netdev - Re: alignment faults in 3.6

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Date:	Thu, 11 Oct 2012 13:54:16 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	mbizon@...ebox.fr
Cc:	Måns Rullgård <mans@...sr.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	netdev@...r.kernel.org, David Laight <David.Laight@...LAB.COM>,
	Jon Masters <jonathan@...masters.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: alignment faults in 3.6

On Thu, 2012-10-11 at 13:47 +0200, Maxime Bizon wrote:
> On Thu, 2012-10-11 at 13:28 +0200, Eric Dumazet wrote:
> 
> > You probably are aware that a driver can use : 
> > 
> > - a fragment to hold the frame given by the hardware, with whatever
> > alignment is needed by the hardware.
> > 
> > Then allocate an skb with enough room (128 bytes) to pull the headers as
> > needed later.
> > 
> > skb = netdev_alloc_skb_ip_align(dev, 128);
> 
> What happen at tx time, supposing that same hardware cannot do SG ?
> 
> Aren't we going to memcpy the data into the head ?
> 

Of course, if you use a forwarding setup, and the tx driver is not SG
capable, performance will be bad (You have to copy the data into a
single skb (linearize the skb)) 

But in 2012, having to use hardware without SG for a router sounds a bad
joke (if cpu speed is _also_ too low)

Adding get_unaligned() everywhere in linux network stacks is not an
option.

We actually want to be able to read the code and fix the bugs, not only
run it on a cheap low end router.



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