netdev - Re: [PATCH] net: update the usage of CHECKSUM

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Message-ID: <1336608301.2499.12.camel@bwh-desktop.uk.solarflarecom.com>
Date:	Thu, 10 May 2012 01:05:01 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>,
	David Miller <davem@...emloft.net>
CC:	Yi Zou <yi.zou@...el.com>, <netdev@...r.kernel.org>,
	<devel@...n-fcoe.org>, <jeffrey.t.kirsher@...el.com>
Subject: Re: [PATCH] net: update the usage of CHECKSUM_UNNECESSARY

On Wed, 2012-05-09 at 09:03 +0300, Michael S. Tsirkin wrote:
> On Tue, May 08, 2012 at 07:20:58PM +0100, Ben Hutchings wrote:
> > On Tue, 2012-05-08 at 20:48 +0300, Michael S. Tsirkin wrote:
> > > On Mon, Mar 19, 2012 at 02:12:41PM -0700, Yi Zou wrote:
> > > > As suggested by Ben, this adds the clarification on the usage of
> > > > CHECKSUM_UNNECESSARY on the outgoing patch. Also add the usage
> > > > description of NETIF_F_FCOE_CRC and CHECKSUM_UNNECESSARY
> > > > for the kernel FCoE protocol driver.
> > > > 
> > > > This is a follow-up to the following:
> > > > http://patchwork.ozlabs.org/patch/147315/
> > > > 
> > > > Signed-off-by: Yi Zou <yi.zou@...el.com>
> > > > Cc: Ben Hutchings <bhutchings@...arflare.com>
> > > > Cc: Jeff Kirsher <jeffrey.t.kirsher@...el.com>
> > > > Cc: www.Open-FCoE.org <devel@...n-fcoe.org>
> > > > ---
> > > > 
> > > >  include/linux/skbuff.h |    7 +++++++
> > > >  1 files changed, 7 insertions(+), 0 deletions(-)
> > > > 
> > > > diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> > > > index 8dc8257..a2b9953 100644
> > > > --- a/include/linux/skbuff.h
> > > > +++ b/include/linux/skbuff.h
> > > > @@ -94,6 +94,13 @@
> > > >   *			  about CHECKSUM_UNNECESSARY. 8)
> > > >   *	NETIF_F_IPV6_CSUM about as dumb as the last one but does IPv6 instead.
> > > >   *
> > > > + *	UNNECESSARY: device will do per protocol specific csum. Protocol drivers
> > > > + *	that do not want net to perform the checksum calculation should use
> > > > + *	this flag in their outgoing skbs.
> > > > + *	NETIF_F_FCOE_CRC  this indicates the device can do FCoE FC CRC
> > > > + *			  offload. Correspondingly, the FCoE protocol driver
> > > > + *			  stack should use CHECKSUM_UNNECESSARY.
> > > > + *
> > > >   *	Any questions? No questions, good. 		--ANK
> > > >   */
> > > >  
> > > 
> > > So just to make sure I understand, you never get
> > > UNNECESSARY packets on tx unless you declared NETIF_F_FCOE_CRC?
> > > 
> > > Maybe the comment says this somehow but could not figure it out.
> > 
> > That's what should happen now.  In future CHECKSUM_UNNECESSARY could be
> > used on output by other protocols which don't use TCP/IP-style
> > checksums, but always dependent on the output device supporting the
> > relevant offload feature.
> > 
> > Ben.
> 
> Isn't there another case: a device passes UNNECESSARY in on rx,
> and the skb is forwarded to another device?
> For example it is handled this way by tun, giving a nice
> performance boost for VMs, see 10a8d94a95742bb15b4e617ee9884bb4381362be

Hmm... I thought UNNECESSARY was supposed to be replaced by NONE on
output, but I don't see where that would happen.  Which would mean we
had an undocumented case here, and now we have ambiguity. :-(

Ben.

-- 
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.

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