Re: [PATCH] net: vhost-user: add QAPI events to report connection state

[Stefano Brivio <sbrivio@redhat.com>]

On Fri, 14 Feb 2025 13:59:20 +0000
Daniel P. Berrangé <berrange@redhat.com> wrote:

> On Fri, Feb 14, 2025 at 11:18:55AM +0100, Laurent Vivier wrote:
> > On 14/02/2025 11:06, Markus Armbruster wrote:  
> > > Laurent Vivier <lvivier@redhat.com> writes:
> > >   
> > > > The netdev reports NETDEV_VHOST_USER_CONNECTED event when
> > > > the chardev is connected, and NETDEV_VHOST_USER_DISCONNECTED
> > > > when it is disconnected.
> > > > 
> > > > The NETDEV_VHOST_USER_CONNECTED event includes the ChardevInfo
> > > > (label, filename and frontend_open).
> > > > 
> > > > This allows a system manager like libvirt to detect when the server
> > > > fails.
> > > > 
> > > > For instance with passt:
> > > > 
> > > > { 'execute': 'qmp_capabilities' }
> > > > { "return": { } }
> > > > 
> > > > [killing passt here]
> > > > 
> > > > { "timestamp": { "seconds": 1739517243, "microseconds": 115081 },
> > > >    "event": "NETDEV_VHOST_USER_DISCONNECTED",
> > > >    "data": { "netdev-id": "netdev0" } }
> > > > 
> > > > [automatic reconnection with reconnect-ms]
> > > > 
> > > > { "timestamp": { "seconds": 1739517290, "microseconds": 343777 },
> > > >    "event": "NETDEV_VHOST_USER_CONNECTED",
> > > >    "data": { "netdev-id": "netdev0",
> > > >              "info": { "frontend-open": true,
> > > >                        "filename": "unix:",
> > > >                        "label": "chr0" } } }
> > > > 
> > > > Signed-off-by: Laurent Vivier <lvivier@redhat.com>  
> > > 
> > > Standard question for events: if a management application misses an
> > > event, say because it restarts and reconnects, is there a way to obtain
> > > the missed information with a query command?
> > >   
> > 
> > query-chardev could help but it doesn't provide the netdev id.  
> 
> It doesn't have to IMHO. The application that created the NIC should know
> what ID it assigned to both the netdev and chardev, and thus should be
> able to use query-chardev to identify the chardev it previously
> associated with the netdev.

It does (of course?), see processNetdevStreamDisconnectedEvent() in
libvirt, src/qemu/qemu_driver.c:

static void
processNetdevStreamDisconnectedEvent(virDomainObj *vm,
                                     const char *netdevId)
{

[...]

    const char *devAlias = STRSKIP(netdevId, "host");

    /* The event sends us the "netdev-id", but we don't store the
     * netdev-id in the NetDef and thus can't use it to find the
     * correct NetDef. We *do* keep the device alias in the NetDef,
     * and by convention the netdev-id is always "host" + devAlias, so
     * we just need to remove "host" from the front of netdev-id to
     * get the alias, which we can then use to find the proper NetDef.
     */

[...]

    if (virDomainDefFindDevice(vm->def, devAlias, &dev, true) < 0) {
        VIR_WARN("NETDEV_STREAM_DISCONNECTED event received for non-existent device %s in domain %s",
                 devAlias, vm->def->name);
        goto endjob;
    }

[...]

-- 
Stefano