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

[Jesse Gross <jesse-l0M0P4e3n4LQT0dZR+AlfA@public.gmane.org>]

On Wed, May 1, 2013 at 3:57 PM, Simon Horman <horms-/R6kz+dDXgpPR4JQBCEnsQ@public.gmane.org> wrote:
> On Wed, May 01, 2013 at 11:16:40AM -0700, Jesse Gross wrote:
>> On Wed, May 1, 2013 at 12:50 AM, Simon Horman <horms-/R6kz+dDXgpPR4JQBCEnsQ@public.gmane.org> wrote:
>> > 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-/R6kz+dDXgpPR4JQBCEnsQ@public.gmane.org> 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-/R6kz+dDXgpPR4JQBCEnsQ@public.gmane.org> 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...?
>>
>> No, I'm talking about netdev features. You can already tell the
>> encapsulation type of a packet by looking at the EtherType.
>
> Now I am completely confused about what are the two fields that you
> refer to in your previous email.

I have always been referring to the netdev features for various
protocol types. This is because considering MPLS as a form of
encapsulation for the purpose of offloads buckets too many protocols
into the same set and NICs will have varying features for those.
Trying to avoid this by having a bit for offloadable encapsulations is
just going to be very confusing and not very future proof.

> In regards to looking ath the ethernet type:
>
> One of the tricky parts of MPLS is that the packet itself does not contain
> the ethernet type or any other way of knowing the type of the inner-packet.
> Information that is needed for GSO.

I'm aware of that. However, you were referring to the type of
encapsulation. It is easy to determine that a packet is MPLS.

> My proposal to get around this is to leave skb->protocol as the
> original, in the case we are interested in non-MPLS, ethernet type.

At the very least, this is not consistent with how it is currently
handled (for example, with VLANs) and seems difficult to do properly.
However, I have not seen any further analysis since the last time that
we discussed this.