netdev - RE: [PATCH net-next v2 1/5] xen-netback: add IPv6 checksum offload to guest

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Date:	Tue, 8 Oct 2013 13:14:30 +0000
From:	Paul Durrant <Paul.Durrant@...rix.com>
To:	Wei Liu <wei.liu2@...rix.com>
CC:	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Wei Liu <wei.liu2@...rix.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Ian Campbell <Ian.Campbell@...rix.com>
Subject: RE: [PATCH net-next v2 1/5] xen-netback: add IPv6 checksum offload
 to guest

> -----Original Message-----
> From: Wei Liu [mailto:wei.liu2@...rix.com]
> Sent: 08 October 2013 14:10
> To: Paul Durrant
> Cc: xen-devel@...ts.xen.org; netdev@...r.kernel.org; Wei Liu; David Vrabel;
> Ian Campbell
> Subject: Re: [PATCH net-next v2 1/5] xen-netback: add IPv6 checksum
> offload to guest
> 
> On Tue, Oct 08, 2013 at 11:58:12AM +0100, Paul Durrant wrote:
> > Check xenstore flag feature-ipv6-csum-offload to determine if a
> > guest is happy to accept IPv6 packets with only partial checksum.
> > Also check analogous feature-ip-csum-offload to determone if a
> 
> determone -> determine
> 

Ok.

> > guest is happy to accept IPv4 packets with only partial checksum
> > as a replacement for a negated feature-no-csum-offload value and
> > add a comment to deprecate use of feature-no-csum-offload.
> >
> [...]
> > @@ -306,7 +308,7 @@ struct xenvif *xenvif_alloc(struct device *parent,
> domid_t domid,
> >  	vif->domid  = domid;
> >  	vif->handle = handle;
> >  	vif->can_sg = 1;
> > -	vif->csum = 1;
> > +	vif->ip_csum = 1;
> 
> Not setting a default value for vif->ipv6_csum?
> 

The default is 0 so no need.

> >  	vif->dev = dev;
> >
> [...]
> > +	/* Before feature-ipv6-csum-offload was introduced, checksum
> offload
> > +	 * was turned on by a zero value in (or lack of)
> > +	 * feature-no-csum-offload. Therefore, for compatibility, assume
> > +	 * that if feature-ip-csum-offload is missing that IP (v4) offload
> > +	 * is turned on. If this is not the case then the flag will be
> > +	 * cleared when we subsequently sample feature-no-csum-offload.
> > +	 */
> > +	if (xenbus_scanf(XBT_NIL, dev->otherend, "feature-ip-csum-
> offload",
> > +			 "%d", &val) < 0)
> > +		val = 1;
> > +	vif->ip_csum = !!val;
> > +
> > +	if (xenbus_scanf(XBT_NIL, dev->otherend, "feature-ipv6-csum-
> offload",
> > +			 "%d", &val) < 0)
> > +		val = 0;
> > +	vif->ipv6_csum = !!val;
> > +
> > +	/* This is deprecated. Frontends should set IP v4 and v6 checksum
> > +	 * offload using feature-ip-csum-offload and
> > +	 * feature-ipv6-csum-offload respectively.
> > +	 */
> 
> These comments on feature flags should also be synchronized to master
> netif.h in Xen and Linux's header. We've been trying to document thing
> along the way for quite some time and the long term benifit is big.
> 

Ok. That sounds like a good idea. I'll add a patch to do that.

> >  	if (xenbus_scanf(XBT_NIL, dev->otherend, "feature-no-csum-
> offload",
> >  			 "%d", &val) < 0)
> >  		val = 0;
> > -	vif->csum = !val;
> > +	if (val)
> > +		vif->ip_csum = 0;
> 
> Do you also need to set vif->ipv6_csum to 0? I don't think a frontend
> which cannot deal with V4 csum offload has the ability to deal with V6
> csum offload.
> 

I was just trying to preserve the existing semantic of feature-no-csum-offload, which only affects v4. Since it's deprecated, should we add a new semantic?

  Paul
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