Subject: Re: [PATCH] net-gre-gro: Fix a bug that breaks the forwarding path

From: Joseph Gasparakis <[email-masked]>
Date: Tue, 4 Mar 2014 17:27:43 -0800 (PST)

On Tue, 4 Mar 2014, Jerry Chu wrote:

> On Tue, Mar 4, 2014 at 5:01 PM, Joseph Gasparakis
> <[email-masked]> wrote:
> >
> >
> > On Tue, 4 Mar 2014, Jerry Chu wrote:
> >
> >> On Tue, Mar 4, 2014 at 2:53 PM, Joseph Gasparakis
> >> <[email-masked]> wrote:
> >> >
> >> >
> >> > On Tue, 4 Mar 2014, Jerry Chu wrote:
> >> >
> >> >> Hi Or,
> >> >>
> >> >> Thanks for writing this up.
> >> >>
> >> >> On Tue, Mar 4, 2014 at 8:13 AM, Or Gerlitz <[email-masked]> wrote:
> >> >> > On 28/02/2014 23:56, David Miller wrote:
> >> >> >>
> >> >> >> The topic of the skb->encapsulation semantics has come up several times in
> >> >> >> the past few weeks. We cannot move forward on any changes in this area until
> >> >> >> the semantics are well defined, and documented. Can someone work on a patch
> >> >> >> which documents skb->encapsulation properly, and then we can come back to
> >> >> >> fixing this bug? Thanks.
> >> >> >
> >> >> >
> >> >> > Lets try... the skb->encapsulation flag was introduced and used in 3.8 by
> >> >> > the
> >> >> > sequence of these three commits
> >> >> >
> >> >> > 0afb166 vxlan: Add capability of Rx checksum offload for inner packet
> >> >> > d6727fe vxlan: capture inner headers during encapsulation
> >> >> > 6a674e9 net: Add support for hardware-offloaded encapsulation
> >> >> >
> >> >> > When discussed earlier on the list in the context of the skb->ip_summed
> >> >> > field,
> >> >> > Tom Herbert came with the following interpretation for the semantics which
> >> >> > Joseph confirmed
> >> >> >
> >> >> > "when skb->encapsulation is set the ip_summed is valid for both the inner
> >> >> > and outer header
> >> >> > (e.g. CHECKSUM_COMPLETE is always assumed okay for both layers). If
> >> >> > skb->encapsulation is not set then ip_summed is only valid for outer header"
> >> >>
> >> >> For GRE encapped pkts is the following interpretation correct?
> >> >>
> >> >> 1) ip_summed == CHECKSUM_COMPLETE
> >> >>     => csum covers IP payload csum of the outer IP datagram
> >> >>
> >> >> 2) ip_summed == CHECKSUM_UNNECESSARY
> >> >> 2.1. skb->encapsulation is on => both GRE csum (if one is present) and TCP/UDP
> >> >> csum have been validated (assuming inner is a TCP or UDP pkt)
> >> >
> >> > i40e also supports SCTP csumming for the inner packet, too.
> >> >
> >> >>
> >> >> 2.2. skb->encapsulation is off => only GRE csum (if one is present) is
> >> >> validated.
> >> >>
> >> >
> >> > Apart for the SCTP request for inclusion, it looks reasonable to me.
> >> >
> >> >> >
> >> >> > As for the TX side of things, the change-log of commit 6a674e9 states
> >> >> >
> >> >> > "For Tx encapsulation offload, the driver will need to set the right bits in
> >> >> > netdev->hw_enc_features. The protocol driver will have to set the
> >> >> > skb->encapsulation bit and populate the inner headers, so the NIC driver
> >> >> > will use those inner headers to calculate the csum in hardware."
> >> >>
> >> >> So we only support/care about csum offload for the inner pkts, which
> >> >> makes sense.
> >> >
> >> > Well, we care about outer too. It is just that the inner headers are for
> >> > the inner csum, the outer headers are for the outer csum. The outer
> >> > headers will be always there anyway, so the patch introduced the inner
> >> > ones. But I guess this is what you meant, right?
> >>
> >> Actually i was wondering that since there is only one set csum_start/csum_offset
> >> fields per skb, which would you support CHECKSUM_PARTIAL for both inner
> >> and outer? You can only support one, right? (I haven't checked the UDP encap
> >> code to see how this works though.)
> >>
> >> Jerry
> >
> > I am not sure I understand the question, Jerry... Are you asking in Tx
> > path if the ip_summed==CHECKSUM_PARTIAL, which headers will the
> > csum_start/csum_offset refer to?
> 
> Yes (if the question makes sense since i haven't read any UDP encap
> code - will the outer (UDP) csum be enabled?).
> 
> Thanks,
> 
> Jerry

I believe they will refer to the inner headers as csum_start and 
csum_offset do not get changed during encapsulation and they refer to the 
original (inner) packet, but please verify this.

> 
> >
> >> >
> >> >>
> >> >> >
> >> >> > So in higher level, it seems that the role of the skb->encapsulation field
> >> >> > is to mark the skb to carry encapsulated packet for the code path between
> >> >> > the time the packet is encapsulated by the protocol driver (e.g vxlan/ipip)
> >> >> > to the time driver xmit is called. Or from the time driver rx code runs till
> >> >> > the the time the packet is decapsulated.
> >> >> >
> >> >> > Further, my personal interpretation was that on the rx path, skb should
> >> >> > carry the encapsulation flag **only** if the HW was able to offload the
> >> >> > inner checksum.
> >> >>
> >> >> SGTM.
> >> >>
> >> >> Jerry
> >> >>
> >> >> >
> >> >> > Joseph, what's your thinking here?
> >> >> >
> >> >> > Or.
> >> >> >
> >> >> >
> >> >>
> >>
>