Subject: Re: [PATCH net-next v5 12/19] xdp: add generic xdp_build_skb_from_buff()

From: Ido Schimmel <[email-masked]>
Date: Thu, 14 Nov 2024 17:16:44 +0200

On Thu, Nov 14, 2024 at 05:06:06PM +0200, Ido Schimmel wrote:
> Looks good (no objections to the patch), but I have a question. See
> below.
> 
> On Wed, Nov 13, 2024 at 04:24:35PM +0100, 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.
> > 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.
> > 
> > Reviewed-by: Toke Høiland-Jørgensen <[email-masked]>
> > Signed-off-by: Alexander Lobakin <[email-masked]>
> 
> Reviewed-by: Ido Schimmel <[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;
> > +	}
> > +
> > +	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 tsize;
> > +
> > +		tsize = sinfo->xdp_frags_truesize ? : nr_frags * xdp->frame_sz;
> > +		xdp_update_skb_shared_info(skb, nr_frags,
> > +					   sinfo->xdp_frags_size, tsize,
> > +					   xdp_buff_is_frag_pfmemalloc(xdp));
> > +	}
> > +
> > +	skb->protocol = eth_type_trans(skb, rxq->dev);
> 
> The device we are working with has more ports (net devices) than Rx
> queues, so each queue can receive packets from different net devices.
> Currently, each Rx queue has its own NAPI instance and its own page
> pool. All the Rx NAPI instances are initialized using the same dummy net
> device which is allocated using alloc_netdev_dummy().
> 
> What are our options with regards to the XDP Rx queue info structure? As
> evident by this patch, it does not seem valid to register one such
> structure per Rx queue and pass the dummy net device. Would it be valid
> to register one such structure per port (net device) and pass zero for
> the queue index and NAPI ID?

Actually, this does not seem to be valid either as we need to associate
an XDP Rx queue info with the correct page pool :/

> 
> To be clear, I understand it is not a common use case.
> 
> Thanks
> 
> > +
> > +	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)
> > -- 
> > 2.47.0
> > 
> >