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Date:	Sat, 17 May 2014 10:32:24 +0900
From:	Simon Horman <horms@...ge.net.au>
To:	Jesse Gross <jesse@...ira.com>
Cc:	Thomas Graf <tgraf@...g.ch>,
	"dev@...nvswitch.org" <dev@...nvswitch.org>,
	netdev <netdev@...r.kernel.org>,
	Pravin B Shelar <pshelar@...ira.com>,
	Ben Pfaff <blp@...ira.com>, Ravi K <rkerur@...il.com>,
	Joe Stringer <joe@...d.net.nz>
Subject: Re: [PATCH v2.57] datapath: Add basic MPLS support to kernel

On Fri, May 16, 2014 at 03:17:18PM -0700, Jesse Gross wrote:
> On Fri, May 16, 2014 at 2:48 PM, Thomas Graf <tgraf@...g.ch> wrote:
> > On 05/16/14 at 02:29pm, Jesse Gross wrote:
> >> On Fri, May 16, 2014 at 1:48 AM, Simon Horman <horms@...ge.net.au> wrote:
> >> > On Fri, May 16, 2014 at 08:07:07AM +0900, Simon Horman wrote:
> >> >> Allow datapath to recognize and extract MPLS labels into flow keys
> >> >> and execute actions which push, pop, and set labels on packets.
> >> >
> >> > [snip]
> >> >
> >> >> diff --git a/datapath/linux/compat/gso.h b/datapath/linux/compat/gso.h
> >> >> index b83a4c3..98b2aba 100644
> >> >> --- a/datapath/linux/compat/gso.h
> >> >> +++ b/datapath/linux/compat/gso.h
> >> >> @@ -4,6 +4,7 @@
> >> >>  #include <linux/version.h>
> >> >>  #if LINUX_VERSION_CODE < KERNEL_VERSION(3,12,0)
> >> >>
> >> >> +#include <linux/netdevice.h>
> >> >>  #include <linux/skbuff.h>
> >> >>  #include <net/protocol.h>
> >> >>
> >> >> @@ -14,6 +15,9 @@ struct ovs_gso_cb {
> >> >>       sk_buff_data_t  inner_network_header;
> >> >>       sk_buff_data_t  inner_mac_header;
> >> >>       void (*fix_segment)(struct sk_buff *);
> >> >> +#if LINUX_VERSION_CODE < KERNEL_VERSION(3,11,0)
> >> >> +     __be16                  inner_protocol;
> >> >> +#endif
> >> >>  };
> >> >>  #define OVS_GSO_CB(skb) ((struct ovs_gso_cb *)(skb)->cb)
> >> >
> >> > Unfortunately it seems that at least for v3.11 on x86_64 the above now
> >> > results in struct ovs_gso_cb being larger than the cb field of struct
> >> > skb_buff.
> >> >
> >> > (It also seems that the call to ovs_skb_set_inner_protocol() is
> >> >  missing from push_mpls() so it is likely that GSO segmentation
> >> >  is broken.)
> >> >
> >> > I apologise for not noticing this earlier.
> >> >
> >> > One idea I had is so make inner_protocol union of some other member of
> >> > either struct ovs_gso_cb or struct ovs_skb_cb. But its not clear to me that
> >> > would fly.
> >> >
> >> > Another possibility is to simply drop compatibility for segmentation of GSO
> >> > packets that become MPLS.  But I gather that is not the preferred way for
> >> > datapath features.
> >> >
> >> > Any suggestions would be much appreciated.
> >>
> >> Argh, I didn't realize that we were completely at the limit here...
> >>
> >> This is a little nasty but for the compatibility code maybe we could
> >> make the inner offsets be 16 bits. I think this is unlikely to ever
> >> cause a problem in practice and so it might be OK for the compat code.
> >
> > We actually hit the 16bit overflow issue with collapsed TCP frames
> > on IB when the headroom grew beyond 64K. See commit 50bceae9bd
> > ''tcp: Reallocate headroom if it would overflow csum_start''
> > for additional details.
> 
> Good point, thanks.
> 
> > What about keeping the mac offset 32bit and basing a 16bit net offset
> > on the mac offset. That would give 16 spare bits.
> 
> I think that could work since we always set both at the same time.

Hi Jesse, Hi Thomas,

thanks for your suggestions. I will see if I can make them fly.
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