netfilter: flowtable: Add IPIP tx sw acceleration

Introduce sw acceleration for tx path of IPIP tunnels relying on the
netfilter flowtable infrastructure.
This patch introduces basic infrastructure to accelerate other tunnel
types (e.g. IP6IP6).
IPIP sw tx acceleration can be tested running the following scenario where
the traffic is forwarded between two NICs (eth0 and eth1) and an IPIP
tunnel is used to access a remote site (using eth1 as the underlay device):

ETH0 -- TUN0 <==> ETH1 -- [IP network] -- TUN1 (192.168.100.2)

$ip addr show
6: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
link/ether 00:00:22:33:11:55 brd ff:ff:ff:ff:ff:ff
inet 192.168.0.2/24 scope global eth0
valid_lft forever preferred_lft forever
7: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
link/ether 00:11:22:33:11:55 brd ff:ff:ff:ff:ff:ff
inet 192.168.1.1/24 scope global eth1
valid_lft forever preferred_lft forever
8: tun0@NONE: <POINTOPOINT,NOARP,UP,LOWER_UP> mtu 1480 qdisc noqueue state UNKNOWN group default qlen 1000
link/ipip 192.168.1.1 peer 192.168.1.2
inet 192.168.100.1/24 scope global tun0
valid_lft forever preferred_lft forever

$ip route show
default via 192.168.100.2 dev tun0
192.168.0.0/24 dev eth0 proto kernel scope link src 192.168.0.2
192.168.1.0/24 dev eth1 proto kernel scope link src 192.168.1.1
192.168.100.0/24 dev tun0 proto kernel scope link src 192.168.100.1

$nft list ruleset
table inet filter {
flowtable ft {
hook ingress priority filter
devices = { eth0, eth1 }
}

chain forward {
type filter hook forward priority filter; policy accept;
meta l4proto { tcp, udp } flow add @ft
}
}

Reproducing the scenario described above using veths I got the following
results:
- TCP stream trasmitted into the IPIP tunnel:
- net-next: (baseline) ~ 85Gbps
- net-next + IPIP flowtable support: ~102Gbps

Co-developed-by: Pablo Neira Ayuso <[e-mail]>
Signed-off-by: Lorenzo Bianconi <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

net/netfilter/nf_flow_table_ip.c

···
 	flow = container_of(tuplehash, struct flow_offload, tuplehash[dir]);
 
 	mtu = flow->tuplehash[dir].tuple.mtu + ctx->offset;
+	if (flow->tuplehash[!dir].tuple.tun_num)
+		mtu -= sizeof(*iph);
+
 	if (unlikely(nf_flow_exceeds_mtu(skb, mtu)))
 		return 0;
 
···
 	return 0;
 }
 
+static int nf_flow_tunnel_ipip_push(struct net *net, struct sk_buff *skb,
+				    struct flow_offload_tuple *tuple,
+				    __be32 *ip_daddr)
+{
+	struct iphdr *iph = (struct iphdr *)skb_network_header(skb);
+	struct rtable *rt = dst_rtable(tuple->dst_cache);
+	u8 tos = iph->tos, ttl = iph->ttl;
+	__be16 frag_off = iph->frag_off;
+	u32 headroom = sizeof(*iph);
+	int err;
+
+	err = iptunnel_handle_offloads(skb, SKB_GSO_IPXIP4);
+	if (err)
+		return err;
+
+	skb_set_inner_ipproto(skb, IPPROTO_IPIP);
+	headroom += LL_RESERVED_SPACE(rt->dst.dev) + rt->dst.header_len;
+	err = skb_cow_head(skb, headroom);
+	if (err)
+		return err;
+
+	skb_scrub_packet(skb, true);
+	skb_clear_hash_if_not_l4(skb);
+
+	/* Push down and install the IP header. */
+	skb_push(skb, sizeof(*iph));
+	skb_reset_network_header(skb);
+
+	iph = ip_hdr(skb);
+	iph->version	= 4;
+	iph->ihl	= sizeof(*iph) >> 2;
+	iph->frag_off	= ip_mtu_locked(&rt->dst) ? 0 : frag_off;
+	iph->protocol	= tuple->tun.l3_proto;
+	iph->tos	= tos;
+	iph->daddr	= tuple->tun.src_v4.s_addr;
+	iph->saddr	= tuple->tun.dst_v4.s_addr;
+	iph->ttl	= ttl;
+	iph->tot_len	= htons(skb->len);
+	__ip_select_ident(net, iph, skb_shinfo(skb)->gso_segs ?: 1);
+	ip_send_check(iph);
+
+	*ip_daddr = tuple->tun.src_v4.s_addr;
+
+	return 0;
+}
+
+static int nf_flow_tunnel_v4_push(struct net *net, struct sk_buff *skb,
+				  struct flow_offload_tuple *tuple,
+				  __be32 *ip_daddr)
+{
+	if (tuple->tun_num)
+		return nf_flow_tunnel_ipip_push(net, skb, tuple, ip_daddr);
+
+	return 0;
+}
+
 static int nf_flow_encap_push(struct sk_buff *skb,
 			      struct flow_offload_tuple *tuple)
 {
···
 	flow = container_of(tuplehash, struct flow_offload, tuplehash[dir]);
 	other_tuple = &flow->tuplehash[!dir].tuple;
 	ip_daddr = other_tuple->src_v4.s_addr;
+
+	if (nf_flow_tunnel_v4_push(state->net, skb, other_tuple, &ip_daddr) < 0)
+		return NF_DROP;
 
