Merge branch 'xsk-tailroom-reservation-and-mtu-validation'

Maciej Fijalkowski says:

====================
xsk: tailroom reservation and MTU validation

here we fix a long-standing issue regarding multi-buffer scenario in ZC
mode - we have not been providing space at the end of the buffer where
multi-buffer XDP works on skb_shared_info. This has been brought to our
attention via [0].

Unaligned mode does not get any specific treatment, it is user's
responsibility to properly handle XSK addresses in queues.

With adjustments included here in this set against xskxceiver I have
been able to pass the full test suite on ice.

[0]: https://community.intel.com/t5/Ethernet-Products/X710-XDP-Packet-Corruption-Issue-DRV-MODE-Zero-Copy-Multi-Buffer/m-p/1724208
====================

Link: https://patch.msgid.link/20260402154958.562179-1-maciej.fijalkowski@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

2 files changed, 23 insertions, 2 deletions

diff --git a/include/net/xdp_sock.h b/include/net/xdp_sock.h
index 23e8861e8b25..ebac60a3d8a1 100644
--- a/include/net/xdp_sock.h
+++ b/include/net/xdp_sock.h
@@ -14,7 +14,7 @@
 #include <linux/mm.h>
 #include <net/sock.h>
 
-#define XDP_UMEM_SG_FLAG (1 << 1)
+#define XDP_UMEM_SG_FLAG BIT(3)
 
 struct net_device;
 struct xsk_queue;
diff --git a/include/net/xdp_sock_drv.h b/include/net/xdp_sock_drv.h
index 6b9ebae2dc95..46797645a0c2 100644
--- a/include/net/xdp_sock_drv.h
+++ b/include/net/xdp_sock_drv.h
@@ -41,16 +41,37 @@ static inline u32 xsk_pool_get_headroom(struct xsk_buff_pool *pool)
 	return XDP_PACKET_HEADROOM + pool->headroom;
 }
 
+static inline u32 xsk_pool_get_tailroom(bool mbuf)
+{
+	return mbuf ? SKB_DATA_ALIGN(sizeof(struct skb_shared_info)) : 0;
+}
+
 static inline u32 xsk_pool_get_chunk_size(struct xsk_buff_pool *pool)
 {
 	return pool->chunk_size;
 }
 
-static inline u32 xsk_pool_get_rx_frame_size(struct xsk_buff_pool *pool)
+static inline u32 __xsk_pool_get_rx_frame_size(struct xsk_buff_pool *pool)
 {
 	return xsk_pool_get_chunk_size(pool) - xsk_pool_get_headroom(pool);
 }
 
+static inline u32 xsk_pool_get_rx_frame_size(struct xsk_buff_pool *pool)
+{
+	u32 frame_size =  __xsk_pool_get_rx_frame_size(pool);
+	struct xdp_umem *umem = pool->umem;
+	bool mbuf;
+
+	/* Reserve tailroom only for zero-copy pools that opted into
+	 * multi-buffer. The reserved area is used for skb_shared_info,
+	 * matching the XDP core's xdp_data_hard_end() layout.
+	 */
+	mbuf = pool->dev && (umem->flags & XDP_UMEM_SG_FLAG);
+	frame_size -= xsk_pool_get_tailroom(mbuf);
+
+	return ALIGN_DOWN(frame_size, 128);
+}
+
 static inline u32 xsk_pool_get_rx_frag_step(struct xsk_buff_pool *pool)
 {
 	return pool->unaligned ? 0 : xsk_pool_get_chunk_size(pool);