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

Simon Horman horms at verge.net.au
Wed May 1 22:57:06 UTC 2013


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 at verge.net.au> 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 at verge.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 at verge.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...?
> 
> 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.


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.

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.

The MPLS ethertype is in in the packet itself, however checking
that seems expensive.




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