Subject: Re: [PATCH] flow-dissector: Fix alignment issue in __skb_flow_get_ports

From: Alexander Duyck <[email-masked]>
Date: Fri, 10 Oct 2014 09:50:17 -0700

On 10/10/2014 08:29 AM, Eric Dumazet wrote:
> 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.

Still it means possibly hurting performance on those platforms that 
don't have it defined.

If I just use get_unaligned that is pretty easy in terms of cleanup for 
the ports and IPv4 addresses, the IPv6 will still be a significant 
hurdle to overcome though.

Thanks,

Alex