Subject: Re: [PATCH 1/2] mactap: Fix checksum errors for non-gso packets in bridge mode

From: Vlad Yasevich <[email-masked]>
Date: Wed, 23 Apr 2014 15:39:44 -0400

On 04/23/2014 03:20 PM, Michael S. Tsirkin wrote:
> On Wed, Apr 23, 2014 at 12:51:40PM -0400, Vlad Yasevich wrote:
>> The following is a problematic configuration:
>>
>>  VM1: virtio-net device connected to macvtap0@eth0
>>  VM2: e1000 device connect to macvtap1@eth0
>>
>> The problem is is that virtio-net supports checksum offloading
>> and thus sends the packets to the host with CHECKSUM_PARTIAL set.
>> On the other hand, e1000 does not support any acceleration.
>>
>> For small TCP packets (and this includes the 3-way handshake),
>> e1000 ends up receiving packets that only have a partial checksum
>> set.  This causes TCP to fail checksum validation and to drop
>> packets.  As a result tcp connections can not be established.
>>
>> Commit 3e4f8b787370978733ca6cae452720a4f0c296b8
>> 	macvtap: Perform GSO on forwarding path.
>> fixes this issue for large packets wthat will end up undergoing GSO.
>> This commit adds a check for the non-GSO case and attempts to
>> compute the checksum for partially checksummed packets in the
>> non-GSO case.
>>
>> CC: Daniel Lezcano <[email-masked]>
>> CC: Patrick McHardy <[email-masked]>
>> CC: Andrian Nord <[email-masked]>
>> CC: Eric Dumazet <[email-masked]>
>> CC: Michael S. Tsirkin <[email-masked]>
>> CC: Jason Wang <[email-masked]>
>> Signed-off-by: Vlad Yasevich <[email-masked]>
>> ---
>>  drivers/net/macvtap.c | 7 +++++++
>>  1 file changed, 7 insertions(+)
>>
>> diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
>> index ff111a8..ba91084 100644
>> --- a/drivers/net/macvtap.c
>> +++ b/drivers/net/macvtap.c
>> @@ -322,6 +322,13 @@ static rx_handler_result_t macvtap_handle_frame(struct sk_buff **pskb)
>>  			segs = nskb;
>>  		}
>>  	} else {
>> +		/* If we receive a partial checksum and the tap side
>> +		 * doesn't support checksum offload, compute the checksum.
>> +		 */
>> +		if (skb->ip_summed == CHECKSUM_PARTIAL &&
>> +		    !(features & NETIF_F_ALL_CSUM) &&
>> +		    skb_checksum_help(skb))
>> +			goto drop;
> 
> Hmm confused by NETIF_F_ALL_CSUM here.
> 
> features come from here:
>                 feature_mask = NETIF_F_HW_CSUM;
> 
>                 if (arg & (TUN_F_TSO4 | TUN_F_TSO6)) {
>                         if (arg & TUN_F_TSO_ECN)
>                                 feature_mask |= NETIF_F_TSO_ECN;
>                         if (arg & TUN_F_TSO4)
>                                 feature_mask |= NETIF_F_TSO;
>                         if (arg & TUN_F_TSO6)
>                                 feature_mask |= NETIF_F_TSO6;
>                 }
> 
>                 if (arg & TUN_F_UFO)
>                         feature_mask |= NETIF_F_UFO;
> 
> 
> okay so why not just check that NETIF_F_HW_CSUM is set?

We can do that, but it doesn't make much difference.

> 
> Also does it matter whether specific offloads are enabled?
> 

No it doesn't matter at all.  The packet is not a GSO packet
so no other acceleration is used.
Also, other offloads are dependent on checksum.

-vlad

> 
>>  		skb_queue_tail(&q->sk.sk_receive_queue, skb);
>>  	}
>>  
>> -- 
>> 1.9.0