netdev - RE: [PATCH net-next v4] openvswitch: enable NSH support

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Date:   Mon, 21 Aug 2017 10:10:38 +0000
From:   Jan Scheurich <jan.scheurich@...csson.com>
To:     Jiri Benc <jbenc@...hat.com>
CC:     "Yang, Yi" <yi.y.yang@...el.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "dev@...nvswitch.org" <dev@...nvswitch.org>,
        "blp@....org" <blp@....org>, "e@...g.me" <e@...g.me>
Subject: RE: [PATCH net-next v4] openvswitch: enable NSH support

> > NSH can be carried over Ethernet with a 14 byte header. In that case
> > the total NSH header would typically be 16-bit aligned, so that all
> > 32-bit members would be misaligned.
> 
> See NET_IP_ALIGN in include/linux/skbuff.h.

If I understand correctly, this is a default definition that can be overridden by drivers so that we still cannot rely on the Ethernet payload always being 32-bit-aligned. 

Furthermore, there seem to be machine architectures that cannot handle misaligned 32-bit access at all (not even with a performance penalty).

Or why else does OVS user space code take so great pain to model possible misalignment and provide/use safe access functions?

Does Linux kernel code generally ignore this risk?

/Jan