netdev - RE: [PATCH v19 bpf-next 03/23] net: mvneta: update mb bit before passing the xdp buffer to eBPF layer

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Date:   Sun, 05 Dec 2021 19:06:52 -0800
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 v19 bpf-next 03/23] 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>

[...]

> @@ -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;

Doesn't really need a respin IMO, but rather an observation. Its not
obvious to me the unlikely/likely pair here is wanted. Seems it could
be relatively common for some applications sending jumbo frames.

Maybe worth some experimenting in the future.

>  
>  	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];