netdev - Re: [PATCH v14 bpf-next 03/18] net: mvneta: update mb bit before passing the xdp buffer to eBPF layer

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Date:   Mon, 20 Sep 2021 10:37:20 +0200
From:   Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
To:     Shay Agroskin <shayagr@...zon.com>
Cc:     Lorenzo Bianconi <lorenzo@...nel.org>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
        ast@...nel.org, daniel@...earbox.net, john.fastabend@...il.com,
        dsahern@...nel.org, brouer@...hat.com, echaudro@...hat.com,
        jasowang@...hat.com, alexander.duyck@...il.com, saeed@...nel.org,
        maciej.fijalkowski@...el.com, magnus.karlsson@...el.com,
        tirthendu.sarkar@...el.com, toke@...hat.com
Subject: Re: [PATCH v14 bpf-next 03/18] net: mvneta: update mb bit before
 passing the xdp buffer to eBPF layer

> 
> Lorenzo Bianconi <lorenzo@...nel.org> writes:
> 
> > ...
> > diff --git a/drivers/net/ethernet/marvell/mvneta.c
> > b/drivers/net/ethernet/marvell/mvneta.c
> > index 9d460a270601..0c7b84ca6efc 100644
> > --- a/drivers/net/ethernet/marvell/mvneta.c
> > +++ b/drivers/net/ethernet/marvell/mvneta.c
> > ...
> > @@ -2320,8 +2325,12 @@ mvneta_swbm_build_skb(struct mvneta_port *pp,
> > struct page_pool *pool,
> >  		      struct xdp_buff *xdp, u32 desc_status)
> >  {
> >  	struct skb_shared_info *sinfo =  xdp_get_shared_info_from_buff(xdp);
> > -	int i, num_frags = sinfo->nr_frags;
> >  	struct sk_buff *skb;
> > +	u8 num_frags;
> > +	int i;
> > +
> > +	if (unlikely(xdp_buff_is_mb(xdp)))
> > +		num_frags = sinfo->nr_frags;
> 
> Hi,
> nit, it seems that the num_frags assignment can be moved after the other
> 'if' condition you added (right before the 'for' for num_frags), or even be
> eliminated completely so that sinfo->nr_frags is used directly.
> Either way it looks like you can remove one 'if'.
> 
> Shay

Hi Shay,

we can't move nr_frags assignement after build_skb() since this field will be
overwritten by that call.

Regards,
Lorenzo

> 
> >  	skb = build_skb(xdp->data_hard_start, PAGE_SIZE);
> >  	if (!skb)
> > @@ -2333,6 +2342,9 @@ mvneta_swbm_build_skb(struct mvneta_port *pp,
> > struct page_pool *pool,
> >  	skb_put(skb, xdp->data_end - xdp->data);
> >  	skb->ip_summed = mvneta_rx_csum(pp, desc_status);
> > +	if (likely(!xdp_buff_is_mb(xdp)))
> > +		goto out;
> > +
> >  	for (i = 0; i < num_frags; i++) {
> >  		skb_frag_t *frag = &sinfo->frags[i];
> >   @@ -2341,6 +2353,7 @@ mvneta_swbm_build_skb(struct mvneta_port *pp,
> > struct page_pool *pool,
> >  				skb_frag_size(frag), PAGE_SIZE);
> >  	}
> > +out:
> >  	return skb;
> >  }
> 

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