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.