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From: Rusty Russell <rusty@rustcorp.com.au>
Date: Thu, 25 Jul 2013 11:25:20 +0930
Message-ID: <87li4v8ktz.fsf@rustcorp.com.au>
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Subject: [Qemu-devel] vhost acceleration broken?
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To: qemu-devel@nongnu.org
Cc: Stefan Hajnoczi <stefanha@redhat.com>, mst@redhat.com

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.