Linux Virtualization - May 2020 - [PATCH net-next 1/2] virtio-net: don't reserve space for vnet header for XDP

On 2020/5/6 ??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 at redhat.com>
>> Signed-off-by: Jason Wang <jasowang at redhat.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

On Wed, May 06, 2020 at 04:19:40PM +0800, Jason Wang
wrote:
> 
> On 2020/5/6 ??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
Then what's the problem?
> 
> > 
> > 
> > > Cc: Jesper Dangaard Brouer <brouer at redhat.com>
> > > Signed-off-by: Jason Wang <jasowang at redhat.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

On 2020/5/6 ??5:54, Michael S. Tsirkin wrote:
> On Wed, May 06, 2020 at 04:19:40PM +0800, Jason Wang wrote:
>> On 2020/5/6 ??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
> Then what's the problem?

Then we can't do anything more than just invalidating vnet header since 
we don't know whether or not the packet has been modified or not.

Technically, XDP can give the driver some hint about whether or not the 
packet has been modified, but AFAIK we haven't implemented this yet.

Thanks

>