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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 14:25:51 +0000
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: 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 09:11 -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's also not acking e.g. MRG_RXBUF, which suggests that it is
selectively acking features, and choosing to ack TSO4

A quick look through the guest driver code should clear up the
confusion. Dustion, got a pointer?

Thanks,
Mark.