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Date:   Wed, 6 May 2020 16:34:36 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     mst@...hat.com, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        bpf@...r.kernel.org, "Jubran, Samih" <sameehj@...zon.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4:21, Jesper Dangaard Brouer wrote:
> On Wed,  6 May 2020 14:16:32 +0800
> Jason Wang <jasowang@...hat.com> 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
> IMHO above statements are wrong. XDP cannot access memory before
> xdp.data_hard_start. Thus, it is safe to store a vnet headers before
> xdp.data_hard_start. (The sfc driver also use this "before" area).


The problem is if we place vnet header before data_hard_start, 
virtio-net will fail any header adjustment.

Or do you mean to copy vnet header before data_hard_start before 
processing XDP?


>
>> there's no way for XDP to know the existence of the vnet header currently.
> It is true that XDP is unaware of this area, which is the way it
> should be.  Currently the area will survive after calling BPF/XDP.
> After your change it will be overwritten in xdp_frame cases.
>
>
>> So let's just not reserve space for vnet header in this case.
> I think this is a wrong approach!
>
> We are working on supporting GRO multi-buffer for XDP.  The vnet header
> contains GRO information (see pahole below sign).


Another note is that since we need reserve room for skb_shared_info, GRO 
for XDP may probably lead more frag list.


>   It is currently not
> used in the XDP case, but we will be working towards using it.


Good to know that, but I think it can only work when the packet is not 
modified by XDP?


> There
> are a lot of unanswered questions on how this will be implemented.
> Thus, I cannot layout how we are going to leverage this info yet, but
> your patch are killing this info, which IHMO is going in the wrong
> direction.


I can copy vnet header ahead of data_hard_start, does it work for you?

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;
>
>

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