ixgbe: add ipsec hw offload note to ixgbe Documentation

Add a short note about using IPsec Hardware Offload with
the ixgbe driver.
Signed-off-by: Shannon Nelson <shannon.nelson@oracle.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

Documentation/networking/ixgbe.rst

@@ -501,6 +501,19 @@ NOTE: This feature can be disabled for a specific Virtual Function (VF)::
 
   ip link set <pf dev> vf <vf id> spoofchk {off|on}
 
+IPsec Offload
+-------------
+The ixgbe driver supports IPsec Hardware Offload.  When creating Security
+Associations with "ip xfrm ..." the 'offload' tag option can be used to
+register the IPsec SA with the driver in order to get higher throughput in
+the secure communications.
+
+The offload is also supported for ixgbe's VFs, but the VF must be set as
+'trusted' and the support must be enabled with::
+
+  ethtool --set-priv-flags eth<x> vf-ipsec on
+  ip link set eth<x> vf <y> trust on
+
 
 Known Issues/Troubleshooting
 ============================