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

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Message-ID: <CAPshTChzL5f5SD2ofH4frhX8QJBgbkw9+qStCMdg2u=WQ8Trqg@mail.gmail.com>
Date:	Tue, 4 Mar 2014 15:11:34 -0800
From:	Jerry Chu <hkchu@...gle.com>
To:	Joseph Gasparakis <joseph.gasparakis@...el.com>
Cc:	Or Gerlitz <ogerlitz@...lanox.com>,
	David Miller <davem@...emloft.net>,
	Pravin B Shelar <pshelar@...ira.com>,
	Eric Dumazet <edumazet@...gle.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Tom Herbert <therbert@...gle.com>
Subject: Re: [PATCH] net-gre-gro: Fix a bug that breaks the forwarding path

On Tue, Mar 4, 2014 at 2:53 PM, Joseph Gasparakis
<joseph.gasparakis@...el.com> 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 <ogerlitz@...lanox.com> 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

>
>>
>> >
>> > 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.
>> >
>> >
>>
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