netdev - Re: [RFC PATCH] virtio_net: XDP support for adjust

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Date:   Thu, 5 Jan 2017 11:10:05 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     John Fastabend <john.fastabend@...il.com>, mst@...hat.com
Cc:     john.r.fastabend@...el.com, netdev@...r.kernel.org,
        alexei.starovoitov@...il.com, daniel@...earbox.net
Subject: Re: [RFC PATCH] virtio_net: XDP support for adjust_head



On 2017年01月05日 02:58, John Fastabend wrote:
> [...]
>
>>> @@ -393,34 +397,39 @@ static u32 do_xdp_prog(struct virtnet_info *vi,
>>>                   struct bpf_prog *xdp_prog,
>>>                   void *data, int len)
>>>    {
>>> -    int hdr_padded_len;
>>>        struct xdp_buff xdp;
>>> -    void *buf;
>>>        unsigned int qp;
>>>        u32 act;
>>>    +
>>>        if (vi->mergeable_rx_bufs) {
>>> -        hdr_padded_len = sizeof(struct virtio_net_hdr_mrg_rxbuf);
>>> -        xdp.data = data + hdr_padded_len;
>>> +        int desc_room = sizeof(struct virtio_net_hdr_mrg_rxbuf);
>>> +
>>> +        /* Allow consuming headroom but reserve enough space to push
>>> +         * the descriptor on if we get an XDP_TX return code.
>>> +         */
>>> +        xdp.data_hard_start = data - vi->headroom + desc_room;
>>> +        xdp.data = data + desc_room;
>>>            xdp.data_end = xdp.data + (len - vi->hdr_len);
>>> -        buf = data;
>>>        } else { /* small buffers */
>>>            struct sk_buff *skb = data;
>>>    -        xdp.data = skb->data;
>>> +        xdp.data_hard_start = skb->data;
>>> +        xdp.data = skb->data + vi->headroom;
>>>            xdp.data_end = xdp.data + len;
>>> -        buf = skb->data;
>>>        }
>>>          act = bpf_prog_run_xdp(xdp_prog, &xdp);
>>>        switch (act) {
>>>        case XDP_PASS:
>>> +        if (!vi->mergeable_rx_bufs)
>>> +            __skb_pull((struct sk_buff *) data,
>>> +                   xdp.data - xdp.data_hard_start);
>> Instead of doing things here and virtnet_xdp_xmit(). How about always making
>> skb->data point to the buffer head like:
>>
>> 1) reserve headroom in add_recvbuf_small()
>> 2) skb_push(xdp->data - xdp_data_hard_start, skb) if we detect xdp->data was
>> modified afer bpf_prog_run_xdp()
>>
>> Then there's no special code in either XDP_PASS or XDP_TX?
>>
> Alternatively moving the pull into the receive_small XDP handler also
> removes the special case. I'll submit a patch shortly let me know what
> you think.
>

I'm ok with this.

Thanks