Re: [PATCH net-next] net/vxlan: Avoid unaligned access in vxlan_build_skb()

On Fri, Sep 23, 2016 at 4:41 PM, Sowmini Varadhan
[off-list ref] wrote:

On (09/23/16 10:38), Alexander Duyck wrote:

quoted

So basically what I was thinking is we do something like reserving
NET_IP_ALIGN and continue writing headers to skb->data, but we force
the tracking for the inner headers into frag[0] so that we can keep
the inner headers aligned without messing up the alignment for outer
headers.  In theory the inner offset and all that would still be
functional but might need a few tweaks.  You could probably even use
the skb->encapsulation bit to indicate you are doing this.  You could
almost think of it as us doing something like the inverse of
pskb_pull_tail.  The general idea here is we want to actually leave
the data in skb->data, but just reference it from frag[0] so that we
don't accidentally pull in the 2 byte padding for alignment when
transmitting the frame.

yes, I think something along this line could do the trick.. I tried
hacking it a bit today for vxlan, and it could be extended for all
these encaps protocols. Let me fix/test this more next week, maybe
we can discuss in Tokyo.

Agreed.  Keep in mind we only really need it for the architectures
that need to set NET_IP_ALIGN so we may want to end up wrapping the
code in ifndef checks for HAVE_EFFICIENT_UNALIGNED_ACCESS.

- Alex