netdev - Re: [net-next 05/19] ixgbe: Add support for UDP-encapsulated tx checksum offload

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Date:	Thu, 3 Sep 2015 00:07:17 +0300
From:	Or Gerlitz <gerlitz.or@...il.com>
To:	Tom Herbert <tom@...bertland.com>
Cc:	"Rustad, Mark D" <mark.d.rustad@...el.com>,
	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
	"David S. Miller" <davem@...emloft.net>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	"nhorman@...hat.com" <nhorman@...hat.com>,
	"sassmann@...hat.com" <sassmann@...hat.com>,
	"jogreene@...hat.com" <jogreene@...hat.com>
Subject: Re: [net-next 05/19] ixgbe: Add support for UDP-encapsulated tx
 checksum offload

On Wed, Sep 2, 2015 at 8:38 PM, Tom Herbert <tom@...bertland.com> wrote:
> On Wed, Sep 2, 2015 at 9:46 AM, Rustad, Mark D <mark.d.rustad@...el.com> wrote:

>> Note: NETIF_F_HW_CSUM is a superset of NETIF_F_IP_CSUM + NETIF_F_IPV6_CSUM.
>> It means that device can fill TCP/UDP-like checksum anywhere in the packets
>> whatever headers there might be.

>> The device can't do whatever, wherever. There is always a limit to the offset to the inner headers that can be handled, for instance.

> If the device does NETIF_F_HW_CSUM then inner/outer headers are
> irrelevant at least in the non-GSO case. All the device needs to do is
> compute the checksum from start and write the answer at the given
> offset. No protocol awareness needed in the device, no need to parse
> headers on transmit.

Tom, could you elaborate a little further on the
semantics/requirements for devices supporting NETIF_F_HW_CSUM,
specifically, AFAIU this isn't a TX equivalent of supporting checksum
complete on RX, right? when you say "write the answer at the given
offset" what non-common answers are you expecting devices to produce?
how the kernel is hinting to the device on the nature on the expected
answer beyond the offset?

Or.
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