netdev - Re: [PATCH v2.57] datapath: Add basic MPLS support to kernel

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Message-ID: <CAEP_g=-BQJTthV3JbgiiMtCEwM+-YX_sN5VpGkiPQLUVOKP0cg@mail.gmail.com>
Date:	Fri, 16 May 2014 14:29:11 -0700
From:	Jesse Gross <jesse@...ira.com>
To:	Simon Horman <horms@...ge.net.au>
Cc:	"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 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.
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