!r) {
 		stimer->msg_pending = false;
 		if (!(stimer->config.periodic))

arch/x86/kvm/trace.h

@@ -1254,24 +1254,26 @@ TRACE_EVENT(kvm_hv_stimer_callback,
  * Tracepoint for stimer_expiration.
  */
 TRACE_EVENT(kvm_hv_stimer_expiration,
-	TP_PROTO(int vcpu_id, int timer_index, int msg_send_result),
-	TP_ARGS(vcpu_id, timer_index, msg_send_result),
+	TP_PROTO(int vcpu_id, int timer_index, int direct, int msg_send_result),
+	TP_ARGS(vcpu_id, timer_index, direct, msg_send_result),
 
 	TP_STRUCT__entry(
 		__field(int, vcpu_id)
 		__field(int, timer_index)
+		__field(int, direct)
 		__field(int, msg_send_result)
 	),
 
 	TP_fast_assign(
 		__entry->vcpu_id = vcpu_id;
 		__entry->timer_index = timer_index;
+		__entry->direct = direct;
 		__entry->msg_send_result = msg_send_result;
 	),
 
-	TP_printk("vcpu_id %d timer %d msg send result %d",
+	TP_printk("vcpu_id %d timer %d direct %d send result %d",
 		  __entry->vcpu_id, __entry->timer_index,
-		  __entry->msg_send_result)
+		  __entry->direct, __entry->msg_send_result)
 );
 
 /*