Drivers: hv: ring_buffer: count on wrap around mappings in get_next_pkt_raw()

BugLink: http://bugs.launchpad.net/bugs/1650059

With wrap around mappings in place we can always provide drivers with
direct links to packets on the ring buffer, even when they wrap around.
Do the required updates to get_next_pkt_raw()/put_pkt_raw()
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Tested-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(back ported from commit bb08d431)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

Conflicts:
	include/linux/hyperv.h
Acked-by: Brad Figg <brad.figg@canonical.com>
Acked-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

include/linux/hyperv.h

@@ -1467,4 +1467,78 @@ static inline  bool hv_need_to_signal_on_read(struct hv_ring_buffer_info *rbi)
 	return false;
 }
 
+/*
+ * An API to support in-place processing of incoming VMBUS packets.
+ */
+#define VMBUS_PKT_TRAILER	8
+
+static inline struct vmpacket_descriptor *
+get_next_pkt_raw(struct vmbus_channel *channel)
+{
+	struct hv_ring_buffer_info *ring_info = &channel->inbound;
+	u32 priv_read_loc = ring_info->priv_read_index;
+	void *ring_buffer = hv_get_ring_buffer(ring_info);
+	u32 dsize = ring_info->ring_datasize;
+	/*
+	 * delta is the difference between what is available to read and
+	 * what was already consumed in place. We commit read index after
+	 * the whole batch is processed.
+	 */
+	u32 delta = priv_read_loc >= ring_info->ring_buffer->read_index ?
+		priv_read_loc - ring_info->ring_buffer->read_index :
+		(dsize - ring_info->ring_buffer->read_index) + priv_read_loc;
+	u32 bytes_avail_toread = (hv_get_bytes_to_read(ring_info) - delta);
+
+	if (bytes_avail_toread < sizeof(struct vmpacket_descriptor))
+		return NULL;
+
+	return ring_buffer + priv_read_loc;
+}
+
+/*
+ * A helper function to step through packets "in-place"
+ * This API is to be called after each successful call
+ * get_next_pkt_raw().
+ */
+static inline void put_pkt_raw(struct vmbus_channel *channel,
+				struct vmpacket_descriptor *desc)
+{
+	struct hv_ring_buffer_info *ring_info = &channel->inbound;
+	u32 packetlen = desc->len8 << 3;
+	u32 dsize = ring_info->ring_datasize;
+
+	/*
+	 * Include the packet trailer.
+	 */
+	ring_info->priv_read_index += packetlen + VMBUS_PKT_TRAILER;
+	ring_info->priv_read_index %= dsize;
+}
+
+/*
+ * This call commits the read index and potentially signals the host.
+ * Here is the pattern for using the "in-place" consumption APIs:
+ *
+ * while (get_next_pkt_raw() {
+ *	process the packet "in-place";
+ *	put_pkt_raw();
+ * }
+ * if (packets processed in place)
+ *	commit_rd_index();
+ */
+static inline void commit_rd_index(struct vmbus_channel *channel)
+{
+	struct hv_ring_buffer_info *ring_info = &channel->inbound;
+	/*
+	 * Make sure all reads are done before we update the read index since
+	 * the writer may start writing to the read area once the read index
+	 * is updated.
+	 */
+	virt_rmb();
+	ring_info->ring_buffer->read_index = ring_info->priv_read_index;
+
+	if (hv_need_to_signal_on_read(ring_info))
+		vmbus_set_event(channel);
+}
+
+
 #endif /* _HYPERV_H */