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Date: Mon, 15 Jun 2015 10:44:39 +0200
From: "Michael S. Tsirkin" <mst@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v3 2/2] vhost user: Add RARP injection for
 legacy guest
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To: Jason Wang <jasowang@redhat.com>
Cc: Thibaut Collet <thibaut.collet@6wind.com>, qemu-devel <qemu-devel@nongnu.org>, Stefan Hajnoczi <stefanha@redhat.com>

On Mon, Jun 15, 2015 at 03:43:13PM +0800, Jason Wang wrote:
> 
> 
> On 06/12/2015 10:28 PM, Michael S. Tsirkin wrote:
> > On Fri, Jun 12, 2015 at 03:55:33PM +0800, Jason Wang wrote:
> >>
> >> On 06/11/2015 08:13 PM, Michael S. Tsirkin wrote:
> >>> On Thu, Jun 11, 2015 at 02:10:48PM +0200, Thibaut Collet wrote:
> >>>> I am not sure to understand your remark:
> >>>>
> >>>>> It needs to be sent when backend is activated by guest kick
> >>>>> (in case of virtio 1, it's possible to use DRIVER_OK for this).
> >>>>> This does not happen when VM still runs on source.
> >>>> Could you confirm rarp can be sent by backend when the 
> >>>> VHOST_USER_SET_VRING_KICK message is received by the backend ?
> >>> No - the time to send pakets is when you start processing
> >>> the rings.
> >>>
> >>> And the time to do that is when you detect a kick on
> >>> an eventfd, not when said fd is set.
> >>>
> >> Probably not. What if guest is only doing receiving?
> > Clarification: the kick can be on any VQs.
> > In your example, guest kicks after adding receive buffers.
> 
> Yes, but refill only happens on we are lacking of receive buffers. It is
> not guaranteed to happen just after migration, we may have still have
> enough rx buffers for device to receive.

I think we also kick the backend after migration, do we not?
Further, DRIVER_OK can be used as a signal to start backend too.

> >
> >> In this case, you
> >> won't detect any kick if you don't send the rarp first.