netdev - Re: [PATCH v2 net-next 1/9] xdp: introduce mb in xdp_buff/xdp

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Date:   Thu, 3 Sep 2020 18:07:05 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Lorenzo Bianconi <lorenzo@...nel.org>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org, davem@...emloft.net,
        lorenzo.bianconi@...hat.com, brouer@...hat.com,
        echaudro@...hat.com, sameehj@...zon.com, kuba@...nel.org,
        john.fastabend@...il.com, daniel@...earbox.net, ast@...nel.org,
        shayagr@...zon.com
Subject: Re: [PATCH v2 net-next 1/9] xdp: introduce mb in xdp_buff/xdp_frame

On Thu, Sep 03, 2020 at 10:58:45PM +0200, Lorenzo Bianconi wrote:
> Introduce multi-buffer bit (mb) in xdp_frame/xdp_buffer to specify
> if shared_info area has been properly initialized for non-linear
> xdp buffers
> 
> Signed-off-by: Lorenzo Bianconi <lorenzo@...nel.org>
> ---
>  include/net/xdp.h | 8 ++++++--
>  net/core/xdp.c    | 1 +
>  2 files changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/include/net/xdp.h b/include/net/xdp.h
> index 3814fb631d52..42f439f9fcda 100644
> --- a/include/net/xdp.h
> +++ b/include/net/xdp.h
> @@ -72,7 +72,8 @@ struct xdp_buff {
>  	void *data_hard_start;
>  	struct xdp_rxq_info *rxq;
>  	struct xdp_txq_info *txq;
> -	u32 frame_sz; /* frame size to deduce data_hard_end/reserved tailroom*/
> +	u32 frame_sz:31; /* frame size to deduce data_hard_end/reserved tailroom*/
> +	u32 mb:1; /* xdp non-linear buffer */
>  };
>  
>  /* Reserve memory area at end-of data area.
> @@ -96,7 +97,8 @@ struct xdp_frame {
>  	u16 len;
>  	u16 headroom;
>  	u32 metasize:8;
> -	u32 frame_sz:24;
> +	u32 frame_sz:23;
> +	u32 mb:1; /* xdp non-linear frame */

Hmm. Last time I checked compilers were generating ugly code with bitfields.
Not performant and not efficient.
frame_sz is used in the fast path.
I suspect the first hunk alone will cause performance degradation.
Could you use normal u8 or u32 flag field?