xdp: add generic xdp_build_skb_from_buff()

The code which builds an skb from an &xdp_buff keeps multiplying itself
around the drivers with almost no changes. Let's try to stop that by
adding a generic function.
Unlike __xdp_build_skb_from_frame(), always allocate an skbuff head
using napi_build_skb() and make use of the available xdp_rxq pointer to
assign the Rx queue index. In case of PP-backed buffer, mark the skb to
be recycled, as every PP user's been switched to recycle skbs.

Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Link: https://patch.msgid.link/20241218174435.1445282-4-aleksander.lobakin@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/xdp.h

@@ -336,6 +336,7 @@ xdp_update_skb_shared_info(struct sk_buff *skb, u8 nr_frags,
 void xdp_warn(const char *msg, const char *func, const int line);
 #define XDP_WARN(msg) xdp_warn(msg, __func__, __LINE__)
 
+struct sk_buff *xdp_build_skb_from_buff(const struct xdp_buff *xdp);
 struct xdp_frame *xdp_convert_zc_to_xdp_frame(struct xdp_buff *xdp);
 struct sk_buff *__xdp_build_skb_from_frame(struct xdp_frame *xdpf,
 					   struct sk_buff *skb,

net/core/xdp.c

@@ -629,6 +629,61 @@ int xdp_alloc_skb_bulk(void **skbs, int n_skb, gfp_t gfp)
 }
 EXPORT_SYMBOL_GPL(xdp_alloc_skb_bulk);
 
+/**
+ * xdp_build_skb_from_buff - create an skb from &xdp_buff
+ * @xdp: &xdp_buff to convert to an skb
+ *
+ * Perform common operations to create a new skb to pass up the stack from
+ * &xdp_buff: allocate an skb head from the NAPI percpu cache, initialize
+ * skb data pointers and offsets, set the recycle bit if the buff is
+ * PP-backed, Rx queue index, protocol and update frags info.
+ *
+ * Return: new &sk_buff on success, %NULL on error.
+ */
+struct sk_buff *xdp_build_skb_from_buff(const struct xdp_buff *xdp)
+{
+	const struct xdp_rxq_info *rxq = xdp->rxq;
+	const struct skb_shared_info *sinfo;
+	struct sk_buff *skb;
+	u32 nr_frags = 0;
+	int metalen;
+
+	if (unlikely(xdp_buff_has_frags(xdp))) {
+		sinfo = xdp_get_shared_info_from_buff(xdp);
+		nr_frags = sinfo->nr_frags;
+	}
+
+	skb = napi_build_skb(xdp->data_hard_start, xdp->frame_sz);
+	if (unlikely(!skb))
+		return NULL;
+
+	skb_reserve(skb, xdp->data - xdp->data_hard_start);
+	__skb_put(skb, xdp->data_end - xdp->data);
+
+	metalen = xdp->data - xdp->data_meta;
+	if (metalen > 0)
+		skb_metadata_set(skb, metalen);
+
+	if (rxq->mem.type == MEM_TYPE_PAGE_POOL)
+		skb_mark_for_recycle(skb);
+
+	skb_record_rx_queue(skb, rxq->queue_index);
+
+	if (unlikely(nr_frags)) {
+		u32 tsize;
+
+		tsize = sinfo->xdp_frags_truesize ? : nr_frags * xdp->frame_sz;
+		xdp_update_skb_shared_info(skb, nr_frags,
+					   sinfo->xdp_frags_size, tsize,
+					   xdp_buff_is_frag_pfmemalloc(xdp));
+	}
+
+	skb->protocol = eth_type_trans(skb, rxq->dev);
+
+	return skb;
+}
+EXPORT_SYMBOL_GPL(xdp_build_skb_from_buff);
+
 struct sk_buff *__xdp_build_skb_from_frame(struct xdp_frame *xdpf,
 					   struct sk_buff *skb,
 					   struct net_device *dev)