xsk: add multi-buffer support for sockets sharing umem

Userspace applications indicate their multi-buffer capability to xsk
using XSK_USE_SG socket bind flag. For sockets using shared umem the
bind flag may contain XSK_USE_SG only for the first socket. For any
subsequent socket the only option supported is XDP_SHARED_UMEM.

Add option XDP_UMEM_SG_FLAG in umem config flags to store the
multi-buffer handling capability when indicated by XSK_USE_SG option in
bing flag by the first socket. Use this to derive multi-buffer capability
for subsequent sockets in xsk core.

Signed-off-by: Tirthendu Sarkar <tirthendu.sarkar@intel.com>
Fixes: 81470b5 ("xsk: introduce XSK_USE_SG bind flag for xsk socket")
Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Link: https://lore.kernel.org/r/20230907035032.2627879-1-tirthendu.sarkar@intel.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: tirthendu-intel

include/net/xdp_sock.h

@@ -14,6 +14,8 @@
 #include <linux/mm.h>
 #include <net/sock.h>
 
+#define XDP_UMEM_SG_FLAG (1 << 1)
+
 struct net_device;
 struct xsk_queue;
 struct xdp_buff;

net/xdp/xsk.c

@@ -1228,7 +1228,7 @@ static int xsk_bind(struct socket *sock, struct sockaddr *addr, int addr_len)
 
 	xs->dev = dev;
 	xs->zc = xs->umem->zc;
-	xs->sg = !!(flags & XDP_USE_SG);
+	xs->sg = !!(xs->umem->flags & XDP_UMEM_SG_FLAG);
 	xs->queue_id = qid;
 	xp_add_xsk(xs->pool, xs);
 

net/xdp/xsk_buff_pool.c

@@ -170,6 +170,9 @@ int xp_assign_dev(struct xsk_buff_pool *pool,
 	if (err)
 		return err;
 
+	if (flags & XDP_USE_SG)
+		pool->umem->flags |= XDP_UMEM_SG_FLAG;
+
 	if (flags & XDP_USE_NEED_WAKEUP)
 		pool->uses_need_wakeup = true;
 	/* Tx needs to be explicitly woken up the first time.  Also