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Date:	Tue, 20 May 2014 07:02:22 +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 Sat, May 17, 2014 at 10:32:24AM +0900, Simon Horman wrote:
> 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.

I have come up with this which seems clean.
On top of this it is easy enough to add inner_protocol
to struct ovs_gso_cb by using the new 16bit hole in that structure.


From: Simon Horman <horms@...ge.net.au>

[PATCH] datapath: 16bit inner_network_header field in struct ovs_gso_cb

The motivation for this is to create a 16bit hole in struct ovs_gso_cb
which may be used for the inner_protocol field which is needed
for the proposed implementation of compatibility for MPLS GSO segmentation.

This should be safe as inner_network_header is now an offset to
the inner_mac_header rather than skb->head.

As pointed out by Thomas Graf simply making both inner offsets 16bis is not
safe as there have been cases of overflow with "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."

This patch is based on suggestions by Thomas Graf and Jesse Gross.

Cc: Thomas Graf <tgraf@...g.ch>
Cc: Jesse Gross <jesse@...ira.com>
Signed-off-by: Simon Horman <horms@...ge.net.au>
---
 datapath/linux/compat/gso.h | 22 +++++++++++++---------
 1 file changed, 13 insertions(+), 9 deletions(-)

diff --git a/datapath/linux/compat/gso.h b/datapath/linux/compat/gso.h
index b83a4c3..f459c18 100644
--- a/datapath/linux/compat/gso.h
+++ b/datapath/linux/compat/gso.h
@@ -11,8 +11,10 @@
 
 struct ovs_gso_cb {
 	struct ovs_skb_cb dp_cb;
-	sk_buff_data_t	inner_network_header;
-	sk_buff_data_t	inner_mac_header;
+	u16		inner_network_header;	/* Offset from
+						 * inner_mac_header */
+	/* 16bit hole */
+	sk_buff_data_t	inner_mac_header;	/* Offset from skb->head */
 	void (*fix_segment)(struct sk_buff *);
 };
 #define OVS_GSO_CB(skb) ((struct ovs_gso_cb *)(skb)->cb)
@@ -20,18 +22,19 @@ struct ovs_gso_cb {
 #define skb_inner_network_header rpl_skb_inner_network_header
 
 #ifdef NET_SKBUFF_DATA_USES_OFFSET
-#define skb_inner_network_header rpl_skb_inner_network_header
-static inline unsigned char *skb_inner_network_header(const struct sk_buff *skb)
-{
-	return skb->head + OVS_GSO_CB(skb)->inner_network_header;
-}
-
 #define skb_inner_mac_header rpl_skb_inner_mac_header
 static inline unsigned char *skb_inner_mac_header(const struct sk_buff *skb)
 {
 	return skb->head + OVS_GSO_CB(skb)->inner_mac_header;
 }
 
+#define skb_inner_network_header rpl_skb_inner_network_header
+static inline unsigned char *skb_inner_network_header(const struct sk_buff *skb)
+{
+	return skb_inner_mac_header(skb) +
+		OVS_GSO_CB(skb)->inner_network_header;
+}
+
 #else
 
 #define skb_inner_network_header rpl_skb_inner_network_header
@@ -64,7 +67,8 @@ static inline int skb_inner_mac_offset(const struct sk_buff *skb)
 static inline void skb_reset_inner_headers(struct sk_buff *skb)
 {
 	BUILD_BUG_ON(sizeof(struct ovs_gso_cb) > FIELD_SIZEOF(struct sk_buff, cb));
-	OVS_GSO_CB(skb)->inner_network_header = skb->network_header;
+	OVS_GSO_CB(skb)->inner_network_header = skb->network_header -
+		skb->mac_header;
 	OVS_GSO_CB(skb)->inner_mac_header = skb->mac_header;
 
 	OVS_GSO_CB(skb)->fix_segment = NULL;
-- 
1.8.4



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