[PATCH net 1/6] xsk: respect tailroom for ZC setups

[Maciej Fijalkowski <maciej.fijalkowski@intel.com>]

Multi-buffer XDP stores information about frags in skb_shared_info that
sits at the tailroom of a packet. The storage space is reserved via
xdp_data_hard_end():

	((xdp)->data_hard_start + (xdp)->frame_sz -	\
	 SKB_DATA_ALIGN(sizeof(struct skb_shared_info)))

and then we refer to it via macro below:

static inline struct skb_shared_info *
xdp_get_shared_info_from_buff(const struct xdp_buff *xdp)
{
        return (struct skb_shared_info *)xdp_data_hard_end(xdp);
}

Currently we do not respect this tailroom space in multi-buffer AF_XDP
ZC scenario. To address this, introduce xsk_pool_get_tailroom() and use
it within xsk_pool_get_rx_frame_size() which is used in ZC drivers to
configure length of HW Rx buffer.

xsk_pool_get_tailroom() is only reserving necessary space when pool is
zc and underlying netdev supports zc multi-buffer. Since this function
relies on pool->umem->zc setting, set it before ndo_bpf during zc
configuration, so that driver that actually calls
xsk_pool_get_rx_frame_size() inside ndo_bpf will get correct tailroom
value.

Fixes: 24ea50127ecf ("xsk: support mbuf on ZC RX")
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
---
 include/net/xdp_sock_drv.h | 21 ++++++++++++++++++++-
 net/xdp/xsk_buff_pool.c    |  3 ++-
 2 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/include/net/xdp_sock_drv.h b/include/net/xdp_sock_drv.h
index 6b9ebae2dc95..13b2aae00737 100644
--- a/include/net/xdp_sock_drv.h
+++ b/include/net/xdp_sock_drv.h
@@ -41,6 +41,19 @@ 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(struct xsk_buff_pool *pool)
+{
+	struct xdp_umem *umem = pool->umem;
+
+	/* 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.
+	 */
+	if (umem->zc && (umem->flags & XDP_UMEM_SG_FLAG))
+		return SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
+	return 0;
+}
+
 static inline u32 xsk_pool_get_chunk_size(struct xsk_buff_pool *pool)
 {
 	return pool->chunk_size;
@@ -48,7 +61,8 @@ static inline u32 xsk_pool_get_chunk_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);
+	return xsk_pool_get_chunk_size(pool) - xsk_pool_get_headroom(pool) -
+	       xsk_pool_get_tailroom(pool);
 }
 
 static inline u32 xsk_pool_get_rx_frag_step(struct xsk_buff_pool *pool)
@@ -332,6 +346,11 @@ static inline u32 xsk_pool_get_headroom(struct xsk_buff_pool *pool)
 	return 0;
 }
 
+static inline u32 xsk_pool_get_tailroom(struct xsk_buff_pool *pool)
+{
+	return 0;
+}
+
 static inline u32 xsk_pool_get_chunk_size(struct xsk_buff_pool *pool)
 {
 	return 0;
diff --git a/net/xdp/xsk_buff_pool.c b/net/xdp/xsk_buff_pool.c
index 37b7a68b89b3..2cfc19e363e3 100644
--- a/net/xdp/xsk_buff_pool.c
+++ b/net/xdp/xsk_buff_pool.c
@@ -213,6 +213,7 @@ int xp_assign_dev(struct xsk_buff_pool *pool,
 	bpf.command = XDP_SETUP_XSK_POOL;
 	bpf.xsk.pool = pool;
 	bpf.xsk.queue_id = queue_id;
+	pool->umem->zc = true;
 
 	netdev_ops_assert_locked(netdev);
 	err = netdev->netdev_ops->ndo_bpf(netdev, &bpf);
@@ -224,13 +225,13 @@ int xp_assign_dev(struct xsk_buff_pool *pool,
 		err = -EINVAL;
 		goto err_unreg_xsk;
 	}
-	pool->umem->zc = true;
 	pool->xdp_zc_max_segs = netdev->xdp_zc_max_segs;
 	return 0;
 
 err_unreg_xsk:
 	xp_disable_drv_zc(pool);
 err_unreg_pool:
+	pool->umem->zc = false;
 	if (!force_zc)
 		err = 0; /* fallback to copy mode */
 	if (err) {
-- 
2.43.0