xfrm: add documentation for xfrm device offload api

Add a writeup on how to use the XFRM device offload API, and
mention this new file in the index.
Signed-off-by: Shannon Nelson <shannon.nelson@oracle.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

Documentation/networking/00-INDEX

@@ -228,6 +228,8 @@ x25.txt
 	- general info on X.25 development.
 x25-iface.txt
 	- description of the X.25 Packet Layer to LAPB device interface.
+xfrm_device.txt
+	- description of XFRM offload API
 xfrm_proc.txt
 	- description of the statistics package for XFRM.
 xfrm_sync.txt

Documentation/networking/xfrm_device.txt

+
+===============================================
+XFRM device - offloading the IPsec computations
+===============================================
+Shannon Nelson <shannon.nelson@oracle.com>
+
+
+Overview
+========
+
+IPsec is a useful feature for securing network traffic, but the
+computational cost is high: a 10Gbps link can easily be brought down
+to under 1Gbps, depending on the traffic and link configuration.
+Luckily, there are NICs that offer a hardware based IPsec offload which
+can radically increase throughput and decrease CPU utilization.  The XFRM
+Device interface allows NIC drivers to offer to the stack access to the
+hardware offload.
+
+Userland access to the offload is typically through a system such as
+libreswan or KAME/raccoon, but the iproute2 'ip xfrm' command set can
+be handy when experimenting.  An example command might look something
+like this:
+
+  ip x s add proto esp dst 14.0.0.70 src 14.0.0.52 spi 0x07 mode transport \
+     reqid 0x07 replay-window 32 \
+     aead 'rfc4106(gcm(aes))' 0x44434241343332312423222114131211f4f3f2f1 128 \
+     sel src 14.0.0.52/24 dst 14.0.0.70/24 proto tcp \
+     offload dev eth4 dir in
+
+Yes, that's ugly, but that's what shell scripts and/or libreswan are for.
+
+
+
+Callbacks to implement
+======================
+
+/* from include/linux/netdevice.h */
+struct xfrmdev_ops {
+	int	(*xdo_dev_state_add) (struct xfrm_state *x);
+	void	(*xdo_dev_state_delete) (struct xfrm_state *x);
+	void	(*xdo_dev_state_free) (struct xfrm_state *x);
+	bool	(*xdo_dev_offload_ok) (struct sk_buff *skb,
+				       struct xfrm_state *x);
+};
+
+The NIC driver offering ipsec offload will need to implement these
+callbacks to make the offload available to the network stack's
+XFRM subsytem.  Additionally, the feature bits NETIF_F_HW_ESP and
+NETIF_F_HW_ESP_TX_CSUM will signal the availability of the offload.
+
+
+
+Flow
+====
+
+At probe time and before the call to register_netdev(), the driver should
+set up local data structures and XFRM callbacks, and set the feature bits.
+The XFRM code's listener will finish the setup on NETDEV_REGISTER.
+
+		adapter->netdev->xfrmdev_ops = &ixgbe_xfrmdev_ops;
+		adapter->netdev->features |= NETIF_F_HW_ESP;
+		adapter->netdev->hw_enc_features |= NETIF_F_HW_ESP;
+
+When new SAs are set up with a request for "offload" feature, the
+driver's xdo_dev_state_add() will be given the new SA to be offloaded
+and an indication of whether it is for Rx or Tx.  The driver should
+	- verify the algorithm is supported for offloads
+	- store the SA information (key, salt, target-ip, protocol, etc)
+	- enable the HW offload of the SA
+
+The driver can also set an offload_handle in the SA, an opaque void pointer
+that can be used to convey context into the fast-path offload requests.
+
+		xs->xso.offload_handle = context;
+
+
+When the network stack is preparing an IPsec packet for an SA that has
+been setup for offload, it first calls into xdo_dev_offload_ok() with
+the skb and the intended offload state to ask the driver if the offload
+will serviceable.  This can check the packet information to be sure the
+offload can be supported (e.g. IPv4 or IPv6, no IPv4 options, etc) and
+return true of false to signify its support.
+
+When ready to send, the driver needs to inspect the Tx packet for the
+offload information, including the opaque context, and set up the packet
+send accordingly.
+
+		xs = xfrm_input_state(skb);
+		context = xs->xso.offload_handle;
+		set up HW for send
+
+The stack has already inserted the appropriate IPsec headers in the
+packet data, the offload just needs to do the encryption and fix up the
+header values.
+
+
+When a packet is received and the HW has indicated that it offloaded a
+decryption, the driver needs to add a reference to the decoded SA into
+the packet's skb.  At this point the data should be decrypted but the
+IPsec headers are still in the packet data; they are removed later up
+the stack in xfrm_input().
+
+	find and hold the SA that was used to the Rx skb
+		get spi, protocol, and destination IP from packet headers
+		xs = find xs from (spi, protocol, dest_IP)
+		xfrm_state_hold(xs);
+
+	store the state information into the skb
+		skb->sp = secpath_dup(skb->sp);
+		skb->sp->xvec[skb->sp->len++] = xs;
+		skb->sp->olen++;
+
+	indicate the success and/or error status of the offload
+		xo = xfrm_offload(skb);
+		xo->flags = CRYPTO_DONE;
+		xo->status = crypto_status;
+
+	hand the packet to napi_gro_receive() as usual
+
+
+When the SA is removed by the user, the driver's xdo_dev_state_delete()
+is asked to disable the offload.  Later, xdo_dev_state_free() is called
+from a garbage collection routine after all reference counts to the state
+have been removed and any remaining resources can be cleared for the
+offload state.  How these are used by the driver will depend on specific
+hardware needs.
+
+As a netdev is set to DOWN the XFRM stack's netdev listener will call
+xdo_dev_state_delete() and xdo_dev_state_free() on any remaining offloaded
+states.
+
+