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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 23 Apr 2014 23:10:50 +0300

On Wed, Apr 23, 2014 at 03:39:44PM -0400, Vlad Yasevich wrote:
> 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.

Seems cleaner to test a single bit otherwise one is left
wondering what happens if only one bit matches.

> > 
> > 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.

Hmm how do we know it's not a gso packet?
All I see is need_gso test which means it needs segmentation.

> Also, other offloads are dependent on checksum.
> 
> -vlad

Right so what if checksum is on, but segmentation is off?
Not the case with e1000 today but can be with other userspace.

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