netdev - Re: [PATCH] flow-dissector: Fix alignment issue in __skb_flow_get

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Date:	Fri, 10 Oct 2014 08:29:33 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	alexander.h.duyck@...hat.com
Cc:	David Laight <David.Laight@...LAB.COM>,
	David Miller <davem@...emloft.net>,
	"alexander.duyck@...il.com" <alexander.duyck@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH] flow-dissector: Fix alignment issue in
 __skb_flow_get_ports

On Fri, 2014-10-10 at 08:14 -0700, Alexander Duyck wrote:

> I think you might be coming to this a little late.  The igb and ixgbe 
> drivers had been working this way for a long time.  We did a memcpy to 
> move the headers from the page and into the skb->data at an aligned 
> offset.  In order to determine the length to memcpy we had a function 
> that could walk through the DMA aligned data to get the header length.  
> The function for that was replaced with the __skb_flow_dissect as it was 
> considered a duplication of code with the flow_dissection functions.  
> However that is obviously not the case now that we are hitting these 
> alignment issues.
> 
> The question I have in all this is do I push forward and make 
> __skb_flow_dissect work with unaligned accesses, or do I back off and 
> put something equivilent to igb/ixgbe_get_headlen functions in the 
> kernel in order to deal with the unaligned accesses as they had no 
> issues with them since they were only concerned with getting the header 
> length and kept all accesses 16b aligned.
> 

I see nothing wrong dealing with unaligned accesses, as these helpers
are nop on x86 or CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS=y arches.




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