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

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Date:	Mon, 01 Feb 2016 09:25:41 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	Alexander Duyck <alexander.duyck@...il.com>,
	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

On Mon, 2016-02-01 at 16:55 +0000, David Laight wrote:
> 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'.

If memcpy_to_io() has problem because it assumed memcpy() had some
properties, it is quite a different problem that should be referred to
x86 maintainers ?

arch/x86/lib/memcpy_64.S certainly contains a suitable variant.