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

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Message-ID: <1454103195.7627.76.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Fri, 29 Jan 2016 13:33:15 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexander Duyck <alexander.duyck@...il.com>
Cc:	Sowmini Varadhan <sowmini.varadhan@...cle.com>,
	David Miller <davem@...emloft.net>,
	Netdev <netdev@...r.kernel.org>
Subject: Re: [RFC] Kernel unaligned access at __skb_flow_dissect

On Fri, 2016-01-29 at 13:16 -0800, Alexander Duyck wrote:

> It has to be something recent.  I know back when I wrote the code I
> tested it on a few different architectures and had to add a few bits
> in __skb_get_poff so that it would read doff as a u8 instead of
> bitfield in a u32.
> 
> Looking at the code it seems like this should be an easy fix.  Just
> swap the two lines that acquire and test the flow label with the check
> for dissector_uses_key(... _KEY_FLOW_LABEL).  Then ixgbe will stop
> trying to grab flow labels.

Note that if we properly align headers, we also align TCP/UDP payload.

Meaning that copying data in recvmsg() is likely faster in some cpus.
(The ones having NET_IP_ALIGN set to 2 presumably)