Re: [Qemu-devel] qemu-kvm-0.11 regression, crashes on older guests with virtio network

[Dustin Kirkland]

On Thu, 2009-10-29 at 20:00 +0800, Scott Tsai wrote:
> Excerpts from Mark McLoughlin's message of Thu Oct 29 17:16:43 +0800 2009:
> > Assuming this is something like the virtio-net in 2.6.26, there was no
> > receivable buffers support so (as Scott points out) it must be that
> > we've read a packet from the tap device which is >1514 bytes (or >1524
> > bytes with IFF_VNET_HDR) but the guest has not supplied buffers which
> > are large enough to take it
> 
> > One thing to check is that the tap device is being initialized by
> > qemu-kvm using TUNSETOFFLOAD with either zero or TUN_F_CSUM - i.e. GSO
> > should not be enabled, because the guest cannot handle large GSO packets
> 
> > Another possibility is that the MTU on the bridge in the host is too
> > large and that's what's causing the large packets to be sent
> 
> Using Dustin's image, I see:
>       virtio_net_set_features(features: 0x00000930)
>       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.
> 
> BTW, I can also reproduce this running Dustin's image inside Fedora 11's 
> qemu-0.10.6-9.fc11.x86_64.
> 
> The patch I posted earlier actually only applies to the 0.10 branch, here's a 
> patch that compiles for 0.11:


Hi Scott,

Thanks for this.  Testing this, kvm doesn't crash.  And the guest has
working network connectivity, until I saturate the network connection
with nc.  At that point, the guest loses network connectivity all
together.  So the fix is not quite ideal, yet.

:-Dustin

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