Subject: Re: [PATCH net-next v2 12/18] xdp: add generic xdp_build_skb_from_buff()

From: Maciej Fijalkowski <[email-masked]>
Date: Thu, 17 Oct 2024 14:34:33 +0200

On Tue, Oct 15, 2024 at 04:53:44PM +0200, Alexander Lobakin wrote:
> 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.
> There's __xdp_build_skb_from_frame() already, so just convert it to take
> &xdp_buff instead, while making the original one a wrapper. The original
> one always took an already allocated skb, allow both variants here -- if
> no skb passed, which is expected when calling from a driver, pick one via
> napi_build_skb().
> 
> 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 19d2b283b845..83e3f4648caa 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..9dc103a09b5c 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;
> +	}
> +
> +	skb = napi_build_skb(xdp->data_hard_start, xdp->frame_sz);
> +	if (unlikely(!skb))
> +		return NULL;
> +
> +	skb_reserve(skb, xdp->data - xdp->data_hard_start);
> +	__skb_put(skb, xdp->data_end - xdp->data);
> +
> +	metalen = xdp->data - xdp->data_meta;
> +	if (metalen > 0)
> +		skb_metadata_set(skb, metalen);
> +
> +	if (is_page_pool_compiled_in() && rxq->mem.type == MEM_TYPE_PAGE_POOL)
> +		skb_mark_for_recycle(skb);
> +
> +	skb_record_rx_queue(skb, rxq->queue_index);
> +
> +	if (unlikely(nr_frags)) {
> +		u32 ts;

nit: spell out truesize? ts confuse my brain with timestamp TBH

> +
> +		ts = sinfo->xdp_frags_truesize ? : nr_frags * xdp->frame_sz;
> +		xdp_update_skb_shared_info(skb, nr_frags,
> +					   sinfo->xdp_frags_size, ts,
> +					   xdp_buff_is_frag_pfmemalloc(xdp));
> +	}
> +
> +	skb->protocol = eth_type_trans(skb, rxq->dev);

could we leave this out to be set by drivers? i see in ice for example
netdev ptr is retrieved in different ways here.

> +
> +	return skb;
> +}
> +EXPORT_SYMBOL_GPL(xdp_build_skb_from_buff);
> +
>  struct sk_buff *__xdp_build_skb_from_frame(struct xdp_frame *xdpf,
>  					   struct sk_buff *skb,
>  					   struct net_device *dev)
> -- 
> 2.46.2
>