netdev - Re: [net-next 07/15] i40e/i40evf: Add TX/RX outer UDP checksum support for X722

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Date:	Thu, 6 Aug 2015 08:12:30 -0700
From:	Tom Herbert <tom@...bertland.com>
To:	Jesse Brandeburg <jesse.brandeburg@...el.com>
Cc:	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	"David S. Miller" <davem@...emloft.net>,
	Anjali Singhai Jain <anjali.singhai@...el.com>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	nhorman@...hat.com, sassmann@...hat.com, jogreene@...hat.com,
	Catherine Sullivan <catherine.sullivan@...el.com>
Subject: Re: [net-next 07/15] i40e/i40evf: Add TX/RX outer UDP checksum
 support for X722

On Wed, Aug 5, 2015 at 5:38 PM, Jesse Brandeburg
<jesse.brandeburg@...el.com> wrote:
> On Wed, 5 Aug 2015 17:13:21 -0700
> Tom Herbert <tom@...bertland.com> wrote:
>
>> On Wed, Aug 5, 2015 at 4:52 PM, Jeff Kirsher
>> <jeffrey.t.kirsher@...el.com> wrote:
>> > From: Anjali Singhai Jain <anjali.singhai@...el.com>
>> >                 if (vsi->back->flags & I40E_FLAG_WB_ON_ITR_CAPABLE)
>> >                         tx_ring->flags = I40E_TXR_FLAGS_WB_ON_ITR;
>> > +               if (vsi->back->flags & I40E_FLAG_OUTER_UDP_CSUM_CAPABLE)
>> > +                       tx_ring->flags |= I40E_TXR_FLAGS_OUTER_UDP_CSUM;
>>
>> Just curious... is there a difference between enabling the outer UDP
>> checksum (of a tunnel) and just enabling checksum offload for UDP
>> packets?
>
> Yes, the hardware knows the difference (or we actually tell it
> the difference) between a UDP packet and a tunnel inside a UDP
> packet.

There should be no difference between handling the UDP checksum for
tunneling versus a non-tunnel packet with a UDP checksum. While VXLAN
(RFC7348) allows for non-zero checksums to be not be verified, this
fundamentally violates UDP specification (RFC1122). Neither is there
any API that allows a driver to indicate to the stack that a non-zero
UDP checksum is accepted but not verified, using checksum unnecessary
for that would also be incorrect.

Not to belabor the point, but if devices provide checksum-complete
this complexity in drivers and devices needing to parse specific UDP
encapsulations goes away.

Tom
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