Re: [PATCH] IB/ipoib: move back the IB LL address into the hard header

[David Miller <davem-fT/PcQaiUtIeIZ0/mPfg9Q@public.gmane.org>]

From: Paolo Abeni <pabeni-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Date: Tue, 11 Oct 2016 19:15:44 +0200

> After the commit 9207f9d45b0a ("net: preserve IP control block
> during GSO segmentation"), the GSO CB and the IPoIB CB conflict.
> That destroy the IPoIB address information cached there,
> causing a severe performance regression, as better described here:
> 
> http://marc.info/?l=linux-kernel&m=146787279825501&w=2
> 
> This change moves the data cached by the IPoIB driver from the
> skb control lock into the IPoIB hard header, as done before
> the commit 936d7de3d736 ("IPoIB: Stop lying about hard_header_len
> and use skb->cb to stash LL addresses").
> In order to avoid GRO issue, on packet reception, the IPoIB driver
> stash into the skb a dummy pseudo header, so that the received
> packets have actually a hard header matching the declared length.
> Also the connected mode maximum mtu is reduced by 16 bytes to
> cope with the increased hard header len.
> 
> After this commit, IPoIB performances are back to pre-regression
> value.
> 
> Fixes: 9207f9d45b0a ("net: preserve IP control block during GSO segmentation")
> Signed-off-by: Paolo Abeni <pabeni-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>

Not providing an accurate hard_header_len causes many problems.

In fact we recently fixed the mlxsw driver to stop doing this.
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