netdev - Re: [PATCH] veth: remove hardware checksum feature

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Date:	Wed, 07 Aug 2013 17:12:32 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Ben Greear <greearb@...delatech.com>
Cc:	"Vitaly E. Lavrov" <lve@...p.ru>, netdev@...r.kernel.org,
	davem@...emloft.net
Subject: Re: [PATCH] veth: remove hardware checksum feature

On Wed, 2013-08-07 at 17:07 -0700, Ben Greear wrote:
> On 07/25/2013 10:20 AM, Vitaly E. Lavrov wrote:
> > The network device VETH can't support the feature NETIF_F_HW_CSUM.
> > All locally generated packets have invalid checksum.
> > Wrong commit http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=8093315a91340bca52549044975d8c7f673b28a1 ( kernel 3.9.0 )
> >
> > Workaround "ethtool -K vethX tx off"
> >
> > Possible patch:
> >
> > diff --git a/drivers/net/veth.c b/drivers/net/veth.c
> > index 177f911..3db97da 100644
> > --- a/drivers/net/veth.c
> > +++ b/drivers/net/veth.c
> > @@ -254,7 +254,7 @@ static const struct net_device_ops veth_netdev_ops = {
> >   };
> >
> >   #define VETH_FEATURES (NETIF_F_SG | NETIF_F_FRAGLIST | NETIF_F_ALL_TSO |    \
> > -                      NETIF_F_HW_CSUM | NETIF_F_RXCSUM | NETIF_F_HIGHDMA | \
> > +                      NETIF_F_RXCSUM | NETIF_F_HIGHDMA | \
> >                         NETIF_F_HW_VLAN_CTAG_TX | NETIF_F_HW_VLAN_CTAG_RX | \
> >                         NETIF_F_HW_VLAN_STAG_TX | NETIF_F_HW_VLAN_STAG_RX )
> 
> I see the same problem.
> 
> My test case is a bit complicated, but the gist is that I have a VETH
> pair, one with IP (veth1), one connected to a bridge-like-thing (veth2).
> 
> The UDP frames sent on veth1 appear on veth2, and when I sniff veth2,
> the packets show broken checksum.  The work-around mentioned in Vitaly's
> email above fixes the problem for me (I did not try the patch yet).
> 
> Eric:  You responded originally that you needed more info.  If
> my explanation above is not sufficient, please let me know what
> you need...
> 

tcpdump is known to display wrong checksums, its not a reason to disable
tx checksums on our interfaces and kill performance.

       -K     Don't  attempt to verify IP, TCP, or UDP checksums.  This
is useful for interfaces that perform some or all of those checksum
calculation in hardware; other‐
              wise, all outgoing TCP checksums will be flagged as bad.


So we need more information than 'my trcpdump says checksums are wrong'

We could also disable TSO because : It sends packets bigger than MTU,
this can not be good ;)



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