net, xdp: Introduce xdp_init_buff utility routine

Introduce xdp_init_buff utility routine to initialize xdp_buff fields
const over NAPI iterations (e.g. frame_sz or rxq pointer). Rely on
xdp_init_buff in all XDP capable drivers.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Shay Agroskin <shayagr@amazon.com>
Acked-by: Martin Habets <habetsm.xilinx@gmail.com>
Acked-by: Camelia Groza <camelia.groza@nxp.com>
Acked-by: Marcin Wojtas <mw@semihalf.com>
Link: https://lore.kernel.org/bpf/7f8329b6da1434dc2b05a77f2e800b29628a8913.1608670965.git.lorenzo@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 508408f..36b0f81 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -689,12 +689,11 @@ static struct sk_buff *receive_small(struct net_device *dev,
 			page = xdp_page;
 		}
 
+		xdp_init_buff(&xdp, buflen, &rq->xdp_rxq);
 		xdp.data_hard_start = buf + VIRTNET_RX_PAD + vi->hdr_len;
 		xdp.data = xdp.data_hard_start + xdp_headroom;
 		xdp.data_end = xdp.data + len;
 		xdp.data_meta = xdp.data;
-		xdp.rxq = &rq->xdp_rxq;
-		xdp.frame_sz = buflen;
 		orig_data = xdp.data;
 		act = bpf_prog_run_xdp(xdp_prog, &xdp);
 		stats->xdp_packets++;
@@ -859,12 +858,11 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
 		 * the descriptor on if we get an XDP_TX return code.
 		 */
 		data = page_address(xdp_page) + offset;
+		xdp_init_buff(&xdp, frame_sz - vi->hdr_len, &rq->xdp_rxq);
 		xdp.data_hard_start = data - VIRTIO_XDP_HEADROOM + vi->hdr_len;
 		xdp.data = data + vi->hdr_len;
 		xdp.data_end = xdp.data + (len - vi->hdr_len);
 		xdp.data_meta = xdp.data;
-		xdp.rxq = &rq->xdp_rxq;
-		xdp.frame_sz = frame_sz - vi->hdr_len;
 
 		act = bpf_prog_run_xdp(xdp_prog, &xdp);
 		stats->xdp_packets++;