Date:	Wed, 1 May 2013 16:50:29 +0900
From:	Simon Horman <horms@...ge.net.au>
To:	Jesse Gross <jesse@...ira.com>
Cc:	Joseph Gasparakis <joseph.gasparakis@...el.com>,
	"dev@...nvswitch.org" <dev@...nvswitch.org>,
	netdev <netdev@...r.kernel.org>,
	Jarno Rajahalme <jarno.rajahalme@....com>,
	Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@...el.com>,
	Alexander Duyck <alexander.h.duyck@...el.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Maciej Żenczykowski <maze@...gle.com>
Subject: Re: [PATCH net-next 1/2] net: More fine-grained support for
 encapsulated GSO features

On Tue, Apr 30, 2013 at 09:19:51AM -0700, Jesse Gross wrote:
> On Mon, Apr 29, 2013 at 8:21 PM, Simon Horman <horms@...ge.net.au> wrote:
> > On Fri, Apr 26, 2013 at 04:03:21PM -0700, Jesse Gross wrote:
> >> On Thu, Apr 25, 2013 at 12:36 AM, Simon Horman <horms@...ge.net.au> wrote:
> >> > On Tue, Apr 23, 2013 at 02:00:19PM -0700, Joseph Gasparakis wrote:
> >> >> Any particular reason to introduce skb->encapsulation_features instead of
> >> >> using the existing skb->encapsulation? Also I don't see it used in your
> >> >> second patch either.
> >> >
> >> > My reasoning is that skb->encapsulation seems to alter the behaviour of
> >> > many different locations and I'm not sure that any of them, other than the
> >> > one in dev_hard_start_xmit() make sense for MPLS.
> >>
> >> The problem is the meaning of skb->encapsulation isn't really defined
> >> clearly and I'm certain that the current implementation is not going
> >> to work in the future. Depending on your perspective, vlans, MPLS,
> >> tunnels, etc. can all be considered forms of encapsulation but clearly
> >> there are many NICs that have different capabilities across those. I
> >> believe the intention here was really to describe L3 tunnels as
> >> encapsulation, in which case MPLS really shouldn't be using this
> >> mechanism at all.
> >>
> >> Now there is some overlap, especially today since most currently
> >> shipping silicon wasn't designed to support tunnels and so is using
> >> some form of offset based offloads. In that case, all forms of
> >> encapsulation are pretty similar. However, in the future that won't be
> >> the case as support for specific protocols is implemented for higher
> >> performance and richer support. When that happens, not only will MPLS
> >> and tunnels have different capabilities but various forms tunnels
> >> might as well.
> >
> > Wouldn't be possible to describe those differences using,
> > dev->hw_enc_features? I assumed that was its purpose.
> 
> If there truly are differences between the offload capabilities of
> MPLS and L3 tunnels then no, it's not possible, because it's a single
> field. It's certainly not a valid assumption that a NIC that can do
> TSO over GRE can also do it over MPLS.
> 
> However, it's unlikely that there are truly significant differences
> between various encapsulation formats on a per-feature basis. I think
> what we need to do is separate out the ability to understand the
> headers from the capabilities so you have two fields: header (none,
> VLAN, QinQ, MPLS, VXLAN, GRE, etc.) and feature (checksum, SG, TSO,
> etc.) rather than the product of each. Otherwise, we end up with a ton
> of different combinations.

I'm not quite sure that I follow.

Is your idea to replace skb->encapsulation (a single bit) with
a field that corresponds to the outer-most (encapsulation) header in use
and has bits for none, VLAN, QinQ, MPLS, VXLAN, GRE, etc...?

If so, I believe that would solve the problem I was trying to address
with this patch.
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