Re: [Qemu-devel] [PATCH] net: Remove vhostforce option in addition to vhost parameter

[Jason Wang <jasowang@redhat.com>]

On 05/28/2015 02:28 PM, Michal Privoznik wrote:
> On 28.05.2015 05:36, Jason Wang wrote:
>>
>> On 05/28/2015 11:21 AM, Jason Wang wrote:
>>> On 05/27/2015 07:57 PM, Michael S. Tsirkin wrote:
>>>> On Wed, May 27, 2015 at 04:45:34PM +0800, Jason Wang wrote:
>>>>>> On 05/27/2015 02:26 PM, Pankaj Gupta wrote:
>>>>>>>> Ping.
>>>>>>>>
>>>>>>>> Can I get any suggestions on this patch.
>>>>>>>>
>>>>>>>> Best regards,
>>>>>>>> Pankaj
>>>>>>>>
>>>>>>>>>> vhostforce was added to enable vhost when
>>>>>>>>>>  guest don't have MSI-X support.
>>>>>>>>>>  Now, we have scenarios like DPDK in Guest which dont use
>>>>>>>>>>  interrupts and still use vhost. Also, performance of guests
>>>>>>>>>>  without MSI-X support is getting less popular.
>>>>>>>>>>
>>>>>>>>>>  Its OK to remove this extra option and enable vhost
>>>>>>>>>>  on the basis of vhost=ON/OFF.
>>>>>>>>>>  Done basic testing with vhost on/off for latest guests
>>>>>>>>>>  and old guests(non-msix).
>>>>>>>>>>
>>>>>>>>>> Signed-off-by: Pankaj Gupta <pagupta@redhat.com>
>>>>>> Looks good. Two questions:
>>>>>>
>>>>>> - Did libvirt use this? if not, we may want to drop vhostfore option
>>>>>> completely.
>>>> Yes, it did.
>>>>
>>> For vhost-user, vhostforce is mandatory. But how about tap? Looks like
>>> it was not used, and I could not even find any option in
>>> http://libvirt.org/formatdomain.html.
>>>
>> CC Michal for the answer.
>>
> No, vhostforce is not used by libvirt at all. Which brings up
> interesting question: for vhost-user libvirt constructs merely the
> follwing cmd line:
>
> -chardev socket,id=charnet0,path=/tmp/vhost0.sock,server \
> -netdev type=vhost-user,id=hostnet0,chardev=charnet0 \
>
> How does vhostforce fit in? I suppose the cmd line is working even
> without libvirt passing vhostforce. Should it do so?

Without this patch,  it works with normal virtio-net drivers but may not
work for guest pmd. Since pmd does not use MSI-X interrupts, so qemu
won't start vhost_net. So libvirt needs vhostforce for vhost-user.

With this patch, libvirt does not need to care about this.

>
> Michal
>