netdev - Re: [PATCH v3 bpf-next 3/8] veth: Avoid drops by oversized packets when XDP is enabled

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Date:   Tue, 24 Jul 2018 10:56:23 +0900
From:   Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Toshiaki Makita <toshiaki.makita1@...il.com>
Cc:     netdev@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <brouer@...hat.com>
Subject: Re: [PATCH v3 bpf-next 3/8] veth: Avoid drops by oversized packets
 when XDP is enabled

On 2018/07/24 9:27, Jakub Kicinski wrote:
> On Mon, 23 Jul 2018 00:13:03 +0900, Toshiaki Makita wrote:
>> From: Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
>>
>> All oversized packets including GSO packets are dropped if XDP is
>> enabled on receiver side, so don't send such packets from peer.
>>
>> Drop TSO and SCTP fragmentation features so that veth devices themselves
>> segment packets with XDP enabled. Also cap MTU accordingly.
>>
>> Signed-off-by: Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
> 
> Is there any precedence for fixing up features and MTU like this?  Most
> drivers just refuse to install the program if settings are incompatible.

I don't know any precedence. I can refuse the program on installing it
when features and MTU are not appropriate. Is it preferred?
Note that with current implementation wanted_features are not touched so
features will be restored when the XDP program is removed. MTU will not
be restored though, as I do not remember the original MTU.


>> diff --git a/drivers/net/veth.c b/drivers/net/veth.c
>> index 78fa08cb6e24..f5b72e937d9d 100644
>> --- a/drivers/net/veth.c
>> +++ b/drivers/net/veth.c
>> @@ -542,6 +542,23 @@ static int veth_get_iflink(const struct net_device *dev)
>>  	return iflink;
>>  }
>>  
>> +static netdev_features_t veth_fix_features(struct net_device *dev,
>> +					   netdev_features_t features)
>> +{
>> +	struct veth_priv *priv = netdev_priv(dev);
>> +	struct net_device *peer;
>> +
>> +	peer = rtnl_dereference(priv->peer);
>> +	if (peer) {
>> +		struct veth_priv *peer_priv = netdev_priv(peer);
>> +
>> +		if (peer_priv->_xdp_prog)
>> +			features &= ~NETIF_F_GSO_SOFTWARE;
>> +	}
>> +
>> +	return features;
>> +}
>> +
>>  static void veth_set_rx_headroom(struct net_device *dev, int new_hr)
>>  {
>>  	struct veth_priv *peer_priv, *priv = netdev_priv(dev);
>> @@ -591,14 +608,33 @@ static int veth_xdp_set(struct net_device *dev, struct bpf_prog *prog,
>>  				goto err;
>>  			}
>>  		}
>> +
>> +		if (!old_prog) {
>> +			peer->hw_features &= ~NETIF_F_GSO_SOFTWARE;
>> +			peer->max_mtu = PAGE_SIZE - VETH_XDP_HEADROOM -
>> +				peer->hard_header_len -
>> +				SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
>> +			if (peer->mtu > peer->max_mtu)
>> +				dev_set_mtu(peer, peer->max_mtu);
>> +		}
>>  	}
>>  
>>  	if (old_prog) {
>> -		if (!prog && dev->flags & IFF_UP)
>> -			veth_disable_xdp(dev);
>> +		if (!prog) {
>> +			if (dev->flags & IFF_UP)
>> +				veth_disable_xdp(dev);
>> +
>> +			if (peer) {
>> +				peer->hw_features |= NETIF_F_GSO_SOFTWARE;
>> +				peer->max_mtu = ETH_MAX_MTU;
>> +			}
>> +		}
>>  		bpf_prog_put(old_prog);
>>  	}
>>  
>> +	if ((!!old_prog ^ !!prog) && peer)
>> +		netdev_update_features(peer);
>> +
>>  	return 0;
>>  err:
>>  	priv->_xdp_prog = old_prog;
>> @@ -643,6 +679,7 @@ static const struct net_device_ops veth_netdev_ops = {
>>  	.ndo_poll_controller	= veth_poll_controller,
>>  #endif
>>  	.ndo_get_iflink		= veth_get_iflink,
>> +	.ndo_fix_features	= veth_fix_features,
>>  	.ndo_features_check	= passthru_features_check,
>>  	.ndo_set_rx_headroom	= veth_set_rx_headroom,
>>  	.ndo_bpf		= veth_xdp,

-- 
Toshiaki Makita