Re: [Qemu-devel] [PATCH] net: Update netdev peer on link change

[Michael S. Tsirkin]

On Wed, Apr 02, 2014 at 11:25:32AM -0400, Vlad Yasevich wrote:
> On 04/02/2014 11:03 AM, Michael S. Tsirkin wrote:
> > On Wed, Apr 02, 2014 at 10:57:08AM -0400, Vlad Yasevich wrote:
> >> On 04/02/2014 06:46 AM, Yan Vugenfirer wrote:
> >>>
> >>> On Apr 2, 2014, at 11:51 AM, Michael S. Tsirkin <address@hidden> wrote:
> >>>
> >>>> On Thu, Nov 21, 2013 at 09:05:51PM -0500, Vlad Yasevich wrote:
> >>>>> When a link change occurs on a backend (like tap), we currently do
> >>>>> not propage such change to the nic.  As a result, when someone turns
> >>>>> off a link on a tap device, for instance, then a guest doesn't see
> >>>>> that change and continues to try to send traffic or run DHCP even
> >>>>> though the lower-layer is disconnected.  This is OK when the network
> >>>>> is set up as a HUB since the the guest may be connected to other HUB
> >>>>> ports too, but when it's set up as a netdev, it makes thinkgs worse.
> >>>>>
> >>>>> The patch addresses this by setting the peers link down only when the
> >>>>> peer is not a HUBPORT device.  With this patch, in the following config
> >>>>>  -netdev tap,id=net0 -device e1000,mac=XXXXX,netdev=net0
> >>>>> when net0 link is turned off, the guest e1000 shows lower-layer link
> >>>>> down. This allows guests to boot much faster in such configurations.
> >>>>> With windows guest, it also allows the network to recover properly
> >>>>> since windows will not configure the link-local IPv4 address, and
> >>>>> when the link is turned on, the proper address address is configured.
> >>>>>
> >>>>> Signed-off-by: Vlad Yasevich <address@hidden>
> >>>>> ---
> >>>>> net/net.c | 26 +++++++++++++++++---------
> >>>>> 1 file changed, 17 insertions(+), 9 deletions(-)
> >>>>>
> >>>>> diff --git a/net/net.c b/net/net.c
> >>>>> index 0a88e68..8a084bf 100644
> >>>>> --- a/net/net.c
> >>>>> +++ b/net/net.c
> >>>>> @@ -1065,15 +1065,23 @@ void qmp_set_link(const char *name, bool up, 
> >>>>> Error **errp)
> >>>>>         nc->info->link_status_changed(nc);
> >>>>>     }
> >>>>>
> >>>>> -    /* Notify peer. Don't update peer link status: this makes it 
> >>>>> possible to
> >>>>> -     * disconnect from host network without notifying the guest.
> >>>>> -     * FIXME: is disconnected link status change operation useful?
> >>>>> -     *
> >>>>> -     * Current behaviour is compatible with qemu vlans where there 
> >>>>> could be
> >>>>> -     * multiple clients that can still communicate with each other in
> >>>>> -     * disconnected mode. For now maintain this compatibility. */
> >>>>
> >>>> Hmm this went in without much discussion.
> >>>>
> >>>> But before this change went in, it was possible to control link state on 
> >>>> NIC
> >>>> and on link separately, without guest noticing.
> >>>>
> >>>> Why did we decide to drop this functionality?
> >>>
> >>> Not sure there was a real need for the patch.
> >>>
> >>> On other hand Windows guest will not receive IP address from DHCP server 
> >>> if you start VM with tap link down and then set it to up without toggling 
> >>> link state of the guest device as well.
> >>
> >> Same for a linux guest.  It a fault scenario.  So we either handle
> >> it by propagating the link event, or we forbid it.  Doing neither
> >> allows for a bad state in common configuration.
> > 
> > It's how networking works.
> > If I turn off my router at home I can't get dhcp either.
> 
> And you device link will show that it has not carrier.

It doesn't as long as the switch it's connected to is up.


> > 
> > We had a way to disable networking  at link or at
> > the router, then removed the ability to turn it
> > off at the router and I'm asking why.
> 
> We didn't remove the ability to turn it off at router/switch.
> We just now propagate carrier loss correctly.
> 
> In fact, if you have a configuration where the VM is connected
> to a hub in qemu, turning off the link on the 'tap' connected
> to the same hub doesn't not change guest state.

So why is it a good idea to make -netdev inconsistent
with this?
It doesn't seem to add anything useful.
It might fix things for users who turned off link
at the wrong place by mistake but I doubt it's
a common scenario.

Where end users getting it wrong and complaining?


> > 
> >> This patch chose to propagate the event under certain configuration.
> >>
> >> -vlad
> > 
> > So don't do this then.
> > If you want guest to know that link is down,
> > put it down on guest side.
> > 
> > It's that simple.
> > 
> 
> Sorry, but that's not always possible.  The host administrator
> may not always be vm administrator and without this patch vm
> administrator has no idea what happened to his network.
> 
> -vlad
> > 
> > 
> > 
> >>>
> >>> Yan. 
> >>>
> >>>>
> >>>>
> >>>>
> >>>>> -    if (nc->peer && nc->peer->info->link_status_changed) {
> >>>>> -        nc->peer->info->link_status_changed(nc->peer);
> >>>>> +    if (nc->peer) {
> >>>>> +        /* Change peer link only if the peer is NIC and then notify 
> >>>>> peer.
> >>>>> +         * If the peer is a HUBPORT or a backend, we do not change the
> >>>>> +         * link status.
> >>>>> +         *
> >>>>> +         * This behavior is compatible with qemu vlans where there 
> >>>>> could be
> >>>>> +         * multiple clients that can still communicate with each other 
> >>>>> in
> >>>>> +         * disconnected mode. For now maintain this compatibility.
> >>>>> +         */
> >>>>> +        if (nc->peer->info->type == NET_CLIENT_OPTIONS_KIND_NIC) {
> >>>>> +            for (i = 0; i < queues; i++) {
> >>>>> +                ncs[i]->peer->link_down = !up;
> >>>>> +            }
> >>>>> +        }
> >>>>> +        if (nc->peer->info->link_status_changed) {
> >>>>> +            nc->peer->info->link_status_changed(nc->peer);
> >>>>> +        }
> >>>>>     }
> >>>>> }
> >>>>>
> >>>>> -- 
> >>>>> 1.8.4.2
> >>>>>
> >>>>
> >>>