netdev - RE: [PATCH v12 bpf-next 03/18] net: mvneta: update mb bit before passing the xdp buffer to eBPF layer

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Message-ID: <612eba0cd2678_6b872081d@john-XPS-13-9370.notmuch>
Date:   Tue, 31 Aug 2021 16:23:56 -0700
From:   John Fastabend <john.fastabend@...il.com>
To:     Lorenzo Bianconi <lorenzo@...nel.org>, bpf@...r.kernel.org,
        netdev@...r.kernel.org
Cc:     lorenzo.bianconi@...hat.com, davem@...emloft.net, kuba@...nel.org,
        ast@...nel.org, daniel@...earbox.net, shayagr@...zon.com,
        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 v12 bpf-next 03/18] net: mvneta: update mb bit before
 passing the xdp buffer to eBPF layer

Lorenzo Bianconi 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. Access
> skb_shared_info only if xdp_buff mb is set in order to avoid possible
> cache-misses.
> 
> Signed-off-by: Lorenzo Bianconi <lorenzo@...nel.org>
> ---
>  drivers/net/ethernet/marvell/mvneta.c | 23 ++++++++++++++++++-----
>  1 file changed, 18 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/net/ethernet/marvell/mvneta.c b/drivers/net/ethernet/marvell/mvneta.c
> index 5d1007e1b5c9..9f4858e35566 100644
> --- a/drivers/net/ethernet/marvell/mvneta.c
> +++ b/drivers/net/ethernet/marvell/mvneta.c
> @@ -2037,9 +2037,14 @@ mvneta_xdp_put_buff(struct mvneta_port *pp, struct mvneta_rx_queue *rxq,
>  {
>  	int i;
>  
> +	if (likely(!xdp_buff_is_mb(xdp)))
> +		goto out;
> +

Wouldn't nr_frags = 0 in the !xdp_buff_is_mb case? Is the
xdp_buff_is_mb check with goto really required?

>  	for (i = 0; i < sinfo->nr_frags; i++)
>  		page_pool_put_full_page(rxq->page_pool,
>  					skb_frag_page(&sinfo->frags[i]), true);
> +
> +out:
>  	page_pool_put_page(rxq->page_pool, virt_to_head_page(xdp->data),
>  			   sync_len, true);
>  }
> @@ -2241,7 +2246,6 @@ mvneta_swbm_rx_frame(struct mvneta_port *pp,
>  	int data_len = -MVNETA_MH_SIZE, len;
>  	struct net_device *dev = pp->dev;
>  	enum dma_data_direction dma_dir;
> -	struct skb_shared_info *sinfo;
>  
>  	if (*size > MVNETA_MAX_RX_BUF_SIZE) {
>  		len = MVNETA_MAX_RX_BUF_SIZE;
> @@ -2261,11 +2265,9 @@ mvneta_swbm_rx_frame(struct mvneta_port *pp,
>  
>  	/* Prefetch header */
>  	prefetch(data);
> +	xdp_buff_clear_mb(xdp);
>  	xdp_prepare_buff(xdp, data, pp->rx_offset_correction + MVNETA_MH_SIZE,
>  			 data_len, false);
> -
> -	sinfo = xdp_get_shared_info_from_buff(xdp);
> -	sinfo->nr_frags = 0;
>  }
>  
>  static void
> @@ -2299,6 +2301,9 @@ mvneta_swbm_add_rx_fragment(struct mvneta_port *pp,
>  		skb_frag_off_set(frag, pp->rx_offset_correction);
>  		skb_frag_size_set(frag, data_len);
>  		__skb_frag_set_page(frag, page);
> +
> +		if (!xdp_buff_is_mb(xdp))
> +			xdp_buff_set_mb(xdp);
>  	} else {
>  		page_pool_put_full_page(rxq->page_pool, page, true);
>  	}
> @@ -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;
>  
>  	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;
> +

Not that I care much, but couldn't you just init num_frags = 0 and
avoid the goto?

Anyways its not my driver so no need to change it if you like it better
the way it is. Mostly just checking my understanding.

>  	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;
>  }
>  
> -- 
> 2.31.1
>