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

[Michael S. Tsirkin]

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.

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.

> 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.




> > 
> > 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
> >>>
> >>
> >