netdev - Re: [PATCH net-next] vrf: Add ethernet header for pass through VRF device

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Date:	Wed, 26 Aug 2015 17:30:58 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	dsa@...ulusnetworks.com
Cc:	netdev@...r.kernel.org, shm@...ulusnetworks.com
Subject: Re: [PATCH net-next] vrf: Add ethernet header for pass through VRF
 device

From: David Ahern <dsa@...ulusnetworks.com>
Date: Wed, 26 Aug 2015 12:36:15 -0700

> As such all we need is to push an eth header to the front of the skb
> for 1 loop through the stack and eth_header via dev_hard_header with
> NULL daddr is the simplest path to accomplish that. Any other path is
> just extra overhead.

And dev_hard_header() is full of conditional code and partial stores,
whereas the hard header cache is a _SINGLE UNCONDITIONAL MEMCPY_.

You're making this data path more expensive than it needs to be just
to placate features which in no way are default situations.

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