Re: [Qemu-devel] [PATCH 0/4] Fix QEMU crash on vhost-user socket disconnect.

[Ilya Maximets <i.maximets@samsung.com>]

> ------- Original Message -------
> Sender : Michael S. Tsirkin<mst@redhat.com>
> Date : Apr 07, 2016 10:01 (GMT+03:00)
> Title : Re: Re: [PATCH 0/4] Fix QEMU crash on vhost-user socket disconnect.
> 
> On Wed, Apr 06, 2016 at 11:52:56PM +0000, Ilya Maximets wrote:
> > ------- Original Message -------
> > Sender : Michael S. Tsirkin
> > Date : Apr 05, 2016 13:46 (GMT+03:00)
> > Title : Re: [PATCH 0/4] Fix QEMU crash on vhost-user socket disconnect.
> > 
> > > On Thu, Mar 31, 2016 at 09:02:01AM +0300, Ilya Maximets wrote:
> > > > On 30.03.2016 20:01, Michael S. Tsirkin wrote:
> > > > > On Wed, Mar 30, 2016 at 06:14:05PM +0300, Ilya Maximets wrote:
> > > > >> Currently QEMU always crashes in following scenario (assume that
> > > > >> vhost-user application is Open vSwitch with 'dpdkvhostuser' port):
> > > > > 
> > > > > In fact, wouldn't the right thing to do be stopping the VM?
> > > > 
> > > > I don't think that is reasonable to stop VM on failure of just one of
> > > > network interfaces.
> > > 
> > > We don't start QEMU until vhost user connects, either.
> > > Making guest run properly with a backend is a much bigger
> > > project, let's tackle this separately.
> > > 
> > > Also, I think handling graceful disconnect is the correct first step.
> > > Handling misbehaving clients is much harder as we have asserts on remote
> > > obeying protocol rules all over the place.
> > > 
> > > > There may be still working vhost-kernel interfaces.
> > > > Even connection can still be established if vhost-user interface was in
> > > > bonding with kernel interface.
> > > 
> > > Could not parse this.
> > >
> > > > Anyway user should be able to save all his data before QEMU restart.
> > >
> > > So reconnect a new backend, and VM will keep going.
> > 
> > We cant't do this because of 2 reasons:
> > 1. Segmentation fault of QEMU on disconnect. (fixed by this patch-set)
> > 2. There is no reconnect functionality in current QEMU version.
> > 
> > So, what are you talking about?
> > 
> > Best regards, Ilya Maximets.
> 
> One can currently disconnect vhost user clients without killing
> guests using migration:
> - save vm
> - quit qemu
> - start new qemu
> - load vm
>
> Or using hotplug
> - request unplug
> - wait for guest eject
> - create new device

This may be fixes second reason mentioned above, but in my scenario (described in cover-letter)
guset will know that backend disconnected only after segmentation fault. So, there will be
no opportunity to save or unplug machine.

> I would like to make sure we do not create an expectation that guests
> keep going unconditionally with device present even with backend
>disconnected. Unfortunately your patchset might create such
> expectation.

It's already so, because guest will never know about crach of backend untill it
tries to communicate via socket.