netdev - Re: regression caused by 1d2024f61ec14bdb0c57a97a3fe73685abc2d198?

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Message-ID: <1360187747.2659.35.camel@bwh-desktop.uk.solarflarecom.com>
Date:	Wed, 6 Feb 2013 21:55:47 +0000
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	Eric Dumazet <eric.dumazet@...il.com>,
	<alexander.h.duyck@...el.com>, <stephen.s.ko@...el.com>,
	<jeffrey.t.kirsher@...el.com>, David Miller <davem@...emloft.net>,
	<netdev@...r.kernel.org>, <sony.chacko@...gic.com>,
	<mchan@...adcom.com>, <jitendra.kalsaria@...gic.com>,
	<eilong@...adcom.com>
Subject: Re: regression caused by 1d2024f61ec14bdb0c57a97a3fe73685abc2d198?

On Wed, 2013-02-06 at 23:45 +0200, Michael S. Tsirkin wrote:
> On Wed, Feb 06, 2013 at 07:58:21PM +0000, Ben Hutchings wrote:
> > On Wed, 2013-02-06 at 17:50 +0200, Michael S. Tsirkin wrote:
> > > On Wed, Feb 06, 2013 at 05:07:39AM -0800, Eric Dumazet wrote:
> > > > On Wed, 2013-02-06 at 13:43 +0200, Michael S. Tsirkin wrote:
> > > > > It seems that starting with kernel 3.3 ixgbe sets gso_size for
> > > > > incoming frames. It seems that this might result in gso_size
> > > > > being set even when gso_type is 0.
> > > > > This in turn leads to a crash at macvtap_skb_to_vnet_hdr
> > > > > drivers/net/macvtap.c:628
> > > > > which has this code:
> > > > > 
> > > > >        if (skb_is_gso(skb)) {
> > > > >                 struct skb_shared_info *sinfo = skb_shinfo(skb);
> > > > > 
> > > > >                 /* This is a hint as to how much should be linear. */
> > > > >                 vnet_hdr->hdr_len = skb_headlen(skb);
> > > > >                 vnet_hdr->gso_size = sinfo->gso_size;
> > > > >                 if (sinfo->gso_type & SKB_GSO_TCPV4)
> > > > >                         vnet_hdr->gso_type = VIRTIO_NET_HDR_GSO_TCPV4;
> > > > >                 else if (sinfo->gso_type & SKB_GSO_TCPV6)
> > > > >                         vnet_hdr->gso_type = VIRTIO_NET_HDR_GSO_TCPV6;
> > > > >                 else if (sinfo->gso_type & SKB_GSO_UDP)
> > > > >                         vnet_hdr->gso_type = VIRTIO_NET_HDR_GSO_UDP;
> > > > >                 else
> > > > >                         BUG();
> > > > >                 if (sinfo->gso_type & SKB_GSO_TCP_ECN)
> > > > >                         vnet_hdr->gso_type |= VIRTIO_NET_HDR_GSO_ECN;
> > > > >         } else
> > > > >                 vnet_hdr->gso_type = VIRTIO_NET_HDR_GSO_NONE;
> > > > > 
> > > > > 
> > > > > Since skb_is_gso tests gso_size.
> > > > > 
> > > > > What's the right way to handle this? Should skb_is_gso be
> > > > > changed to test gso_type != 0?
> > > > > 
> > > > 
> > > > Or fix ixgbe to set gso_type in ixgbe_get_headlen(), as it does all the
> > > > dissection.
> > > 
> > > 
> > > Hmm, ixgbe_get_headlen isn't run on linear skbs though.
> > > 
> > > Also, I'm not sure I understand when should drivers set gso size
> > > for incoming messages and what is a reasonable value.
> > > Commit log talks about improved performance for lossy connections,
> > > in this case, isn't this something net core should set?
> > [...]
> > 
> > It should be set to the segment size on the wire, so TCP gets a correct
> > picture of packet loss.  The networking core has no idea what hardware/
> > firmware LRO did.
> > 
> > I've previously raised this issue of macvlan vs LRO (which is the same
> > issue we previously had with IP forwarding and with bridging):
> >      http://thread.gmane.org/gmane.linux.network/221695
> > 
> > Ben.
> 
> I see, you proposed disabling LRO the moment a macvlan is attached.
> If I understand correctly, the difference as compared to bridge is that
> bridge normally consumes all incoming packets, macvlan is often used in
> parallel with the underlying interface.

Not so different from IP forwarding, though, in that a single interface
can both forward packets and deliver them locally.

> BTW are there other issues with forwarding/bridging and LRO? If everyone
> sets gso_type in packets it seems we can leave LRO set even with
> bridging?

Some implementations of LRO violate the end-to-end principle by merging
segments with varying lengths and TCP timestamps.  GRO is very careful
to ensure that the original packets can be recovered from the skbs it
produces.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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