[libvirt] How to enable vhost for virtIO NIC?

[Ruben Kerkhof]

Hi all,

On Wed, May 19, 2010 at 18:22, Laine Stump <laine at laine.org> wrote:
> On 05/19/2010 01:49 AM, Huang, Zhiteng wrote:
>>
>> Hi gurus on the list,
>>
>> I'm trying to bring up a Linux with virtio vhost backend.  My system is
>> running on kernel 2.6.34-rc7, libvirt 0.8.0.
>>
>> Two questions regarding to vhost:
>>
>> 1) XML format for vhost
>> I didn't find any documents on libvirt.org description the XML or QMU
>> argument format for vhost.
>
> If you're using virtio networking, and vhost-net support is available on
> your system, libvirt will automatically alter the qemu commandline arguments
> to take advantage of it. libvirt checks for two things:
>
> 1) it runs "qemu-kvm --help" and looks for ",vhost=" in the output
> (demonstrating that this build of qemu supports vhost)
>
> 2) it attempts to open "/dev/vhost-net" in rw mode (demonstrating that the
> kernel support is there, either built into the kernel or loaded as a
> module).

I just tried this with libvirt 0.8.1, on a 2.6.34 kernel, and it
doesn't seem to work for me.
Setting a breakpoint on the qemudOpenVhostNet function shows that it
returns -1, because QEMUD_CMD_FLAG_NETDEV isn't set.

-netdev is disabled on qemu 0.12

Ruben