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

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Message-ID: <7fb99df47a9eae1fd0fc8dc85336f7df2c120744.camel@mellanox.com>
Date:   Thu, 9 Apr 2020 00:50:02 +0000
From:   Saeed Mahameed <saeedm@...lanox.com>
To:     "brouer@...hat.com" <brouer@...hat.com>,
        "sameehj@...zon.com" <sameehj@...zon.com>
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Subject: Re: [PATCH RFC v2 01/33] xdp: add frame size to xdp_buff

On Wed, 2020-04-08 at 13:50 +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@...hat.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 */
> +

I think it is wrong to make xdp.h depend on skbuff.h
we must keep xdp.h minimal and independent,
the new macros should be defined in skbuff.h 

>  /**
>   * 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*/

why u32 ? u16 should be more than enough.. 

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

this macro is not safe when unary operators are being used

> +/* 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;
> 
>