Re: [Qemu-devel] [PATCH v3 2/2] vhost user: Add RARP injection for legacy guest

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

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.


> At this time the migration is completed and there is no risk of confusing the
> switch.
> In this case:
>   - there are nothing to do in QEMU to manage legacy guest with
> no GUEST_ANNOUNCE.
>   - All the job is done by the backend on the VHOST_USER_SET_VRING_KICK
> reception. Maybe switch notification of live migration is done with a small
> delay but it works
>   - This patch can be discarded.
> 
> 
> On Thu, Jun 11, 2015 at 12:38 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> 
>     On Thu, Jun 11, 2015 at 01:54:22PM +0800, Jason Wang wrote:
>     >
>     >
>     > On 06/11/2015 01:49 PM, Thibaut Collet wrote:
>     > > > Yes, but still need a mechanism to notify the backend of migration
>     > > > completion from qemu side if GUEST_ANNOUNCE is not negotiated.
>     > >
>     > > backend is aware of a connection with the guest (with the feature
>     > > negociation) and can send a rarp. This rarp will be always sent by the
>     > > backend when a VM is launched (first start or live migration
>     > > completion) if the GUEST_ANOUNCE is not supported.
>     > > In this case the issue is solved without done everything by QEMU.
>     >
>     > The issue is during migration guest network is still active. So sending
>     > rarp too early in the destination (e.g during VM is launched) may
>     > confuse the switch.  We want it to be sent exactly when the migration is
>     > completed in destination.
> 
>     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.
> 
>     > > If sending a rarp message on the start of te VM is not accceptable, we
>     > > must provide a mechanism similar of the one I have implemented. The
>     > > message content can be empty as the backend is able to create the rarp
>     > > message.
>     >
>     > Yes.
> 
>