Re: [PATCH RFC v2 01/33] xdp: add frame size to xdp_buff

[Jakub Kicinski <kuba@kernel.org>]

On Wed, 08 Apr 2020 13:50:39 +0200 Jesper Dangaard Brouer wrote:
> XDP have evolved to support several frame sizes, but xdp_buff was not
> updated with this information. The frame size (frame_sz) member of
> xdp_buff is introduced to know the real size of the memory the frame is
> delivered in.
> 
> When introducing this also make it clear that some tailroom is
> reserved/required when creating SKBs using build_skb().
> 
> It would also have been an option to introduce a pointer to
> data_hard_end (with reserved offset). The advantage with frame_sz is
> that (like rxq) drivers only need to setup/assign this value once per
> NAPI cycle. Due to XDP-generic (and some drivers) it's not possible to
> store frame_sz inside xdp_rxq_info, because it's varies per packet as it
> can be based/depend on packet length.
> 
> Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
> ---
>  include/net/xdp.h |   17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/include/net/xdp.h b/include/net/xdp.h
> index 40c6d3398458..99f4374f6214 100644
> --- a/include/net/xdp.h
> +++ b/include/net/xdp.h
> @@ -6,6 +6,8 @@
>  #ifndef __LINUX_NET_XDP_H__
>  #define __LINUX_NET_XDP_H__
>  
> +#include <linux/skbuff.h> /* skb_shared_info */
> +
>  /**
>   * DOC: XDP RX-queue information
>   *
> @@ -70,8 +72,23 @@ struct xdp_buff {
>  	void *data_hard_start;
>  	unsigned long handle;
>  	struct xdp_rxq_info *rxq;
> +	u32 frame_sz; /* frame size to deduct data_hard_end/reserved tailroom*/

Perhaps

/* length of packet buffer, starting at data_hard_start */

?

>  };
>  
> +/* Reserve memory area at end-of data area.

I wouldn't say this reserves anything. It just computes the end
pointer, no?

> + *
> + * This macro reserves tailroom in the XDP buffer by limiting the
> + * XDP/BPF data access to data_hard_end.  Notice same area (and size)
> + * is used for XDP_PASS, when constructing the SKB via build_skb().
> + */
> +#define xdp_data_hard_end(xdp)				\
> +	((xdp)->data_hard_start + (xdp)->frame_sz -	\
> +	 SKB_DATA_ALIGN(sizeof(struct skb_shared_info)))

I think it should be said somewhere that the drivers are expected to
DMA map memory up to xdp_data_hard_end(xdp).

> +
> +/* Like skb_shinfo */
> +#define xdp_shinfo(xdp)	((struct skb_shared_info *)(xdp_data_hard_end(xdp)))
> +// XXX: Above likely belongs in later patch
> +
>  struct xdp_frame {
>  	void *data;
>  	u16 len;
> 
>