netdev - Re: [PATCH net-next 3/6] net: mvneta: update mb bit before passing the xdp buffer to eBPF layer

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Message-ID: <20200820100215.1b93464f@carbon>
Date:   Thu, 20 Aug 2020 10:02:15 +0200
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Lorenzo Bianconi <lorenzo@...nel.org>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org, davem@...emloft.net,
        lorenzo.bianconi@...hat.com, echaudro@...hat.com,
        sameehj@...zon.com, kuba@...nel.org, brouer@...hat.com
Subject: Re: [PATCH net-next 3/6] net: mvneta: update mb bit before passing
 the xdp buffer to eBPF layer

On Wed, 19 Aug 2020 15:13:48 +0200
Lorenzo Bianconi <lorenzo@...nel.org> wrote:

> Update multi-buffer bit (mb) in xdp_buff to notify XDP/eBPF layer and
> XDP remote drivers if this is a "non-linear" XDP buffer
> 
> Signed-off-by: Lorenzo Bianconi <lorenzo@...nel.org>
> ---
>  drivers/net/ethernet/marvell/mvneta.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/net/ethernet/marvell/mvneta.c b/drivers/net/ethernet/marvell/mvneta.c
> index 832bbb8b05c8..36a3defa63fa 100644
> --- a/drivers/net/ethernet/marvell/mvneta.c
> +++ b/drivers/net/ethernet/marvell/mvneta.c
> @@ -2170,11 +2170,14 @@ mvneta_run_xdp(struct mvneta_port *pp, struct mvneta_rx_queue *rxq,
>  	       struct bpf_prog *prog, struct xdp_buff *xdp,
>  	       u32 frame_sz, struct mvneta_stats *stats)
>  {
> +	struct skb_shared_info *sinfo = xdp_get_shared_info_from_buff(xdp);
>  	unsigned int len, data_len, sync;
>  	u32 ret, act;
>  
>  	len = xdp->data_end - xdp->data_hard_start - pp->rx_offset_correction;
>  	data_len = xdp->data_end - xdp->data;
> +
> +	xdp->mb = !!sinfo->nr_frags;
>  	act = bpf_prog_run_xdp(prog, xdp);

Reading the memory sinfo->nr_frags could be a performance issue for our
baseline case of no-multi-buffer.  As you are reading a cache-line that
you don't need to (and driver have not touch yet).

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer