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

From: Lorenzo Bianconi <[email-masked]>
Date: Thu, 20 Aug 2020 10:11:51 +0200

> On Wed, 19 Aug 2020 15:13:48 +0200
> Lorenzo Bianconi <[email-masked]> 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 <[email-masked]>
> > ---
> >  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).

ack, I will rework it in v2 to remove this access.

Regards,
Lorenzo

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