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Date:	Fri, 21 Nov 2014 18:46:52 +0000
From:	Ben Hutchings <ben@...adent.org.uk>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	netdev@...r.kernel.org,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v2 net 1/2] drivers/net: Disable UFO through virtio

On Wed, 2014-11-19 at 11:14 +0200, Michael S. Tsirkin wrote:
> On Thu, Oct 30, 2014 at 06:27:12PM +0000, Ben Hutchings wrote:
> > IPv6 does not allow fragmentation by routers, so there is no
> > fragmentation ID in the fixed header.  UFO for IPv6 requires the ID to
> > be passed separately, but there is no provision for this in the virtio
> > net protocol.
> > 
> > Until recently our software implementation of UFO/IPv6 generated a new
> > ID, but this was a bug.  Now we will use ID=0 for any UFO/IPv6 packet
> > passed through a tap, which is even worse.
> > 
> > Unfortunately there is no distinction between UFO/IPv4 and v6
> > features, so disable UFO on taps and virtio_net completely until we
> > have a proper solution.
> > 
> > We cannot depend on VM managers respecting the tap feature flags, so
> > keep accepting UFO packets but log a warning the first time we do
> > this.
> > 
> > Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
> > Fixes: 916e4cf46d02 ("ipv6: reuse ip6_frag_id from ip6_ufo_append_data")
> 
> 
> There's something I don't understand here. I see:
> 
>         NETIF_F_UFO_BIT,                /* ... UDPv4 fragmentation */
> 
> this comment is wrong then?

Yes.

> The patches drastically regress performance for UDPv4 for VMs only, but
> isn't it likely many other devices based their code on this comment?

There's only one hardware driver that implements UFO (s2io), and it does
handle IPv6.

> How about we disable UFO for IPv6 globally, and put the
> flag back in?
> We can then gradually add NETIF_F_UFO6_BIT for devices that
> actually support UFO for IPv6.

Since the corresponding virtio feature bit is understood to include
UFO/IPv6, and existing VMs rely on that, I don't see what this solves.

Ben.

-- 
Ben Hutchings
Beware of bugs in the above code;
I have only proved it correct, not tried it. - Donald Knuth

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