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

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Date:	Tue, 4 Mar 2014 14:13:20 -0800
From:	Jerry Chu <hkchu@...gle.com>
To:	Or Gerlitz <ogerlitz@...lanox.com>
Cc:	David Miller <davem@...emloft.net>,
	Joseph Gasparakis <joseph.gasparakis@...el.com>,
	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

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)

2.2. skb->encapsulation is off => only GRE csum (if one is present) is
validated.

>
> 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.

>
> 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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