netdev - Re: [PATCH net-next-2.6] net: Fix some corner cases in dev_can

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Date:	Mon, 25 Oct 2010 17:21:08 -0700
From:	Jesse Gross <jesse@...ira.com>
To:	Ben Hutchings <bhutchings@...arflare.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH net-next-2.6] net: Fix some corner cases in dev_can_checksum()

On Fri, Oct 22, 2010 at 7:12 AM, Ben Hutchings
<bhutchings@...arflare.com> wrote:
> dev_can_checksum() incorrectly returns true in these cases:
>
> 1. The skb has both out-of-band and in-band VLAN tags and the device
>   supports checksum offload for the encapsulated protocol but only with
>   one layer of encapsulation.
> 2. The skb has a VLAN tag and the device supports generic checksumming
>   but not in conjunction with VLAN encapsulation.
>
> Rearrange the VLAN tag checks to avoid these.
>
> Signed-off-by: Ben Hutchings <bhutchings@...arflare.com>

If we assume that cards cannot handle offloading for double tagged
packets, which is obviously the most conservative approach, we
probably also need to change the checks for TSO/SG.  There's no issue
with extracting the protocol from the right header but we might assume
that the card can handle double tag offloading when it can't.  For
both TSO/SG we check if there is either an in-band tag or out-of-band
tag and use dev->vlan_features if that is the case.  Maybe we need to
handle it in software if it is double tagged.

On the other hand, I don't know whether it's true that cards can't
handle offloading for packets tagged in both manners.  I suppose that
it depends on where the offloading and tagging are in the pipeline.
For example, when it comes to SG I doubt that the cards care about
vlan tags much at all.
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