Subject: Re: [PATCH net-next 1/2] macvtap: Add support of packet capture on macvtap device.

From: Vlad Yasevich <[email-masked]>
Date: Thu, 12 Dec 2013 12:23:46 -0500

On 12/11/2013 10:45 PM, Jason Wang wrote:
> On 12/12/2013 02:27 AM, Vlad Yasevich wrote:
>> Macvtap device currently doesn not allow a user to capture
>> traffic on due to the fact that it steals the packets
>> from the network stack before the skb->dev is set correctly
>> on the receive side, and that use uses macvlan transmit
>> path directly on the send side.  As a result, we never
>> get a change to give traffic to the taps while the correct
>> device is set in the skb.
>>
>> This patch makes macvtap device behave almost exaclty like
>> macvlan.  On the send side, we switch to using dev_queue_xmit().
>> On the receive side, to deliver packets to macvtap, we now
>> use netif_rx and dev_forward_skb just like macvlan.  The only
>> differnce now is that macvtap has its own rx_handler which is
>> attached to the macvtap netdev.  It is here that we now steal
>> the packet and provide it to the socket.
>>
>> As a result, we can now capture traffic on the macvtap device:
>>    tcpdump -i macvtap0
>>
>> It also gives us the abilit to add tc actions to the macvtap
>> device and actually utilize different bandwidth management
>> queues on output.
>>
>> Signed-off-by: Vlad Yasevich <[email-masked]>
>> ---
>>  drivers/net/macvtap.c | 59 ++++++++++++++++++++++++++++-----------------------
>>  1 file changed, 32 insertions(+), 27 deletions(-)
>>
>> diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
>> index 4c6f84c..f9847da 100644
>> --- a/drivers/net/macvtap.c
>> +++ b/drivers/net/macvtap.c
>> @@ -70,6 +70,11 @@ static const struct proto_ops macvtap_socket_ops;
>>  #define RX_OFFLOADS (NETIF_F_GRO | NETIF_F_LRO)
>>  #define TAP_FEATURES (NETIF_F_GSO | NETIF_F_SG)
>>  
> [...]
>>  
>>  static void macvtap_dellink(struct net_device *dev,
>>  			    struct list_head *head)
>>  {
>> +	netdev_rx_handler_unregister(dev);
>>  	macvtap_del_queues(dev);
>>  	macvlan_dellink(dev, head);
>>  }
>> @@ -725,9 +731,8 @@ static ssize_t macvtap_get_user(struct macvtap_queue *q, struct msghdr *m,
>>  		skb_shinfo(skb)->tx_flags |= SKBTX_SHARED_FRAG;
>>  	}
>>  	if (vlan) {
>> -		local_bh_disable();
>> -		macvlan_start_xmit(skb, vlan->dev);
>> -		local_bh_enable();
>> +		skb->dev = vlan->dev;
>> +		dev_queue_xmit(skb);
>>  	} else {
>>  		kfree_skb(skb);
>>  	}
> 
> An issue here: macvlan is a NETIF_F_LLTX device with only one transmit
> queue, we may get contention on qdisc lock of macvtap when transmitting
> through multiple tx queues.

Hmm... Yes, it looks like we could.  I'll can change this to
    dev_start_hard_xmit(skb, vlan->dev, NULL, NULL);
for now.

To use dev_queue_xmit(), we'd need to expose the macvtap queues
as real device queues (similar to what we do with tap).  Would this
something we would to do.

I could see a desire to do attach qdiscs to macvtap device itself.
Right not that's not really possible.

Thanks
-vlad
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