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From: John Fastabend <john.fastabend@gmail.com>
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Date: Thu, 10 Nov 2016 15:02:43 -0800
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Subject: [Qemu-devel] qemu virtio_net abort bisect
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To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: stefanha@redhat.com, qemu-devel@nongnu.org

Hi Michael,

I'm getting a qemu crash from a load/unload of virtio_net.ko in guest
kernel with the following,

> ./x86_64-softmmu/qemu-system-x86_64 -hda /var/lib/libvirt/images/Fedora-test0.img \
       -m 4096 -enable-kvm -smp 4 -netdev tap,id=hn0,queues=4,vhost=on \
       -device virtio-net-pci,netdev=hn0,mq=on,vectors=9

in the host I do a simple

# rmmod virtio_net
# modprobe virtio_net

And bisecting points at this merge which just from the commit message
looks like it could be the culprit,

> Merge: 4eb28ab 5300063
> Author: Stefan Hajnoczi <stefanha@redhat.com>
> Date: Thu Nov 3 14:41:53 2016 +0000
> 
> Merge remote-tracking branch 'remotes/mst/tags/for_upstream' into staging
> 
> virtio, pc: fixes and features
> 
> nvdimm hotplug support
> virtio migration and ioeventfd rework    <--- ioeventfd bits?
> virtio crypto device
> ipmi fixes
> 
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

Running without the above and everything works no issues. I started to
debug but admittedly got a bit lost in the notifier hooks maybe someone
will have a better idea. Here is the error I get,

> kvm_io_ioeventfd_add: error adding ioeventfd: File exists
> Aborted (core dumped)

I had a gdb backtrace at one point and can post that if its helpful.

Thanks,
John