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Subject: Re: [Qemu-devel] qemu-kvm-0.11 regression, crashes on older guests
	with virtio network
From: Mark McLoughlin <markmc@redhat.com>
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Date: Thu, 29 Oct 2009 14:48:39 +0000
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: Rusty Russell <rusty@rustcorp.com.au>, Scott Tsai <scottt.tw@gmail.com>, qemu-devel <qemu-devel@nongnu.org>, kvm <kvm@vger.kernel.org>, Dustin Kirkland <kirkland@canonical.com>

On Thu, 2009-10-29 at 09:39 -0500, Anthony Liguori wrote:
> Mark McLoughlin wrote:
> > On Thu, 2009-10-29 at 09:11 -0500, Anthony Liguori wrote:
> >   
> >> Mark McLoughlin wrote:
> >>     
> >>>> 	tap_set_offload(csum: 1, tso4: 1, tso6: 1, ecn: 1)
> >>>> being called and get an mtu of 1500 on virbr0 using his birdge.sh script.
> >>>>
> >>>> virtio_net_receive2 was trying to transfer a 1534 byte packet (1524 'size' + 10 'virtio_net_hdr')
> >>>> and the guest only had 1524 bytes of space in its input descriptors.
> >>>>     
> >>>>         
> >>> Okay, that sounds like a bug in Dustin's version of the guest virtio-net
> >>> driver - if it is only supplying 1524 byte buffers, it should not be
> >>> saying it supports the VIRTIO_NET_F_GUEST_TSO4 feature
> >>>   
> >>>       
> >> See:
> >>
> >> commit 8eca6b1bc770982595db2f7207c65051572436cb
> >> Author: aliguori <aliguori@c046a42c-6fe2-441c-8c8c-71466251a162>
> >> Date:   Sun Apr 5 17:40:08 2009 +0000
> >>
> >>     Fix oops on 2.6.25 guest (Rusty Russell)
> >>    
> >>     I believe this is behind the following:
> >>     https://bugs.edge.launchpad.net/ubuntu/jaunty/+source/linux/+bug/331128
> >>    
> >>     virtio_pci in 2.6.25 didn't do feature negotiation correctly: it 
> >> acked every
> >>     bit.  Fortunately, we can detect this.
> >>    
> >>     Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
> >>     Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
> >>
> >> It looks like Rusty's fix wasn't enough.  If I change virtio-net to only 
> >> advertise F_MAC, we don't run into this problem.
> >>     
> >
> > If it's not acking VBAD_FEATURE, then it doesn't sound like the same
> > issue
> >   
> 
> It was acking VBAD_FEATURE when I tested it.
> 
> But if you look at the patch, it whitelists the following features:
> 
>     features |= (1 << VIRTIO_NET_F_MAC);
>     features |= (1 << VIRTIO_NET_F_GUEST_CSUM);
>     features |= (1 << VIRTIO_NET_F_GUEST_TSO4);
>     features |= (1 << VIRTIO_NET_F_GUEST_TSO6);
>     features |= (1 << VIRTIO_NET_F_GUEST_ECN);

Ah, it all makes sense now.

I was getting confused between HOST_* and GUEST_*

this should have been:

    features |= (1 << VIRTIO_NET_F_MAC);
    features |= (1 << VIRTIO_NET_F_HOST_CSUM);
    features |= (1 << VIRTIO_NET_F_HOST_TSO4);
    features |= (1 << VIRTIO_NET_F_HOST_TSO6);
    features |= (1 << VIRTIO_NET_F_HOST_ECN);

Could you try that Dustin?

> Which is why it's ack'ing TSO4.   Removing TSO4 didn't seem to fix it 
> for me.

Odd.

Cheers,
Mark.