hv_netvsc: Add a comment clarifying batching logic

The batching logic in netvsc_send is non-trivial, due to
a combination of the Linux API and the underlying hypervisor
interface. Add a comment explaining why the code is written this
way.

Signed-off-by: Shachar Raindel <shacharr@microsoft.com>
Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Reviewed-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/device_drivers/ethernet/microsoft/netvsc.rst

@@ -87,11 +87,15 @@ Receive Buffer
   contain one or more packets. The number of receive sections may be changed
   via ethtool Rx ring parameters.
 
-  There is a similar send buffer which is used to aggregate packets for sending.
-  The send area is broken into chunks of 6144 bytes, each of section may
-  contain one or more packets. The send buffer is an optimization, the driver
-  will use slower method to handle very large packets or if the send buffer
-  area is exhausted.
+  There is a similar send buffer which is used to aggregate packets
+  for sending.  The send area is broken into chunks, typically of 6144
+  bytes, each of section may contain one or more packets. Small
+  packets are usually transmitted via copy to the send buffer. However,
+  if the buffer is temporarily exhausted, or the packet to be transmitted is
+  an LSO packet, the driver will provide the host with pointers to the data
+  from the SKB. This attempts to achieve a balance between the overhead of
+  data copy and the impact of remapping VM memory to be accessible by the
+  host.
 
 XDP support
 -----------

drivers/net/hyperv/netvsc.c

@@ -1017,6 +1017,26 @@ static inline void move_pkt_msd(struct hv_netvsc_packet **msd_send,
 }
 
 /* RCU already held by caller */
+/* Batching/bouncing logic is designed to attempt to optimize
+ * performance.
+ *
+ * For small, non-LSO packets we copy the packet to a send buffer
+ * which is pre-registered with the Hyper-V side. This enables the
+ * hypervisor to avoid remapping the aperture to access the packet
+ * descriptor and data.
+ *
+ * If we already started using a buffer and the netdev is transmitting
+ * a burst of packets, keep on copying into the buffer until it is
+ * full or we are done collecting a burst. If there is an existing
+ * buffer with space for the RNDIS descriptor but not the packet, copy
+ * the RNDIS descriptor to the buffer, keeping the packet in place.
+ *
+ * If we do batching and send more than one packet using a single
+ * NetVSC message, free the SKBs of the packets copied, except for the
+ * last packet. This is done to streamline the handling of the case
+ * where the last packet only had the RNDIS descriptor copied to the
+ * send buffer, with the data pointers included in the NetVSC message.
+ */
 int netvsc_send(struct net_device *ndev,
 		struct hv_netvsc_packet *packet,
 		struct rndis_message *rndis_msg,