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

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

On Tue, Dec 15, 2015 at 12:47:47PM +0100, Thibaut Collet wrote:
> On Tue, Dec 15, 2015 at 12:43 PM, Thibaut Collet <thibaut.collet@6wind.com>
> wrote:
> 
> >
> >
> > On Tue, Dec 15, 2015 at 11:05 AM, Peter Xu <peterx@redhat.com> wrote:
> >
> >> On Tue, Dec 15, 2015 at 11:45:56AM +0300, Pavel Fedin wrote:
> >> >  To tell the truth, i don't know. I am also learning qemu internals on
> >> the fly. Indeed, i see that it should announce itself. But
> >> > this brings up a question: why do we need special announce procedure in
> >> vhost-user then?
> >>
> >> I have the same question. Here is my guess...
> >>
> >> In customized networks, maybe people are not using ARP at all? When
> >> we use DPDK, we directly pass through the network logic inside
> >> kernel itself. So logically all the network protocols could be
> >> customized by the user of it. In the customized network, maybe there
> >> is some other protocol (rather than RARP) that would do the same
> >> thing as what ARP/RARP does. So, this SEND_RARP request could give
> >> the vhost-user backend a chance to format its own announce packet
> >> and broadcast (in the SEND_RARP request, the guest's mac address
> >> will be appended).
> >>
> >> CCing Victor to better know the truth...
> >>
> >> Peter
> >>
> >

Hey Thibaut,

First of all, thanks a lot for your lengthy explanation.

> > Hi,
> >
> > After a migration, to avoid network outage, the guest must announce its
> > new location to the L2 layer, typically with a GARP. Otherwise requests
> > sent to the guest arrive to the old host until a ARP request is sent (after
> > 30 seconds) or the guest sends some data.
> >
> > QEMU implementation of self announce after a migration with a vhost
> > backend is the following:
> >  - If the VIRTIO_GUEST_ANNOUNCE feature has been negotiated the guest
> > sends automatically a GARP.

I'm kind of clear how VIRTIO_GUEST_ANNOUNCE works so far, except that I
met a bug, which I will describe in another email.

> >  - Else if the vhost backend implements VHOST_USER_SEND_RARP this request
> > is sent to the vhost backend. When this message is received the vhost
> > backend must act as it receives a RARP from the guest (purpose of this RARP

Can you be more specific about this? Say, what kind of acts the vhost
backend should do exactly?

> > is to update switches' MAC->port maaping as a GARP).

Isn't it vhost library is not aware of swtich at all? How could we
update switches's MAC-port mapping inside vhost library?

> This RARP is a false
> > one, created by the vhost backend,

I'm a bit confused now. You were just saying "vhost backend must act
as it __recevives__ a RARP from the guest", and you are now saying
"the RARP is a false one __created__ by the vhost backend".

Thanks.

	--yliu