netdev - Re: [PATCH v2] MPLS: Add limited GSO support

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Date:	Wed, 3 Apr 2013 16:51:38 -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>,
	Jarno Rajahalme <jarno.rajahalme@....com>,
	Pravin B Shelar <pshelar@...ira.com>
Subject: Re: [PATCH v2] MPLS: Add limited GSO support

On Wed, Apr 3, 2013 at 2:11 AM, Simon Horman <horms@...ge.net.au> wrote:
> In the case where a non-MPLS packet is recieved and an MPLS stack is
> added it may well be the case that the original skb is GSO but the
> NIC used for transmit does not support GSO of MPLS packets.
>
> The aim of this code is to provide GSO in software for MPLS packets
> whose skbs are GSO.
>
> When an implementation adds an MPLS stack to a non-MPLS packet it should do
> the following to skb metadata:
>
> * Set skb_mac_header(skb)->protocol to the new MPLS ethertype.
>   That is, either ETH_P_MPLS_UC or ETH_P_MPLS_MC.
>
> * Leave skb->protocol as the old non-MPLS ethertype.
>
> * Set skb->encapsulation = 1.
>
>   This may not strictly be necessary as I believe that checking
>   skb_mac_header(skb)->protocol and skb->protocol should be necessary and
>   sufficient.
>
>   However, it does seem to fit nicely with the current implementation of
>   dev_hard_start_xmit() where the more expensive check of
>   skb_mac_header(skb)->protocol may be guarded by an existing check of
>   skb->encapsulation.
>
> One aspect of this patch that I am unsure about is the modification I have
> made to skb_segment(). This seems to be necessary as checskum accelearation
> may no longer be possible as the packet has changed to be MPLS from some
> other packet type which may have been supported by the hardware in use.
>
> I will post a patch, "[PATCH v3.24] datapath: Add basic MPLS support to
> kernel" which adds MPLS support to the kernel datapath of Open vSwtich.
> That patch sets the above requirements in
> datapath/actions.c:set_ethertype() and was used to exercise the MPLS GSO
> code. The datapath patch is against the Open vSwtich tree but it is
> intended that it be added to the Open vSwtich code present in the mainline
> Linux kernel at some point.
>
> Suggested by Jesse Gross. Based heavily on "v4 GRE: Add TCP segmentation
> offload for GRE" by Pravin B Shelar.
>
> Cc: Jesse Gross <jesse@...ira.com>
> Cc: Pravin B Shelar <pshelar@...ira.com>
> Signed-off-by: Simon Horman <horms@...ge.net.au>

MPLS is very similar to both the Ethernet header and vlans in that GSO
only requires replication without any modification.  That means that
if we look at the mac_len as containing all three then we can just
copy it without any special knowledge.  I don't know that we carefully
maintain mac_len in all places but you are already doing that in your
MPLS patches.

The other piece at that point is getting the inner protocol.  I'm
worried that using skb->protocol for this will break things that try
to use it for filtering.  It also means that depending on whether an
MPLS packet is locally sourced or not skb->protocol may be different
because we won't always be able to find the inner header.  I think
this will require a careful definition of that field to make it
consistent.
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