RE: [PATCH] flow-dissector: Fix alignment issue in __skb_flow_get_ports

From: Eric Dumazet [mailto:eric.dumazet@gmail.com]

Sent: 10 October 2014 16:34
To: David Laight
Cc: 'alexander.h.duyck@redhat.com'; David Miller; alexander.duyck@gmail.com; netdev@vger.kernel.org
Subject: Re: [PATCH] flow-dissector: Fix alignment issue in __skb_flow_get_ports

On Fri, 2014-10-10 at 14:57 +0000, David Laight wrote:

quoted

I think there is code to copy the IP and TCP headers to aligned memory
before they are parsed.

There is no such thing. You are here on netdev list, please read the
code before doing such claims.

I did say 'I think'...
I must be thinking of some similar code somewhere else.
Possibly just the code that ensures the header isn't fragmented.

quoted

quoted

The problem is the igb / ixgbe / fm10k hardware doesn't have a means of
inserting padding from its side...

Shoot the hardware engineers.

You aren't going to get the performance you expect from a 10Ge card
unless the rx buffers are 'correctly' aligned.

That is simply not true on current x86 cpus. They simply dont care at
all.

I was referring to using them on sparc64, not x86.

I know that current intel x86 cpu have support for misaligned 'rep movsd',
but I thought there was still a small cost (maybe one clock) for
single word transfers.
So maybe they care 'just a little bit'.

You cannot blame Intel for other arches.

True, but this does mean that you don't really want to use these adapters
on a system that can't to unaligned accesses.

	David