netdev - RE: [RFC] Kernel unaligned access at __skb_flow

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Date:	Mon, 1 Feb 2016 16:55:02 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Eric Dumazet' <eric.dumazet@...il.com>,
	Alexander Duyck <alexander.duyck@...il.com>
CC:	Sowmini Varadhan <sowmini.varadhan@...cle.com>,
	David Miller <davem@...emloft.net>,
	Netdev <netdev@...r.kernel.org>
Subject: RE: [RFC] Kernel unaligned access at __skb_flow_dissect

From: Eric Dumazet
> Sent: 29 January 2016 22:29
...
> On modern intel cpus, this does not matter at all, sure. It took a while
> before "rep movsb" finally did the right thing.

Unfortunately memcpy_to_io() etc now map to 'rep movsb', and that can
only be optimisied for cached addresses.

So copies to/from pcie space get done as byte copies, slower than slow.

The same is true when usespace has used mmap() to directly access
pcie memory.

There isn't a standard wrapper than generates 'rep movsd'.

	David