netdev - Re: [PATCH net-next 1/2] net: More fine-grained support for encapsulated GSO features

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Date:	Tue, 30 Apr 2013 09:19:51 -0700
From:	Jesse Gross <jesse@...ira.com>
To:	Simon Horman <horms@...ge.net.au>
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 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.
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