Re: [Qemu-devel] vhost acceleration broken?

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jul 25, 2013 at 11:25:20AM +0930, Rusty Russell wrote:
> Hi all,
> 
>         Using latest kernel and master qemu, the following doesn't use
> vhost acceleration:
> 
> sudo qemu-system-x86_64 -machine pc,accel=kvm $ARGS -m 1024 -net tap,script=/home/rusty/bin/kvm-ifup,downscript=no,vhost=on -net nic,model=virtio -drive file=$QEMUIMAGE,index=0,media=disk,if=virtio -kernel arch/x86/boot/bzImage -append "root=/dev/vda1 $KARGS $*"
> 
> Culprit is here:
> 
> hw/net/virtio-net.c:virtio_net_vhost_status():
>     if (nc->peer->info->type != NET_CLIENT_OPTIONS_KIND_TAP) {
>         return;
>     }
> 
> info->type is NET_CLIENT_OPTIONS_KIND_HUBPORT.
> 
> At a glance, it seems like vlan is always enabled, and that means a hub,
> so that change silently disabled vhost acceleration.
> 
> It's quite possible that I've screwed up qemu's impenetrable command
> line (-net or -netdev, who knows what's better?).
> 
> Frustrated,
> Rusty.

That's just it.
Replace -net with -netdev, and things will work.