xsk: support mbuf on ZC RX

Given that skb_shared_info relies on skb_frag_t, in order to support
xskb chaining, introduce xdp_buff_xsk::xskb_list_node and
xsk_buff_pool::xskb_list.

This is needed so ZC drivers can add frags as xskb nodes which will make
it possible to handle it both when producing AF_XDP Rx descriptors as
well as freeing/recycling all the frags that a single frame carries.

Speaking of latter, update xsk_buff_free() to take care of list nodes.
For the former (adding as frags), introduce xsk_buff_add_frag() for ZC
drivers usage that is going to be used to add a frag to xskb list from
pool.

xsk_buff_get_frag() will be utilized by XDP_TX and, on contrary, will
return xdp_buff.

One of the previous patches added a wrapper for ZC Rx so implement xskb
list walk and production of Rx descriptors there.

On bind() path, bail out if socket wants to use ZC multi-buffer but
underlying netdev does not support it.

Signed-off-by: Maciej Fijalkowski <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

include/net/xdp_sock_drv.h

···
 static inline void xsk_buff_free(struct xdp_buff *xdp)
 {
 	struct xdp_buff_xsk *xskb = container_of(xdp, struct xdp_buff_xsk, xdp);
+	struct list_head *xskb_list = &xskb->pool->xskb_list;
+	struct xdp_buff_xsk *pos, *tmp;
 
+	if (likely(!xdp_buff_has_frags(xdp)))
+		goto out;
+
+	list_for_each_entry_safe(pos, tmp, xskb_list, xskb_list_node) {
+		list_del(&pos->xskb_list_node);
+		xp_free(pos);
+	}
+
+	xdp_get_shared_info_from_buff(xdp)->nr_frags = 0;
+out:
 	xp_free(xskb);
+}
+
+static inline void xsk_buff_add_frag(struct xdp_buff *xdp)
+{
+	struct xdp_buff_xsk *frag = container_of(xdp, struct xdp_buff_xsk, xdp);
+
+	list_add_tail(&frag->xskb_list_node, &frag->pool->xskb_list);
+}
+
+static inline struct xdp_buff *xsk_buff_get_frag(struct xdp_buff *first)
+{
+	struct xdp_buff_xsk *xskb = container_of(first, struct xdp_buff_xsk, xdp);
+	struct xdp_buff *ret = NULL;
+	struct xdp_buff_xsk *frag;
+
+	frag = list_first_entry_or_null(&xskb->pool->xskb_list,
+					struct xdp_buff_xsk, xskb_list_node);
+	if (frag) {
+		list_del(&frag->xskb_list_node);
+		ret = &frag->xdp;
+	}
+
+	return ret;
 }
 
 static inline void xsk_buff_set_size(struct xdp_buff *xdp, u32 size)
···
 
 static inline void xsk_buff_free(struct xdp_buff *xdp)
 {
+}
+
+static inline void xsk_buff_add_frag(struct xdp_buff *xdp)
+{
+}
+
+static inline struct xdp_buff *xsk_buff_get_frag(struct xdp_buff *first)
+{
+	return NULL;
 }
 
 static inline void xsk_buff_set_size(struct xdp_buff *xdp, u32 size)

include/net/xsk_buff_pool.h

···
 	struct xsk_buff_pool *pool;
 	u64 orig_addr;
 	struct list_head free_list_node;
+	struct list_head xskb_list_node;
 };
 
 #define XSK_CHECK_PRIV_TYPE(t) BUILD_BUG_ON(sizeof(t) > offsetofend(struct xdp_buff_xsk, cb))
···
 	struct xdp_umem *umem;
 	struct work_struct work;
 	struct list_head free_list;
+	struct list_head xskb_list;
 	u32 heads_cnt;
 	u16 queue_id;
 

net/xdp/xsk.c

···
 static int xsk_rcv_zc(struct xdp_sock *xs, struct xdp_buff *xdp, u32 len)
 {
 	struct xdp_buff_xsk *xskb = container_of(xdp, struct xdp_buff_xsk, xdp);
+	u32 frags = xdp_buff_has_frags(xdp);
+	struct xdp_buff_xsk *pos, *tmp;
+	struct list_head *xskb_list;
+	u32 contd = 0;
+	int err;
 
-	return __xsk_rcv_zc(xs, xskb, len, 0);
+	if (frags)
+		contd = XDP_PKT_CONTD;
+
+	err = __xsk_rcv_zc(xs, xskb, len, contd);
+	if (err || likely(!frags))
+		goto out;
+
+	xskb_list = &xskb->pool->xskb_list;
+	list_for_each_entry_safe(pos, tmp, xskb_list, xskb_list_node) {
+		if (list_is_singular(xskb_list))
+			contd = 0;
+		len = pos->xdp.data_end - pos->xdp.data;
+		err = __xsk_rcv_zc(xs, pos, len, contd);
+		if (err)
+			return err;
+		list_del(&pos->xskb_list_node);
+	}
+
+out:
+	return err;
 }
 
 static void *xsk_copy_xdp_start(struct xdp_buff *from)

net/xdp/xsk_buff_pool.c

···
 	pool->umem = umem;
 	pool->addrs = umem->addrs;
 	INIT_LIST_HEAD(&pool->free_list);
+	INIT_LIST_HEAD(&pool->xskb_list);
 	INIT_LIST_HEAD(&pool->xsk_tx_list);
 	spin_lock_init(&pool->xsk_tx_list_lock);
 	spin_lock_init(&pool->cq_lock);
···
 		xskb->pool = pool;
 		xskb->xdp.frame_sz = umem->chunk_size - umem->headroom;
 		INIT_LIST_HEAD(&xskb->free_list_node);
+		INIT_LIST_HEAD(&xskb->xskb_list_node);
 		if (pool->unaligned)
 			pool->free_heads[i] = xskb;
 		else
···
 		return 0;
 
 	if ((netdev->xdp_features & NETDEV_XDP_ACT_ZC) != NETDEV_XDP_ACT_ZC) {
+		err = -EOPNOTSUPP;
+		goto err_unreg_pool;
+	}
+
+	if (netdev->xdp_zc_max_segs == 1 && (flags & XDP_USE_SG)) {
 		err = -EOPNOTSUPP;
 		goto err_unreg_pool;
 	}