netdev - RE: [net 2/2] net: fix a bug of dropping FCoE frames when disabling tx ip checksum

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Date:	Wed, 14 Mar 2012 23:23:17 +0000
From:	"Zou, Yi" <yi.zou@...el.com>
To:	Ben Hutchings <bhutchings@...arflare.com>
CC:	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"gospo@...hat.com" <gospo@...hat.com>,
	"sassmann@...hat.com" <sassmann@...hat.com>
Subject: RE: [net 2/2] net: fix a bug of dropping FCoE frames when disabling
 tx ip checksum

> On Wed, 2012-03-14 at 20:48 +0000, Zou, Yi wrote:
> > >
> > > On Wed, 2012-03-14 at 00:01 -0700, Jeff Kirsher wrote:
> > > > From: Yi Zou <yi.zou@...el.com>
> > > >
> > > > Fix a bug when using 'ethtool -K ethx tx off' to turn off tx ip
> > > checksum,
> > > > FCoE CRC offload should not be impacte. The skb_checksum_help() is
> > > needed
> > > > only if it's not FCoE traffic for ip checksum, regardless of
> ethtool
> > > toggling
> > > > the tx ip checksum on or off.
> > > [...]
> > >
> > > I think the bug is more fundamental, and it's not just a problem for
> > > FCoE.  For the transmit path, the ip_summed values are specified as:
> > >
> > > [forwarding]
> > >  *	COMPLETE: the most generic way. Device supplied checksum of
> _all_
> > >  *	    the packet as seen by netif_rx in skb->csum.
> > >  *	    NOTE: Even if device supports only some protocols, but
> > >  *	    is able to produce some skb->csum, it MUST use COMPLETE,
> > >  *	    not UNNECESSARY.
> > >
> > > [locally-generated]
> > >  *	NONE: skb is checksummed by protocol or csum is not required.
> > >  *
> > >  *	PARTIAL: device is required to csum packet as seen by
> > > hard_start_xmit
> > >  *	from skb->csum_start to the end and to record the checksum
> > >  *	at skb->csum_start + skb->csum_offset.
> > >
> > > It's implicit that the checksum algorithm for CHECKSUM_PARTIAL is as
> > > specified for TCP/IP.  So none of those is correct when a different
> > > algorithm is to be used.
> > >
> > > It seems like we may need another ip_summed value for FCoE, SCTP or
> any
> > > other protocol with a different checksum algorithm that will be
> > > offloaded.  Maybe allow CHECKSUM_UNNECESSARY to be used on output in
> > > that case?
> > >
> > > Ben.
> >
> > CHECKSUM_UNNECESSARY sounds good to me, if it's ok to be used on the tx
> path
> > as well,
> 
> I don't believe it is yet.
> 
> > I think so but I am not 100% sure, it'd resolve this for fcoe and sctp
> > like, downside is it's a bigger change that requires corresponding
> changes in
> > these protocol driver stacks as well.
> 
> Yes, but this should be done properly rather than patched up with a
> bunch of checks for specific protocols.
> 
> Ben.
Agreed, however, I will still have to fix the netif_needs_gso() like below
to be able to use CHECKSUM_UNNECESSARY to not go down the dev_gso_segment()
path for FCoE, which is using SKB_GSO_FCOE.

So, anyone see this would break anything? i.e., using CHECKSUM_UNNECESSARY
but still want to do dev_gso_segment? If ok, I will resend the patch to
do the following.

Thanks,
yi

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 0eac07c..c1b2b5f 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -2636,7 +2636,8 @@ static inline int netif_needs_gso(struct sk_buff *skb,
        netdev_features_t features)
 {
        return skb_is_gso(skb) && (!skb_gso_ok(skb, features) ||
-               unlikely(skb->ip_summed != CHECKSUM_PARTIAL));
+               unlikely((skb->ip_summed != CHECKSUM_PARTIAL) &&
+                        (skb->ip_summed != CHECKSUM_UNNECESSARY)));
 }



> 
> --
> Ben Hutchings, Staff Engineer, Solarflare
> Not speaking for my employer; that's the marketing department's job.
> They asked us to note that Solarflare product names are trademarked.