Drivers: hv: Implement flow management on the send side

Implement flow management on the send side. When the sender is blocked, the reader
can potentially signal the sender to indicate there is now room to send.
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Reviewed-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/hv/channel.c

@@ -735,6 +735,7 @@ int vmbus_recvpacket(struct vmbus_channel *channel, void *buffer,
 	u32 packetlen;
 	u32 userlen;
 	int ret;
+	bool signal = false;
 
 	*buffer_actual_len = 0;
 	*requestid = 0;
@@ -761,8 +762,10 @@ int vmbus_recvpacket(struct vmbus_channel *channel, void *buffer,
 
 	/* Copy over the packet to the user buffer */
 	ret = hv_ringbuffer_read(&channel->inbound, buffer, userlen,
-			     (desc.offset8 << 3));
+			     (desc.offset8 << 3), &signal);
 
+	if (signal)
+		vmbus_setevent(channel);
 
 	return 0;
 }
@@ -779,6 +782,7 @@ int vmbus_recvpacket_raw(struct vmbus_channel *channel, void *buffer,
 	u32 packetlen;
 	u32 userlen;
 	int ret;
+	bool signal = false;
 
 	*buffer_actual_len = 0;
 	*requestid = 0;
@@ -805,7 +809,11 @@ int vmbus_recvpacket_raw(struct vmbus_channel *channel, void *buffer,
 	*requestid = desc.trans_id;
 
 	/* Copy over the entire packet to the user buffer */
-	ret = hv_ringbuffer_read(&channel->inbound, buffer, packetlen, 0);
+	ret = hv_ringbuffer_read(&channel->inbound, buffer, packetlen, 0,
+				 &signal);
+
+	if (signal)
+		vmbus_setevent(channel);
 
 	return 0;
 }

drivers/hv/hyperv_vmbus.h

@@ -550,7 +550,7 @@ int hv_ringbuffer_peek(struct hv_ring_buffer_info *ring_info, void *buffer,
 int hv_ringbuffer_read(struct hv_ring_buffer_info *ring_info,
 		   void *buffer,
 		   u32 buflen,
-		   u32 offset);
+		   u32 offset, bool *signal);
 
 
 void hv_ringbuffer_get_debuginfo(struct hv_ring_buffer_info *ring_info,

drivers/hv/ring_buffer.c

@@ -84,6 +84,50 @@ static bool hv_need_to_signal(u32 old_write, struct hv_ring_buffer_info *rbi)
 	return false;
 }
 
+/*
+ * To optimize the flow management on the send-side,
+ * when the sender is blocked because of lack of
+ * sufficient space in the ring buffer, potential the
+ * consumer of the ring buffer can signal the producer.
+ * This is controlled by the following parameters:
+ *
+ * 1. pending_send_sz: This is the size in bytes that the
+ *    producer is trying to send.
+ * 2. The feature bit feat_pending_send_sz set to indicate if
+ *    the consumer of the ring will signal when the ring
+ *    state transitions from being full to a state where
+ *    there is room for the producer to send the pending packet.
+ */
+
+static bool hv_need_to_signal_on_read(u32 old_rd,
+					 struct hv_ring_buffer_info *rbi)
+{
+	u32 prev_write_sz;
+	u32 cur_write_sz;
+	u32 r_size;
+	u32 write_loc = rbi->ring_buffer->write_index;
+	u32 read_loc = rbi->ring_buffer->read_index;
+	u32 pending_sz = rbi->ring_buffer->pending_send_sz;
+
+	/*
+	 * If the other end is not blocked on write don't bother.
+	 */
+	if (pending_sz == 0)
+		return false;
+
+	r_size = rbi->ring_datasize;
+	cur_write_sz = write_loc >= read_loc ? r_size - (write_loc - read_loc) :
+			read_loc - write_loc;
+
+	prev_write_sz = write_loc >= old_rd ? r_size - (write_loc - old_rd) :
+			old_rd - write_loc;
+
+
+	if ((prev_write_sz < pending_sz) && (cur_write_sz >= pending_sz))
+		return true;
+
+	return false;
+}
 
 /*
  * hv_get_next_write_location()
@@ -461,13 +505,14 @@ int hv_ringbuffer_peek(struct hv_ring_buffer_info *Inring_info,
  *
  */
 int hv_ringbuffer_read(struct hv_ring_buffer_info *inring_info, void *buffer,
-		   u32 buflen, u32 offset)
+		   u32 buflen, u32 offset, bool *signal)
 {
 	u32 bytes_avail_towrite;
 	u32 bytes_avail_toread;
 	u32 next_read_location = 0;
 	u64 prev_indices = 0;
 	unsigned long flags;
+	u32 old_read;
 
 	if (buflen <= 0)
 		return -EINVAL;
@@ -478,6 +523,8 @@ int hv_ringbuffer_read(struct hv_ring_buffer_info *inring_info, void *buffer,
 				&bytes_avail_toread,
 				&bytes_avail_towrite);
 
+	old_read = bytes_avail_toread;
+
 	/* Make sure there is something to read */
 	if (bytes_avail_toread < buflen) {
 		spin_unlock_irqrestore(&inring_info->ring_lock, flags);
@@ -508,5 +555,7 @@ int hv_ringbuffer_read(struct hv_ring_buffer_info *inring_info, void *buffer,
 
 	spin_unlock_irqrestore(&inring_info->ring_lock, flags);
 
+	*signal = hv_need_to_signal_on_read(old_read, inring_info);
+
 	return 0;
 }