[PATCH v3 net 2/8] 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. Rely on pool->dev
state when configuring tailroom. xsk_pool_get_rx_frame_size() inside
ndo_bpf is usually called when bringing up queues and before xsk's dma
mappings have been configured, which makes it valid to rely on
pool->dev.

Typically drivers on Rx Hw buffers side work on 128 byte alignment so
let us align the value returned by xsk_pool_get_rx_frame_size() in order
to avoid addressing this on driver's side. This addresses the fact that
idpf uses mentioned function *before* pool->dev being set so we were at
risk that after subtracting tailroom we would not provide 128-byte
aligned value to HW.

Reviewed-by: Björn Töpel <bjorn@kernel.org>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Fixes: 24ea50127ecf ("xsk: support mbuf on ZC RX")
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
---
 include/net/xdp_sock_drv.h | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/include/net/xdp_sock_drv.h b/include/net/xdp_sock_drv.h
index 6b9ebae2dc95..dcf811c45b22 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 (pool->dev && (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 +61,9 @@ 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 ALIGN_DOWN(xsk_pool_get_chunk_size(pool) -
+			  xsk_pool_get_headroom(pool) -
+			  xsk_pool_get_tailroom(pool), 128);
 }
 
 static inline u32 xsk_pool_get_rx_frag_step(struct xsk_buff_pool *pool)
-- 
2.43.0