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Date:	Mon, 27 Oct 2014 03:21:50 +0000
From:	Ben Hutchings <ben@...adent.org.uk>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	netdev <netdev@...r.kernel.org>,
	virtualization@...ts.linux-foundation.org
Subject: Re: IPv6 UFO for VMs

On Wed, 2014-10-22 at 11:35 +0200, Hannes Frederic Sowa wrote:
> On Mi, 2014-10-22 at 00:44 +0100, Ben Hutchings wrote:
> > There are several ways that VMs can take advantage of UFO and get the
> > host to do fragmentation for them:
> > 
> > drivers/net/macvtap.c:                  gso_type = SKB_GSO_UDP;
> > drivers/net/tun.c:                      skb_shinfo(skb)->gso_type = SKB_GSO_UDP;
> > drivers/net/virtio_net.c:                       skb_shinfo(skb)->gso_type = SKB_GSO_UDP;
> > 
> > Our implementation of UFO for IPv6 does:
> > 
> > 		fptr = (struct frag_hdr *)(skb_network_header(skb) + unfrag_ip6hlen);
> > 		fptr->nexthdr = nexthdr;
> > 		fptr->reserved = 0;
> > 		fptr->identification = skb_shinfo(skb)->ip6_frag_id;
> > 
> > which assumes ip6_frag_id has been set.  That's only true if the local
> > stack constructed the skb; otherwise it appears we get zero.
> > 
> > This seems to be a regression as a result of:
> > 
> > commit 916e4cf46d0204806c062c8c6c4d1f633852c5b6
> > Author: Hannes Frederic Sowa <hannes@...essinduktion.org>
> > Date:   Fri Feb 21 02:55:35 2014 +0100
> > 
> >     ipv6: reuse ip6_frag_id from ip6_ufo_append_data
> > 
> > However, that change seems reasonable - we *shouldn't* be choosing IDs
> > for any other stack.  Any paravirt net driver that can use IPv6 UFO
> > needs to have some way of passing a fragmentation ID to put in
> > skb_shared_info::ip6_frag_id.
> 
> Do we really gain a lot of performance by enabling UFO on those devices
> or would it make sense to just drop support? It only helps fragmenting
> large UDP packets, so I don't think it is worth it.

It's not been important enough for anyone to bother implementing it in
hardware/firmware aside from Neterion.

I'll shortly post patches to disable it.

Ben.

> Otherwise I agree with Ben, we need to pass a fragmentation id from the
> host over to the system segmenting the gso frame. Fragmentation ids must
> be generated by the end system.
> 
> Hmm...

-- 
Ben Hutchings
Theory and practice are closer in theory than in practice.
                                - John Levine, moderator of comp.compilers

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