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 20:48:03 +0000
From:	"Zou, Yi" <yi.zou@...el.com>
To:	Ben Hutchings <bhutchings@...arflare.com>,
	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>
CC:	"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 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 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.

yi

> 
> --
> 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.