Subject: Re: [PATCH net-next v4 12/19] xdp: add generic xdp_build_skb_from_buff()

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Mon, 11 Nov 2024 17:39:51 +0100

Alexander Lobakin <[email-masked]> writes:

> The code which builds an skb from an &xdp_buff keeps multiplying itself
> around the drivers with almost no changes. Let's try to stop that by
> adding a generic function.
> Unlike __xdp_build_skb_from_frame(), always allocate an skbuff head
> using napi_build_skb() and make use of the available xdp_rxq pointer to
> assign the Rx queue index. In case of PP-backed buffer, mark the skb to
> be recycled, as every PP user's been switched to recycle skbs.
>
> Signed-off-by: Alexander Lobakin <[email-masked]>
> ---
>  include/net/xdp.h |  1 +
>  net/core/xdp.c    | 55 +++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 56 insertions(+)
>
> diff --git a/include/net/xdp.h b/include/net/xdp.h
> index 4c19042adf80..b0a25b7060ff 100644
> --- a/include/net/xdp.h
> +++ b/include/net/xdp.h
> @@ -330,6 +330,7 @@ xdp_update_skb_shared_info(struct sk_buff *skb, u8 nr_frags,
>  void xdp_warn(const char *msg, const char *func, const int line);
>  #define XDP_WARN(msg) xdp_warn(msg, __func__, __LINE__)
>  
> +struct sk_buff *xdp_build_skb_from_buff(const struct xdp_buff *xdp);
>  struct xdp_frame *xdp_convert_zc_to_xdp_frame(struct xdp_buff *xdp);
>  struct sk_buff *__xdp_build_skb_from_frame(struct xdp_frame *xdpf,
>  					   struct sk_buff *skb,
> diff --git a/net/core/xdp.c b/net/core/xdp.c
> index b1b426a9b146..3a9a3c14b080 100644
> --- a/net/core/xdp.c
> +++ b/net/core/xdp.c
> @@ -624,6 +624,61 @@ int xdp_alloc_skb_bulk(void **skbs, int n_skb, gfp_t gfp)
>  }
>  EXPORT_SYMBOL_GPL(xdp_alloc_skb_bulk);
>  
> +/**
> + * xdp_build_skb_from_buff - create an skb from an &xdp_buff
> + * @xdp: &xdp_buff to convert to an skb
> + *
> + * Perform common operations to create a new skb to pass up the stack from
> + * an &xdp_buff: allocate an skb head from the NAPI percpu cache, initialize
> + * skb data pointers and offsets, set the recycle bit if the buff is PP-backed,
> + * Rx queue index, protocol and update frags info.
> + *
> + * Return: new &sk_buff on success, %NULL on error.
> + */
> +struct sk_buff *xdp_build_skb_from_buff(const struct xdp_buff *xdp)
> +{
> +	const struct xdp_rxq_info *rxq = xdp->rxq;
> +	const struct skb_shared_info *sinfo;
> +	struct sk_buff *skb;
> +	u32 nr_frags = 0;
> +	int metalen;
> +
> +	if (unlikely(xdp_buff_has_frags(xdp))) {
> +		sinfo = xdp_get_shared_info_from_buff(xdp);
> +		nr_frags = sinfo->nr_frags;
> +	}

Why this separate branch at the start of the function? nr_frags is no
used until the other branch below, so why not just make that branch on
xdp_buff_has_frags() and keep everything frags-related together in one
block?

-Toke