x86: kvm: hyperv: don't retry message delivery for periodic timers

The SynIC message delivery protocol allows the message originator to
request, should the message slot be busy, to be notified when it's free.

However, this is unnecessary and even undesirable for messages generated
by SynIC timers in periodic mode: if the period is short enough compared
to the time the guest spends in the timer interrupt handler, so the
timer ticks start piling up, the excessive interactions due to this
notification and retried message delivery only makes the things worse.

[This was observed, in particular, with Windows L2 guests setting
(temporarily) the periodic timer to 2 kHz, and spending hundreds of
microseconds in the timer interrupt handler due to several L2->L1 exits;
under some load in L0 this could exceed 500 us so the timer ticks
started to pile up and the guest livelocked.]

Relieve the situation somewhat by not retrying message delivery for
periodic SynIC timers.  This appears to remain within the "lazy" lost
ticks policy for SynIC timers as implemented in KVM.

Note that it doesn't solve the fundamental problem of livelocking the
guest with a periodic timer whose period is smaller than the time needed
to process a tick, but it makes it a bit less likely to be triggered.

Signed-off-by: Roman Kagan <rkagan@virtuozzo.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/hyperv.c

@@ -557,7 +557,7 @@ static int stimer_get_count(struct kvm_vcpu_hv_stimer *stimer, u64 *pcount)
 }
 
 static int synic_deliver_msg(struct kvm_vcpu_hv_synic *synic, u32 sint,
-			     struct hv_message *src_msg)
+			     struct hv_message *src_msg, bool no_retry)
 {
 	struct kvm_vcpu *vcpu = synic_to_vcpu(synic);
 	int msg_off = offsetof(struct hv_message_page, sint_message[sint]);
@@ -584,6 +584,9 @@ static int synic_deliver_msg(struct kvm_vcpu_hv_synic *synic, u32 sint,
 		return r;
 
 	if (hv_hdr.message_type != HVMSG_NONE) {
+		if (no_retry)
+			return 0;
+
 		hv_hdr.message_flags.msg_pending = 1;
 		r = kvm_vcpu_write_guest_page(vcpu, msg_page_gfn,
 					      &hv_hdr.message_flags,
@@ -617,10 +620,17 @@ static int stimer_send_msg(struct kvm_vcpu_hv_stimer *stimer)
 	struct hv_timer_message_payload *payload =
 			(struct hv_timer_message_payload *)&msg->u.payload;
 
+	/*
+	 * To avoid piling up periodic ticks, don't retry message
+	 * delivery for them (within "lazy" lost ticks policy).
+	 */
+	bool no_retry = stimer->config & HV_STIMER_PERIODIC;
+
 	payload->expiration_time = stimer->exp_time;
 	payload->delivery_time = get_time_ref_counter(vcpu->kvm);
 	return synic_deliver_msg(vcpu_to_synic(vcpu),
-				 HV_STIMER_SINT(stimer->config), msg);
+				 HV_STIMER_SINT(stimer->config), msg,
+				 no_retry);
 }
 
 static void stimer_expiration(struct kvm_vcpu_hv_stimer *stimer)