netdev - Re: [PATCH net] net: Allow flow dissector to handle non 4-byte aligned headers

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Message-ID: <1454286240.7627.160.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Sun, 31 Jan 2016 16:24:00 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Tom Herbert <tom@...bertland.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org,
	sowmini.varadhan@...cle.com, kernel-team@...com
Subject: Re: [PATCH net] net: Allow flow dissector to handle non 4-byte
 aligned headers

On Sun, 2016-01-31 at 13:37 -0800, Tom Herbert wrote:
> Call get_unaligned_be32 when we access 32-bit fields in
> __skb_flow_dissect. At the beginning check for unlikely case of
> 1-byte aligned packet.
> 
> Note that flow_dissector may be asked to parse packet unaligned
> fields in two instances:
> 
> 1) Packet from a driver which is aligned to Ethernet header
>    (2-byte alignment)
> 2) Parsing inner headers of a received GRE-TEB packet
> 
> Testing: Ran super_netperf tests did not see a regression. This was on
> x86 which does not have problems with unaligned data.

But this test is absolutely useless, what about testing arches that
actually care ?

I am told all these MIPS based boxes have already not enough cpu power.

It is sad, because none of them use the drivers that might call flow
dissection (mlx4 and 4 intel drivers)

So I would rather fix the cases where flow dissection called from
eth_get_headlen() with non aligned stuff.

And maybe restrict GRE-TEB to platforms with
CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS=y, since otherwise we need to add
these unaligned macros in thousands of places in our stacks.