netdev - Re: [PATCH 0/2] Disable forwarding of LRO skbs

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Message-ID: <20080501110225.GG14219@solarflare.com>
Date:	Thu, 1 May 2008 12:02:27 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	davem@...emloft.net, kmansley@...arflare.com,
	shemminger@...tta.com, netdev@...r.kernel.org
Subject: Re: [PATCH 0/2] Disable forwarding of LRO skbs

Herbert Xu wrote:
> Ben Hutchings <bhutchings@...arflare.com> wrote:
> > Large Receive Offload (LRO) destroys packet headers that should be
> > preserved when forwarding.  Currently it also triggers a BUG() or WARN()
> > in skb_gso_segment().  We should disable it wherever forwarding is
> > enabled, and discard LRO skbs with a warning if it is turned back on.
> 
> I don't think forwarding GSO packets is bad per se.  However,
> doing LRO on forwarded traffic is broken because it breaks the
> end-to-end connection.  So I agree that we should detect this
> but preferrably not based on the GSO flag as LRO isn't the only
> source of GSO traffic and the rest of them can be forwarded just
> fine.

My understanding is that at the points I'm adding the check, we're only
considering received skbs and gso_size will only be set if LRO was used.

I was going to raise the virtualisation issue because Kieran has been
in contact with XenSource over this and they believed LRO was not a
problem in Xen.  So long as the bridges in dom0 each connect a single
physical interface to other domains running paravirtualised Linux, this
is probably true.  But I have no idea how to "whitelist" this sort of
case.

Ben.

-- 
Ben Hutchings, Senior Software Engineer, Solarflare Communications
Not speaking for my employer; that's the marketing department's job.
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