xsk: prepare both copy and zero-copy modes to co-exist

Currently, __xsk_rcv_zc() is a function that is responsible for
producing AF_XDP Rx descriptors. It is used by both copy and zero-copy
mode. Both of these modes are going to differ when multi-buffer support
is going to be added. ZC will work on a chain of xdp_buff_xsk structs
whereas copy-mode is going to utilize skb_shared_info contents. This
means that ZC-specific changes would affect the copy mode.

Let's modify __xsk_rcv_zc() to work directly on xdp_buff_xsk so the
callsites have to retrieve this from xdp_buff. Also, introduce
xsk_rcv_zc() which will carry all the needed later changes for
supporting multi-buffer on ZC side that do not apply to copy mode.
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Link: https://lore.kernel.org/r/20230719132421.584801-4-maciej.fijalkowski@intel.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

net/xdp/xsk.c

@@ -135,9 +135,9 @@ int xsk_reg_pool_at_qid(struct net_device *dev, struct xsk_buff_pool *pool,
 	return 0;
 }
 
-static int __xsk_rcv_zc(struct xdp_sock *xs, struct xdp_buff *xdp, u32 len, u32 flags)
+static int __xsk_rcv_zc(struct xdp_sock *xs, struct xdp_buff_xsk *xskb, u32 len,
+			u32 flags)
 {
-	struct xdp_buff_xsk *xskb = container_of(xdp, struct xdp_buff_xsk, xdp);
 	u64 addr;
 	int err;
 
@@ -152,6 +152,13 @@ static int __xsk_rcv_zc(struct xdp_sock *xs, struct xdp_buff *xdp, u32 len, u32
 	return 0;
 }
 
+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);
+
+	return __xsk_rcv_zc(xs, xskb, len, 0);
+}
+
 static void xsk_copy_xdp(struct xdp_buff *to, struct xdp_buff *from, u32 len)
 {
 	void *from_buf, *to_buf;
@@ -172,6 +179,7 @@ static void xsk_copy_xdp(struct xdp_buff *to, struct xdp_buff *from, u32 len)
 
 static int __xsk_rcv(struct xdp_sock *xs, struct xdp_buff *xdp)
 {
+	struct xdp_buff_xsk *xskb;
 	struct xdp_buff *xsk_xdp;
 	int err;
 	u32 len;
@@ -189,7 +197,8 @@ static int __xsk_rcv(struct xdp_sock *xs, struct xdp_buff *xdp)
 	}
 
 	xsk_copy_xdp(xsk_xdp, xdp, len);
-	err = __xsk_rcv_zc(xs, xsk_xdp, len, 0);
+	xskb = container_of(xsk_xdp, struct xdp_buff_xsk, xdp);
+	err = __xsk_rcv_zc(xs, xskb, len, 0);
 	if (err) {
 		xsk_buff_free(xsk_xdp);
 		return err;
@@ -259,7 +268,7 @@ static int xsk_rcv(struct xdp_sock *xs, struct xdp_buff *xdp)
 
 	if (xdp->rxq->mem.type == MEM_TYPE_XSK_BUFF_POOL) {
 		len = xdp->data_end - xdp->data;
-		return __xsk_rcv_zc(xs, xdp, len, 0);
+		return xsk_rcv_zc(xs, xdp, len);
 	}
 
 	err = __xsk_rcv(xs, xdp);