Re: [PATCH 0/4 for 2.3] vhost-user live migration support

[Yuanhan Liu <yuanhan.liu@linux.intel.com>]

On Wed, Dec 16, 2015 at 10:38:03AM +0800, Peter Xu wrote:
> On Tue, Dec 15, 2015 at 04:07:57PM +0100, Thibaut Collet wrote:
> > After a migration, to avoid netwotk outage, all interfaces of the guest
> > must send a packet to update switches mapping (ideally a GARP).
> > As some interfaces do not do it QEMU does it in behalf of the guest by
> > sending a RARP (his RARP is not forged by the guest but by QEMU). This is
> > the qemu_self_announce purpose that "spoofs" a RARP to all backend of guest
> > ethernet interfaces. For vhost-user backend, QEMU can not do it directly
> > and asks to the vhost-user backend to do it with the VHOST_USER_SEND_RARP
> > request that contains the MAC address of the guest interface.
> > 
> > Thibaut.
> 
> Hi, Thibaut,
> 
> Thanks for the explaination.
> 
> Two more questions:
> 
> 1. if vhost-user backend (or say, DPDK) supports GUEST_ANNOUNCE, and
>    send another RARP (or say, GARP, I will use RARP as example),
>    then there will be two RARP later on the line, right? (since the
>    QEMU one is sent unconditionally from qemu_announce_self).

The one send by qemu_announce_self() will be caught by
vhost_user_receive(), which ends up invoking vhost_user_migration_done().
And it will be dropped when VIRTIO_NET_F_GUEST_ANNOUNCE is negotiated
there.

> 2. if the only thing vhost-user backend is to send another same RARP
>    when got SEND_RARP request, why would it bother if QEMU will
>    unconditionally send one? (or say, I still do not know why we
>    need this SEND_RARP request, if the vhost-user backend is going
>    to do the same thing again as QEMU already does)

Because that one is caught by vhost-user, and vhost-user just relays
it to the backend when necessary (say when GUEST_ANNOUNCE is not
supported)?

	--yliu