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
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