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

From: Lorenzo Bianconi <[email-masked]>
Date: Mon, 20 Sep 2021 11:00:52 +0200

> 
> Lorenzo Bianconi <[email-masked]> writes:
> 
> > > 
> > > Lorenzo Bianconi <[email-masked]> 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
> > 
> 
> Sorry, silly mistake of me.
> 
> Guess this assignment can be done anyway since there doesn't seem to be new
> cache misses introduced by it.
> Anyway, nice catch, sorry for misleading you

actually we probably have a cache miss in this case for the single-buffer use case
since skb_shared_info will not be in the same cache-line.

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;
> > > >  }
> > > 
>