net: Generic XDP

This provides a generic SKB based non-optimized XDP path which is used
if either the driver lacks a specific XDP implementation, or the user
requests it via a new IFLA_XDP_FLAGS value named XDP_FLAGS_SKB_MODE.

It is arguable that perhaps I should have required something like
this as part of the initial XDP feature merge.

I believe this is critical for two reasons:

1) Accessibility.  More people can play with XDP with less
   dependencies.  Yes I know we have XDP support in virtio_net, but
   that just creates another depedency for learning how to use this
   facility.

   I wrote this to make life easier for the XDP newbies.

2) As a model for what the expected semantics are.  If there is a pure
   generic core implementation, it serves as a semantic example for
   driver folks adding XDP support.

One thing I have not tried to address here is the issue of
XDP_PACKET_HEADROOM, thanks to Daniel for spotting that.  It seems
incredibly expensive to do a skb_cow(skb, XDP_PACKET_HEADROOM) or
whatever even if the XDP program doesn't try to push headers at all.
I think we really need the verifier to somehow propagate whether
certain XDP helpers are used or not.

v5:
 - Handle both negative and positive offset after running prog
 - Fix mac length in XDP_TX case (Alexei)
 - Use rcu_dereference_protected() in free_netdev (kbuild test robot)

v4:
 - Fix MAC header adjustmnet before calling prog (David Ahern)
 - Disable LRO when generic XDP is installed (Michael Chan)
 - Bypass qdisc et al. on XDP_TX and record the event (Alexei)
 - Do not perform generic XDP on reinjected packets (DaveM)

v3:
 - Make sure XDP program sees packet at MAC header, push back MAC
   header if we do XDP_TX.  (Alexei)
 - Elide GRO when generic XDP is in use.  (Alexei)
 - Add XDP_FLAG_SKB_MODE flag which the user can use to request generic
   XDP even if the driver has an XDP implementation.  (Alexei)
 - Report whether SKB mode is in use in rtnl_xdp_fill() via XDP_FLAGS
   attribute.  (Daniel)

v2:
 - Add some "fall through" comments in switch statements based
   upon feedback from Andrew Lunn
 - Use RCU for generic xdp_prog, thanks to Johannes Berg.

Tested-by: Andy Gospodarek <andy@greyhouse.net>
Tested-by: Jesper Dangaard Brouer <brouer@redhat.com>
Tested-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/rtnetlink.c

@@ -896,15 +896,13 @@ static size_t rtnl_port_size(const struct net_device *dev,
 		return port_self_size;
 }
 
-static size_t rtnl_xdp_size(const struct net_device *dev)
+static size_t rtnl_xdp_size(void)
 {
 	size_t xdp_size = nla_total_size(0) +	/* nest IFLA_XDP */
-			  nla_total_size(1);	/* XDP_ATTACHED */
+			  nla_total_size(1) +	/* XDP_ATTACHED */
+			  nla_total_size(4);	/* XDP_FLAGS */
 
-	if (!dev->netdev_ops->ndo_xdp)
-		return 0;
-	else
-		return xdp_size;
+	return xdp_size;
 }
 
 static noinline size_t if_nlmsg_size(const struct net_device *dev,
@@ -943,7 +941,7 @@ static noinline size_t if_nlmsg_size(const struct net_device *dev,
 	       + nla_total_size(MAX_PHYS_ITEM_ID_LEN) /* IFLA_PHYS_PORT_ID */
 	       + nla_total_size(MAX_PHYS_ITEM_ID_LEN) /* IFLA_PHYS_SWITCH_ID */
 	       + nla_total_size(IFNAMSIZ) /* IFLA_PHYS_PORT_NAME */
-	       + rtnl_xdp_size(dev) /* IFLA_XDP */
+	       + rtnl_xdp_size() /* IFLA_XDP */
 	       + nla_total_size(1); /* IFLA_PROTO_DOWN */
 
 }
@@ -1251,23 +1249,35 @@ static int rtnl_fill_link_ifmap(struct sk_buff *skb, struct net_device *dev)
 
 static int rtnl_xdp_fill(struct sk_buff *skb, struct net_device *dev)
 {
-	struct netdev_xdp xdp_op = {};
 	struct nlattr *xdp;
+	u32 xdp_flags = 0;
+	u8 val = 0;
 	int err;
 
-	if (!dev->netdev_ops->ndo_xdp)
-		return 0;
 	xdp = nla_nest_start(skb, IFLA_XDP);
 	if (!xdp)
 		return -EMSGSIZE;
-	xdp_op.command = XDP_QUERY_PROG;
-	err = dev->netdev_ops->ndo_xdp(dev, &xdp_op);
-	if (err)
-		goto err_cancel;
-	err = nla_put_u8(skb, IFLA_XDP_ATTACHED, xdp_op.prog_attached);
+	if (rcu_access_pointer(dev->xdp_prog)) {
+		xdp_flags = XDP_FLAGS_SKB_MODE;
+		val = 1;
+	} else if (dev->netdev_ops->ndo_xdp) {
+		struct netdev_xdp xdp_op = {};
+
+		xdp_op.command = XDP_QUERY_PROG;
+		err = dev->netdev_ops->ndo_xdp(dev, &xdp_op);
+		if (err)
+			goto err_cancel;
+		val = xdp_op.prog_attached;
+	}
+	err = nla_put_u8(skb, IFLA_XDP_ATTACHED, val);
 	if (err)
 		goto err_cancel;
 
+	if (xdp_flags) {
+		err = nla_put_u32(skb, IFLA_XDP_FLAGS, xdp_flags);
+		if (err)
+			goto err_cancel;
+	}
 	nla_nest_end(skb, xdp);
 	return 0;