Re: [PATCH bpf-next V2 1/7] net: xdp: Add xdp_rx_meta structure

On Thu, Jul 17, 2025 at 04:40 PM +02, Jesper Dangaard Brouer wrote:

On 17/07/2025 11.19, Jakub Sitnicki wrote:

quoted

On Wed, Jul 02, 2025 at 04:58 PM +02, Jesper Dangaard Brouer wrote:

quoted

From: Lorenzo Bianconi <lorenzo@kernel.org>

Introduce the `xdp_rx_meta` structure to serve as a container for XDP RX
hardware hints within XDP packet buffers. Initially, this structure will
accommodate `rx_hash` and `rx_vlan` metadata. (The `rx_timestamp` hint will
get stored in `skb_shared_info`).

A key design aspect is making this metadata accessible both during BPF
program execution (via `struct xdp_buff`) and later if an `struct
xdp_frame` is materialized (e.g., for XDP_REDIRECT).
To achieve this:
   - The `struct xdp_frame` embeds an `xdp_rx_meta` field directly for
     storage.
   - The `struct xdp_buff` includes an `xdp_rx_meta` pointer. This pointer
     is initialized (in `xdp_prepare_buff`) to point to the memory location
     within the packet buffer's headroom where the `xdp_frame`'s embedded
     `rx_meta` field would reside.

This setup allows BPF kfuncs, operating on `xdp_buff`, to populate the
metadata in the precise location where it will be found if an `xdp_frame`
is subsequently created.

The availability of this metadata storage area within the buffer is
indicated by the `XDP_FLAGS_META_AREA` flag in `xdp_buff->flags` (and
propagated to `xdp_frame->flags`). This flag is only set if sufficient
headroom (at least `XDP_MIN_HEADROOM`, currently 192 bytes) is present.
Specific hints like `XDP_FLAGS_META_RX_HASH` and `XDP_FLAGS_META_RX_VLAN`
will then denote which types of metadata have been populated into the
`xdp_rx_meta` structure.

This patch is a step for enabling the preservation and use of XDP RX
hints across operations like XDP_REDIRECT.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Jesper Dangaard Brouer <hawk@kernel.org>
---
  include/net/xdp.h       |   57 +++++++++++++++++++++++++++++++++++------------
  net/core/xdp.c          |    1 +
  net/xdp/xsk_buff_pool.c |    4 ++-
  3 files changed, 47 insertions(+), 15 deletions(-)

diff --git a/include/net/xdp.h b/include/net/xdp.h
index b40f1f96cb11..f52742a25212 100644
--- a/include/net/xdp.h
+++ b/include/net/xdp.h

@@ -71,11 +71,31 @@ struct xdp_txq_info {
  	struct net_device *dev;
  };
  +struct xdp_rx_meta {
+	struct xdp_rx_meta_hash {
+		u32 val;
+		u32 type; /* enum xdp_rss_hash_type */
+	} hash;
+	struct xdp_rx_meta_vlan {
+		__be16 proto;
+		u16 tci;
+	} vlan;
+};
+
+/* Storage area for HW RX metadata only available with reasonable headroom
+ * available. Less than XDP_PACKET_HEADROOM due to Intel drivers.
+ */
+#define XDP_MIN_HEADROOM	192
+
  enum xdp_buff_flags {
  	XDP_FLAGS_HAS_FRAGS		= BIT(0), /* non-linear xdp buff */
  	XDP_FLAGS_FRAGS_PF_MEMALLOC	= BIT(1), /* xdp paged memory is under
  						   * pressure
  						   */
+	XDP_FLAGS_META_AREA		= BIT(2), /* storage area available */

Idea: Perhaps this could be called *HW*_META_AREA to differentiate from
the existing custom metadata area:

I agree, that calling it META_AREA can easily be misunderstood and confused with
metadata or data_meta.

What do you think about renaming this to "hints" ?
 E.g. XDP_FLAGS_HINTS_AREA
 or   XDP_FLAGS_HINTS_AVAIL

And also renaming XDP_FLAGS_META_RX_* to
 e.g XDP_FLAGS_META_RX_HASH -> XDP_FLAGS_HINT_RX_HASH
                           or  XDP_FLAGS_HW_HINT_RX_HASH

Any name that doesn't lean on the already overloaded "metadata" term is
a better alternative, in my mind :-)