 	if (nf_flow_encap_push(skb, other_tuple) < 0)
 		return NF_DROP;

net/netfilter/nf_flow_table_path.c

···
 	return found;
 }
 
-static void nft_dev_forward_path(struct nf_flow_route *route,
+static int nft_flow_tunnel_update_route(const struct nft_pktinfo *pkt,
+					struct flow_offload_tunnel *tun,
+					struct nf_flow_route *route,
+					enum ip_conntrack_dir dir)
+{
+	struct dst_entry *cur_dst = route->tuple[dir].dst;
+	struct dst_entry *tun_dst = NULL;
+	struct flowi fl = {};
+
+	switch (nft_pf(pkt)) {
+	case NFPROTO_IPV4:
+		fl.u.ip4.daddr = tun->dst_v4.s_addr;
+		fl.u.ip4.saddr = tun->src_v4.s_addr;
+		fl.u.ip4.flowi4_iif = nft_in(pkt)->ifindex;
+		fl.u.ip4.flowi4_dscp = ip4h_dscp(ip_hdr(pkt->skb));
+		fl.u.ip4.flowi4_mark = pkt->skb->mark;
+		fl.u.ip4.flowi4_flags = FLOWI_FLAG_ANYSRC;
+		break;
+	case NFPROTO_IPV6:
+		fl.u.ip6.daddr = tun->dst_v6;
+		fl.u.ip6.saddr = tun->src_v6;
+		fl.u.ip6.flowi6_iif = nft_in(pkt)->ifindex;
+		fl.u.ip6.flowlabel = ip6_flowinfo(ipv6_hdr(pkt->skb));
+		fl.u.ip6.flowi6_mark = pkt->skb->mark;
+		fl.u.ip6.flowi6_flags = FLOWI_FLAG_ANYSRC;
+		break;
+	}
+
+	nf_route(nft_net(pkt), &tun_dst, &fl, false, nft_pf(pkt));
+	if (!tun_dst)
+		return -ENOENT;
+
+	route->tuple[dir].dst = tun_dst;
+	dst_release(cur_dst);
+
+	return 0;
+}
+
+static void nft_dev_forward_path(const struct nft_pktinfo *pkt,
+				 struct nf_flow_route *route,
 				 const struct nf_conn *ct,
 				 enum ip_conntrack_dir dir,
 				 struct nft_flowtable *ft)
···
 		route->tuple[!dir].in.encap[i].proto = info.encap[i].proto;
 	}
 
-	if (info.num_tuns) {
+	if (info.num_tuns &&
+	    !nft_flow_tunnel_update_route(pkt, &info.tun, route, dir)) {
 		route->tuple[!dir].in.tun.src_v6 = info.tun.dst_v6;
 		route->tuple[!dir].in.tun.dst_v6 = info.tun.src_v6;
 		route->tuple[!dir].in.tun.l3_proto = info.tun.l3_proto;
···
 	nft_default_forward_path(route, other_dst, !dir);
 
 	if (route->tuple[dir].xmit_type	== FLOW_OFFLOAD_XMIT_NEIGH)
-		nft_dev_forward_path(route, ct, dir, ft);
+		nft_dev_forward_path(pkt, route, ct, dir, ft);
 	if (route->tuple[!dir].xmit_type == FLOW_OFFLOAD_XMIT_NEIGH)
-		nft_dev_forward_path(route, ct, !dir, ft);
+		nft_dev_forward_path(pkt, route, ct, !dir, ft);
 
 	return 0;
 }