netdev - Re: [PATCH] net: Add ndo_gso

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Date:	Mon, 29 Sep 2014 13:13:47 -0700
From:	Jesse Gross <jesse@...ira.com>
To:	Tom Herbert <therbert@...gle.com>
Cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Or Gerlitz <gerlitz.or@...il.com>
Subject: Re: [PATCH] net: Add ndo_gso_check

On Sun, Sep 28, 2014 at 8:50 PM, Tom Herbert <therbert@...gle.com> wrote:
> Add ndo_gso_check which a device can define to indicate whether is
> is capable of doing GSO on a packet. This funciton would be called from
> the stack to determine whether software GSO is needed to be done. A
> driver should populate this function if it advertises GSO types for
> which there are combinations that it wouldn't be able to handle. For
> instance a device that performs UDP tunneling might only implement
> support for transparent Ethernet bridging type of inner packets
> or might have limitations on lengths of inner headers.
>
> Signed-off-by: Tom Herbert <therbert@...gle.com>

Offloads that might have limitations extend beyond GSO - checksum is
another possibility that could need something like a length check.

In addition, while I would also like to be optimistic about the
capabilities of existing NICs it's unlikely that any of them that are
advertising SKB_GSO_UDP_TUNNEL can actually do it with full
generality. So unless we can get the driver writers to chime in about
their capabilities (maybe we can, there's only a handful of them right
now), we probably need to provide a more conservative implementation
for those drivers. I guess I would probably do length equal to VXLAN
and perhaps containing Ethernet as a balance between the most
conservative and the most optimistic.
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