[bpf-next PATCH] xdp: accept that XDP headroom isn't always equal XDP_PACKET_HEADROOM

[Jesper Dangaard Brouer <brouer@redhat.com>]

The Intel based drivers (ixgbe + i40e) have implemented XDP with
headroom 192 bytes and not the recommended 256 bytes defined by
XDP_PACKET_HEADROOM.  For generic-XDP, accept that this headroom
is also a valid size.

Still for generic-XDP if headroom is less, still expand headroom to
XDP_PACKET_HEADROOM as this is the default in most XDP drivers.

Tested on ixgbe with xdp_rxq_info --skb-mode and --action XDP_DROP:
- Before: 4,816,430 pps
- After : 7,749,678 pps
(Note that ixgbe in native mode XDP_DROP 14,704,539 pps)

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
---
 include/uapi/linux/bpf.h |    1 +
 net/core/dev.c           |    4 ++--
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 906e9f2752db..14dc4f9fb3c8 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -3312,6 +3312,7 @@ struct bpf_xdp_sock {
 };
 
 #define XDP_PACKET_HEADROOM 256
+#define XDP_PACKET_HEADROOM_MIN 192
 
 /* User return codes for XDP prog type.
  * A valid XDP program must return one of these defined values. All other
diff --git a/net/core/dev.c b/net/core/dev.c
index 4770dde3448d..9c941cd38b13 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4518,11 +4518,11 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb,
 		return XDP_PASS;
 
 	/* XDP packets must be linear and must have sufficient headroom
-	 * of XDP_PACKET_HEADROOM bytes. This is the guarantee that also
+	 * of XDP_PACKET_HEADROOM_MIN bytes. This is the guarantee that also
 	 * native XDP provides, thus we need to do it here as well.
 	 */
 	if (skb_cloned(skb) || skb_is_nonlinear(skb) ||
-	    skb_headroom(skb) < XDP_PACKET_HEADROOM) {
+	    skb_headroom(skb) < XDP_PACKET_HEADROOM_MIN) {
 		int hroom = XDP_PACKET_HEADROOM - skb_headroom(skb);
 		int troom = skb->tail + skb->data_len - skb->end;