netdev - Re: [net-next PATCH 3/5] bpf: cpumap xdp_buff to skb conversion and allocation

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Message-ID: <1c37f945-0e2f-1eec-fe88-a740815026d3@redhat.com>
Date:   Fri, 29 Sep 2017 17:49:23 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>, netdev@...r.kernel.org
Cc:     jakub.kicinski@...ronome.com,
        "Michael S. Tsirkin" <mst@...hat.com>, mchan@...adcom.com,
        John Fastabend <john.fastabend@...il.com>,
        peter.waskiewicz.jr@...el.com,
        Daniel Borkmann <borkmann@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Andy Gospodarek <andy@...yhouse.net>
Subject: Re: [net-next PATCH 3/5] bpf: cpumap xdp_buff to skb conversion and
 allocation



On 2017年09月28日 20:57, Jesper Dangaard Brouer wrote:
> +};
> +
> +/* Convert xdp_buff to xdp_pkt */
> +static struct xdp_pkt *convert_to_xdp_pkt(struct xdp_buff *xdp)
> +{
> +	struct xdp_pkt *xdp_pkt;
> +	int headroom;
> +
> +	/* Assure headroom is available for storing info */
> +	headroom = xdp->data - xdp->data_hard_start;
> +	if (headroom < sizeof(*xdp_pkt))
> +		return NULL;

Hi Jesper:

Do you consider this as a trick or a long term solution? Is it better to 
store XDP in a circular buffer? (I'm asking since I meet similar issue 
when doing xdp_xmit for tun).

> +
> +	/* Store info in top of packet */
> +	xdp_pkt = xdp->data_hard_start;
> +
> +	xdp_pkt->data = xdp->data;
> +	xdp_pkt->len  = xdp->data_end - xdp->data;
> +	xdp_pkt->headroom = headroom - sizeof(*xdp_pkt);
> +

Is wmb() needed here?

> +	return xdp_pkt;
> +}

Thanks