1.. SPDX-License-Identifier: GPL-2.0
2
3===============================================
4XFRM device - offloading the IPsec computations
5===============================================
6
7Shannon Nelson <shannon.nelson@oracle.com>
8
9
10Overview
11========
12
13IPsec is a useful feature for securing network traffic, but the
14computational cost is high: a 10Gbps link can easily be brought down
15to under 1Gbps, depending on the traffic and link configuration.
16Luckily, there are NICs that offer a hardware based IPsec offload which
17can radically increase throughput and decrease CPU utilization.  The XFRM
18Device interface allows NIC drivers to offer to the stack access to the
19hardware offload.
20
21Userland access to the offload is typically through a system such as
22libreswan or KAME/raccoon, but the iproute2 'ip xfrm' command set can
23be handy when experimenting.  An example command might look something
24like this::
25
26  ip x s add proto esp dst 14.0.0.70 src 14.0.0.52 spi 0x07 mode transport \
27     reqid 0x07 replay-window 32 \
28     aead 'rfc4106(gcm(aes))' 0x44434241343332312423222114131211f4f3f2f1 128 \
29     sel src 14.0.0.52/24 dst 14.0.0.70/24 proto tcp \
30     offload dev eth4 dir in
31
32Yes, that's ugly, but that's what shell scripts and/or libreswan are for.
33
34
35
36Callbacks to implement
37======================
38
39::
40
41  /* from include/linux/netdevice.h */
42  struct xfrmdev_ops {
43	int	(*xdo_dev_state_add) (struct xfrm_state *x);
44	void	(*xdo_dev_state_delete) (struct xfrm_state *x);
45	void	(*xdo_dev_state_free) (struct xfrm_state *x);
46	bool	(*xdo_dev_offload_ok) (struct sk_buff *skb,
47				       struct xfrm_state *x);
48	void    (*xdo_dev_state_advance_esn) (struct xfrm_state *x);
49  };
50
51The NIC driver offering ipsec offload will need to implement these
52callbacks to make the offload available to the network stack's
53XFRM subsytem.  Additionally, the feature bits NETIF_F_HW_ESP and
54NETIF_F_HW_ESP_TX_CSUM will signal the availability of the offload.
55
56
57
58Flow
59====
60
61At probe time and before the call to register_netdev(), the driver should
62set up local data structures and XFRM callbacks, and set the feature bits.
63The XFRM code's listener will finish the setup on NETDEV_REGISTER.
64
65::
66
67		adapter->netdev->xfrmdev_ops = &ixgbe_xfrmdev_ops;
68		adapter->netdev->features |= NETIF_F_HW_ESP;
69		adapter->netdev->hw_enc_features |= NETIF_F_HW_ESP;
70
71When new SAs are set up with a request for "offload" feature, the
72driver's xdo_dev_state_add() will be given the new SA to be offloaded
73and an indication of whether it is for Rx or Tx.  The driver should
74
75	- verify the algorithm is supported for offloads
76	- store the SA information (key, salt, target-ip, protocol, etc)
77	- enable the HW offload of the SA
78	- return status value:
79
80		===========   ===================================
81		0             success
82		-EOPNETSUPP   offload not supported, try SW IPsec
83		other         fail the request
84		===========   ===================================
85
86The driver can also set an offload_handle in the SA, an opaque void pointer
87that can be used to convey context into the fast-path offload requests::
88
89		xs->xso.offload_handle = context;
90
91
92When the network stack is preparing an IPsec packet for an SA that has
93been setup for offload, it first calls into xdo_dev_offload_ok() with
94the skb and the intended offload state to ask the driver if the offload
95will serviceable.  This can check the packet information to be sure the
96offload can be supported (e.g. IPv4 or IPv6, no IPv4 options, etc) and
97return true of false to signify its support.
98
99When ready to send, the driver needs to inspect the Tx packet for the
100offload information, including the opaque context, and set up the packet
101send accordingly::
102
103		xs = xfrm_input_state(skb);
104		context = xs->xso.offload_handle;
105		set up HW for send
106
107The stack has already inserted the appropriate IPsec headers in the
108packet data, the offload just needs to do the encryption and fix up the
109header values.
110
111
112When a packet is received and the HW has indicated that it offloaded a
113decryption, the driver needs to add a reference to the decoded SA into
114the packet's skb.  At this point the data should be decrypted but the
115IPsec headers are still in the packet data; they are removed later up
116the stack in xfrm_input().
117
118	find and hold the SA that was used to the Rx skb::
119
120		get spi, protocol, and destination IP from packet headers
121		xs = find xs from (spi, protocol, dest_IP)
122		xfrm_state_hold(xs);
123
124	store the state information into the skb::
125
126		sp = secpath_set(skb);
127		if (!sp) return;
128		sp->xvec[sp->len++] = xs;
129		sp->olen++;
130
131	indicate the success and/or error status of the offload::
132
133		xo = xfrm_offload(skb);
134		xo->flags = CRYPTO_DONE;
135		xo->status = crypto_status;
136
137	hand the packet to napi_gro_receive() as usual
138
139In ESN mode, xdo_dev_state_advance_esn() is called from xfrm_replay_advance_esn().
140Driver will check packet seq number and update HW ESN state machine if needed.
141
142When the SA is removed by the user, the driver's xdo_dev_state_delete()
143is asked to disable the offload.  Later, xdo_dev_state_free() is called
144from a garbage collection routine after all reference counts to the state
145have been removed and any remaining resources can be cleared for the
146offload state.  How these are used by the driver will depend on specific
147hardware needs.
148
149As a netdev is set to DOWN the XFRM stack's netdev listener will call
150xdo_dev_state_delete() and xdo_dev_state_free() on any remaining offloaded
151states.
152