netdev - Re: Checksum offload queries

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Date:	Thu, 10 Dec 2015 08:26:57 -0800
From:	Tom Herbert <tom@...bertland.com>
To:	Edward Cree <ecree@...arflare.com>
Cc:	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>
Subject: Re: Checksum offload queries

On Thu, Dec 10, 2015 at 7:49 AM, Edward Cree <ecree@...arflare.com> wrote:
> On 09/12/15 18:00, Tom Herbert wrote:
>> That is not at all true. If the stack has set up VXLAN RCO and the > device decides to set the inner checksum itself then the checksum > will be bad. The checksum interface is very specific please read it > carefully (sk_buff.h), if the driver/device thinks it is smarter > than the stack and tries to do set its own rules on how checksum > offload works then things will eventually break miserably.
>
> Ok, I've passed that on to the guy working on this bit of the driver.
>
> It looks like the best way to support the capabilities of NICs like the
> sfc 8000 series (which can fill in two checksums but uses packet parsing
> to figure out what to do, rather than using csum start/offset) is:
>
> (core / stack)
> * add NETIF_F_HW_2CSUMS (or whatever name)
> * squeeze a second csum start/offset pair into the skb (as you mention,
>  we can do this without size increase)
> * Modify (Tx) CHECKSUM_PARTIAL generation to use both csum pairs.
>  Presumably by creating CHECKSUM_PARTIAL_2CSUMS to indicate that the
>  second csum pair has been filled in as well.
>
> (sfc driver)
> * declare 2CSUMS support
> * on getting an skb for xmit, check whether the csum pairs match what our
>  eeevil packet parsing hardware will do.  If so, send it with appropriate
>  csum offload settings (we can enable/disable inner & outer offload
>  independently, with TX Option descriptors).  Any csum pair that doesn't
>  match, we call skb_checksum_help to do it in software, and tell the
>  hardware not to do that one.
>

> Does that seem reasonable?

It sounds like potentially interesting work. You'll probably want my
patches that provider helper functions that allow a driver to verify
that it can offload a checksum. We'll have to update those also to
allow two checksums.
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