Re: [Qemu-devel] Question about VM inner route entry's lost when vhost-user reconnect

[Lilijun (Jerry, Cloud Networking)]

Hi Stefan,

After more detail test, I found two results:
1) This route entry's lost  can be reproduced on both virtio-net and 
pass-through physical devices.
2) The link down event is handled by a service named NetworkManager in my VM 
(CentOS linux 3.10.0-514-e17.x86_64).  If I stop or kill this service,  the 
issue got disappeared.

So, if some customer's VM guest OS works like my VM,  this problem will cause 
some unexpected TCP connection disconnected when the backend process crashed.

Thanks

> -----Original Message-----
> From: Stefan Hajnoczi [mailto:address@hidden
> Sent: Wednesday, March 13, 2019 10:55 PM
> To: Lilijun (Jerry, Cloud Networking) <address@hidden>
> Cc: address@hidden; wangxin (U)
> <address@hidden>; wangyunjian
> <address@hidden>
> Subject: Re: [Qemu-devel] Question about VM inner route entry's lost when
> vhost-user reconnect
> 
> On Tue, Mar 12, 2019 at 02:01:04AM +0000, Lilijun (Jerry, Cloud Networking)
> wrote:
> >
> >
> > > -----Original Message-----
> > > From: Stefan Hajnoczi [mailto:address@hidden
> > > Sent: Monday, March 11, 2019 5:47 PM
> > > To: Lilijun (Jerry, Cloud Networking) <address@hidden>
> > > Cc: address@hidden; wangxin (U)
> <address@hidden>;
> > > wangyunjian <address@hidden>
> > > Subject: Re: [Qemu-devel] Question about VM inner route entry's lost
> > > when vhost-user reconnect
> > >
> > > On Fri, Mar 08, 2019 at 02:31:12AM +0000, Lilijun (Jerry, Cloud
> > > Networking)
> > > wrote:
> > > > This problem is related with backend vhost-user socket abnormal
> > > > cases, we
> > > shouldn't ask customers to configure it manually for backend's
> > > issues or depends on guest OS's network configuration.
> > >
> > > In Step 1 you said:
> > >
> > > > > >      1) In the VM, I add one route entry manually on the vNIC
> > > > > > eth0 using the
> > > > > linux tool route.
> > >
> > > You configured the route manually inside the guest.  Seems like a
> > > guest problem to me.
> > >
> > > If this was a physical machine that lost connectivity due to a link
> > > event, what would happen?
> >
> > Yes, the configuration can be recovered manually by customers.
> >
> > But in the virtualization machines, this configuration's lost is a result of
> backend process's software unexpected bugs or version update. So I think
> we need hide this change to customers.
> 
> My question is:
> 
> Do manually added routes disappear on a physical machine when the link
> goes down?
> 
> If yes, then the VM is acting correctly and this issue can be solved by
> configuring the guest appropriately.  (Hiding the link down event might seem
> nice in this particular situation but other users might need the event.  
> Usually
> it's best to follow how physical machines behave and rely on existing
> solutions instead of implementing different behavior for VMs because that
> leads to new problems that are hard to foresee.)
> 
> If no, then why is the guest treating the virtio-net link down differently?
> Could be a bug.
> 
> Stefan