Subject: Re: [PATCH bpf-next 01/21] xsk: prepare 'options' in xdp_desc for multi-buffer use

From: Stanislav Fomichev <[email-masked]>
Date: Thu, 18 May 2023 12:22:16 -0700

On 05/18, Maciej Fijalkowski wrote:
> From: Tirthendu Sarkar <[email-masked]>
> 
> Use the 'options' field in xdp_desc as a packet continuity marker. Since
> 'options' field was unused till now and was expected to be set to 0, the
> 'eop' descriptor will have it set to 0, while the non-eop descriptors
> will have to set it to 1. This ensures legacy applications continue to
> work without needing any change for single-buffer packets.
> 
> Add helper functions and extend xskq_prod_reserve_desc() to use the
> 'options' field.
> 
> Signed-off-by: Tirthendu Sarkar <[email-masked]>
> ---
>  include/uapi/linux/if_xdp.h | 16 ++++++++++++++++
>  net/xdp/xsk.c               |  8 ++++----
>  net/xdp/xsk_queue.h         | 12 +++++++++---
>  3 files changed, 29 insertions(+), 7 deletions(-)
> 
> diff --git a/include/uapi/linux/if_xdp.h b/include/uapi/linux/if_xdp.h
> index a78a8096f4ce..4acc3a9430f3 100644
> --- a/include/uapi/linux/if_xdp.h
> +++ b/include/uapi/linux/if_xdp.h
> @@ -108,4 +108,20 @@ struct xdp_desc {
>  
>  /* UMEM descriptor is __u64 */
>  
> +/* Flag indicating that the packet continues with the buffer pointed out by the
> + * next frame in the ring. The end of the packet is signalled by setting this
> + * bit to zero. For single buffer packets, every descriptor has 'options' set
> + * to 0 and this maintains backward compatibility.
> + */
> +#define XDP_PKT_CONTD (1 << 0)
> +
> +/* Maximum number of descriptors supported as frags for a packet. So the total
> + * number of descriptors supported for a packet is XSK_DESC_MAX_FRAGS + 1. The
> + * max frags supported by skb is 16 for page sizes greater than 4K and 17 or

This is now a config option CONFIG_MAX_SKB_FRAGS. Can we use it
directly?