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

From: Jakub Sitnicki <[email-masked]>
Date: Thu, 17 Jul 2025 11:19:59 +0200

On Wed, Jul 02, 2025 at 04:58 PM +02, Jesper Dangaard Brouer wrote:
> From: Lorenzo Bianconi <[email-masked]>
>
> 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 <[email-masked]>
> Signed-off-by: Jesper Dangaard Brouer <[email-masked]>
> ---
>  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:

https://docs.kernel.org/networking/xdp-rx-metadata.html#af-xdp

> +	XDP_FLAGS_META_RX_HASH		= BIT(3), /* hw rx hash */
> +	XDP_FLAGS_META_RX_VLAN		= BIT(4), /* hw rx vlan */
> +	XDP_FLAGS_META_RX_TS		= BIT(5), /* hw rx timestamp */
>  };
>  

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