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

[Stefano Brivio <sbrivio@redhat.com>]

On Fri, 14 Feb 2025 14:31:33 +0000
Daniel P. Berrangé <berrange@redhat.com> wrote:

> On Fri, Feb 14, 2025 at 03:28:34PM +0100, Stefano Brivio wrote:
> > On Fri, 14 Feb 2025 14:19:13 +0000
> > Daniel P. Berrangé <berrange@redhat.com> wrote:
> >   
> > > On Fri, Feb 14, 2025 at 01:59:20PM +0000, Daniel P. Berrangé 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.    
> > > 
> > > That said I kind of wonder whether we should be adding events against
> > > the chardev directly, instead of against the netdev use of chardev.  
> > 
> > What is the advantage? This already works and is consistent with the
> > existing non-vhost-user mechanism.  
> 
> The advantage is that chardevs are used in many places across QEMU, so
> emitting the events against the chardev is more broadly useful to
> consumers of QEMU, than doing a special case that only benefit vhostuser
> NICS.

It's not so much of a special case because it would be perfectly in
line with the NETDEV_STREAM_DISCONNECTED event and its usage in
applications (the only current user being libvirt).

That's a proven approach and the matching implementation for passt is
trivial because of that.

-- 
Stefano