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Date:   Wed, 6 May 2020 16:19:40 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
        Jesper Dangaard Brouer <brouer@...hat.com>
Subject: Re: [PATCH net-next 1/2] virtio-net: don't reserve space for vnet
 header for XDP


On 2020/5/6 下午3:53, Michael S. Tsirkin wrote:
> On Wed, May 06, 2020 at 02:16:32PM +0800, Jason Wang wrote:
>> We tried to reserve space for vnet header before
>> xdp.data_hard_start. But this is useless since the packet could be
>> modified by XDP which may invalidate the information stored in the
>> header and there's no way for XDP to know the existence of the vnet
>> header currently.
> What do you mean? Doesn't XDP_PASS use the header in the buffer?


We don't, see 436c9453a1ac0 ("virtio-net: keep vnet header zeroed after 
processing XDP")

If there's other place, it should be a bug.


>
>> So let's just not reserve space for vnet header in this case.
> In any case, we can find out XDP does head adjustments
> if we need to.


But XDP program can modify the packets without adjusting headers.

Thanks


>
>
>> Cc: Jesper Dangaard Brouer <brouer@...hat.com>
>> Signed-off-by: Jason Wang <jasowang@...hat.com>
>> ---
>>   drivers/net/virtio_net.c | 6 +++---
>>   1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>> index 11f722460513..98dd75b665a5 100644
>> --- a/drivers/net/virtio_net.c
>> +++ b/drivers/net/virtio_net.c
>> @@ -684,8 +684,8 @@ static struct sk_buff *receive_small(struct net_device *dev,
>>   			page = xdp_page;
>>   		}
>>   
>> -		xdp.data_hard_start = buf + VIRTNET_RX_PAD + vi->hdr_len;
>> -		xdp.data = xdp.data_hard_start + xdp_headroom;
>> +		xdp.data_hard_start = buf + VIRTNET_RX_PAD;
>> +		xdp.data = xdp.data_hard_start + xdp_headroom + vi->hdr_len;
>>   		xdp.data_end = xdp.data + len;
>>   		xdp.data_meta = xdp.data;
>>   		xdp.rxq = &rq->xdp_rxq;
>> @@ -845,7 +845,7 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>>   		 * the descriptor on if we get an XDP_TX return code.
>>   		 */
>>   		data = page_address(xdp_page) + offset;
>> -		xdp.data_hard_start = data - VIRTIO_XDP_HEADROOM + vi->hdr_len;
>> +		xdp.data_hard_start = data - VIRTIO_XDP_HEADROOM;
>>   		xdp.data = data + vi->hdr_len;
>>   		xdp.data_end = xdp.data + (len - vi->hdr_len);
>>   		xdp.data_meta = xdp.data;
>> -- 
>> 2.20.1

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