netdev - [PATCH net-next v2 4/9] xsk: extend xsk_build_skb() to support passing an already allocated skb

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Message-Id: <20250825135342.53110-5-kerneljasonxing@gmail.com>
Date: Mon, 25 Aug 2025 21:53:37 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: davem@...emloft.net,
	edumazet@...gle.com,
	kuba@...nel.org,
	pabeni@...hat.com,
	bjorn@...nel.org,
	magnus.karlsson@...el.com,
	maciej.fijalkowski@...el.com,
	jonathan.lemon@...il.com,
	sdf@...ichev.me,
	ast@...nel.org,
	daniel@...earbox.net,
	hawk@...nel.org,
	john.fastabend@...il.com,
	horms@...nel.org,
	andrew+netdev@...n.ch
Cc: bpf@...r.kernel.org,
	netdev@...r.kernel.org,
	Jason Xing <kernelxing@...cent.com>
Subject: [PATCH net-next v2 4/9] xsk: extend xsk_build_skb() to support passing an already allocated skb

From: Jason Xing <kernelxing@...cent.com>

Batch xmit mode needs to allocate and build skbs at one time. To avoid
reinvent the wheel, use xsk_build_skb() as the second half process of
the whole initialization of each skb.

The original xsk_build_skb() itself allocates a new skb by calling
sock_alloc_send_skb whether in copy mode or zerocopy mode. Add a new
parameter allocated skb to let other callers to pass an already
allocated skb to support later xmit batch feature. At that time,
another building skb function will generate a new skb and pass it to
xsk_build_skb() to finish the rest of building process, like
initializing structures and copying data.

Signed-off-by: Jason Xing <kernelxing@...cent.com>
---
 include/net/xdp_sock.h |  4 ++++
 net/xdp/xsk.c          | 23 ++++++++++++++++-------
 2 files changed, 20 insertions(+), 7 deletions(-)

diff --git a/include/net/xdp_sock.h b/include/net/xdp_sock.h
index c2b05268b8ad..cbba880c27c3 100644
--- a/include/net/xdp_sock.h
+++ b/include/net/xdp_sock.h
@@ -123,6 +123,10 @@ struct xsk_tx_metadata_ops {
 	void	(*tmo_request_launch_time)(u64 launch_time, void *priv);
 };
 
+
+struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
+			      struct sk_buff *allocated_skb,
+			      struct xdp_desc *desc);
 #ifdef CONFIG_XDP_SOCKETS
 
 int xsk_generic_rcv(struct xdp_sock *xs, struct xdp_buff *xdp);
diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index 173ad49379c3..213d6100e405 100644
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c
@@ -605,6 +605,7 @@ static void xsk_drop_skb(struct sk_buff *skb)
 }
 
 static struct sk_buff *xsk_build_skb_zerocopy(struct xdp_sock *xs,
+					      struct sk_buff *allocated_skb,
 					      struct xdp_desc *desc)
 {
 	struct xsk_buff_pool *pool = xs->pool;
@@ -618,7 +619,10 @@ static struct sk_buff *xsk_build_skb_zerocopy(struct xdp_sock *xs,
 	if (!skb) {
 		hr = max(NET_SKB_PAD, L1_CACHE_ALIGN(xs->dev->needed_headroom));
 
-		skb = sock_alloc_send_skb(&xs->sk, hr, 1, &err);
+		if (!allocated_skb)
+			skb = sock_alloc_send_skb(&xs->sk, hr, 1, &err);
+		else
+			skb = allocated_skb;
 		if (unlikely(!skb))
 			return ERR_PTR(err);
 
@@ -657,8 +661,9 @@ static struct sk_buff *xsk_build_skb_zerocopy(struct xdp_sock *xs,
 	return skb;
 }
 
-static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
-				     struct xdp_desc *desc)
+struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
+			      struct sk_buff *allocated_skb,
+			      struct xdp_desc *desc)
 {
 	struct xsk_tx_metadata *meta = NULL;
 	struct net_device *dev = xs->dev;
@@ -667,7 +672,7 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
 	int err;
 
 	if (dev->priv_flags & IFF_TX_SKB_NO_LINEAR) {
-		skb = xsk_build_skb_zerocopy(xs, desc);
+		skb = xsk_build_skb_zerocopy(xs, allocated_skb, desc);
 		if (IS_ERR(skb)) {
 			err = PTR_ERR(skb);
 			goto free_err;
@@ -683,8 +688,12 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
 			first_frag = true;
 
 			hr = max(NET_SKB_PAD, L1_CACHE_ALIGN(dev->needed_headroom));
-			tr = dev->needed_tailroom;
-			skb = sock_alloc_send_skb(&xs->sk, hr + len + tr, 1, &err);
+			if (!allocated_skb) {
+				tr = dev->needed_tailroom;
+				skb = sock_alloc_send_skb(&xs->sk, hr + len + tr, 1, &err);
+			} else {
+				skb = allocated_skb;
+			}
 			if (unlikely(!skb))
 				goto free_err;
 
@@ -818,7 +827,7 @@ static int __xsk_generic_xmit(struct sock *sk)
 			goto out;
 		}
 
-		skb = xsk_build_skb(xs, &desc);
+		skb = xsk_build_skb(xs, NULL, &desc);
 		if (IS_ERR(skb)) {
 			err = PTR_ERR(skb);
 			if (err != -EOVERFLOW)
-- 
2.41.3