netdev - Re: [RFC] Kernel unaligned access at __skb_flow

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Date:	Fri, 29 Jan 2016 16:47:46 -0800
From:	Tom Herbert <tom@...bertland.com>
To:	Sowmini Varadhan <sowmini.varadhan@...cle.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Michael Dalton <mwdalton@...gle.com>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [RFC] Kernel unaligned access at __skb_flow_dissect

On Fri, Jan 29, 2016 at 3:58 PM, Sowmini Varadhan
<sowmini.varadhan@...cle.com> wrote:
> On (01/29/16 15:31), Tom Herbert wrote:
>> But even within flow dissector, to be completely correct, we need to
>> replace all 32-bit accesses with the mempy (flow_label, mpls label,
>> keyid) and be vigilant about new ones coming in. For that matter, ..
>
> well, one question that came to my mind when I was looking at this is:
> why does eth_get_headlen try to compute the flow hash even before the
> driver has had a chance to pull things into an aligned buffer? Isn't
> that just risking even more unaligned accesses?
>
Hmm, thanks for pointing that out. It looks like this is called by a
couple drivers as part of pulling up the data to get alignment. I am
actually surprised they are doing this. Flow dissector is not the
cheapest function in the world and it seems like a shame to be calling
it this early and not even getting a hash for the skb. Also, this is
another instance of touching the data so if we do get to the point of
trying steering packets without cache miss (another thread); this
undermines that. Seems like it might be just as well to pull up a
fixed number of bytes (ixgbe want min of 60 bytes), or don't pull up
anything avoid the cache miss here and let the stack pull up as
needed.

Anyway, I think you do have a point that flow_dissector does not
assume alignment or pull up data like the rest of the stack. Seems
like we need a utility function like align_access (maybe there is one
already) to get 32-bit fields and probably do need the sanity check
for odd alignment.

Tom