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

["Michael S. Tsirkin" <mst@redhat.com>]

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.

> > 
> >> 1. # Check that link in guest is in a normal state.
> >>    [guest]# ip link show eth0
> >>     2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc <...>
> >>         link/ether 00:16:35:af:aa:4b brd ff:ff:ff:ff:ff:ff
> >>
> >> 2. # Kill vhost-user application (using SIGSEGV just to be sure).
> >>    [host]# kill -11 `pgrep ovs-vswitchd`
> >>
> >> 3. # Check that guest still thinks that all is good.
> >>    [guest]# ip link show eth0
> >>     2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc <...>
> >>         link/ether 00:16:35:af:aa:4b brd ff:ff:ff:ff:ff:ff
> >>
> >> 4. # Try to unbind virtio-pci driver and observe QEMU crash.
> >>    [guest]# echo -n '0000:00:01.0' > /sys/bus/pci/drivers/virtio-pci/unbind
> >>     qemu: Failed to read msg header. Read 0 instead of 12. Original request 11.
> >>     qemu: Failed to read msg header. Read 0 instead of 12. Original request 11.
> >>     qemu: Failed to read msg header. Read 0 instead of 12. Original request 11.
> >>
> >>     Child terminated with signal = 0xb (SIGSEGV)
> >>     GDBserver exiting
> >>
> >> After the applying of this patch-set:
> >>
> >> 4. # Try to unbind virtio-pci driver. Unbind works fine with only few errors.
> >>    [guest]# echo -n '0000:00:01.0' > /sys/bus/pci/drivers/virtio-pci/unbind
> >>     qemu: Failed to read msg header. Read 0 instead of 12. Original request 11.
> >>     qemu: Failed to read msg header. Read 0 instead of 12. Original request 11.
> >>
> >> 5. # Bind virtio-pci driver back.
> >>    [guest]# echo -n '0000:00:01.0' > /sys/bus/pci/drivers/virtio-pci/bind
> >>
> >> 6. # Check link in guest. No crashes here, link in DOWN state.


We already have interfaces to control link state.
I don't think we should necessarily tie link state to backend
state. Could be an option but probably not the default.

> >>    [guest]# ip link show eth0
> >>     7: eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc <...>
> >>         link/ether 00:16:35:af:aa:4b brd ff:ff:ff:ff:ff:ff
> >>
> >> 7. QEMU may be gracefully restarted to restore communication after restarting
> >>    of vhost-user application.
> >>
> >> Ilya Maximets (4):
> >>   vhost-user: fix crash on socket disconnect.
> >>   vhost: prevent double stop of vhost_net device.
> >>   vhost: check for vhost_net device validity.
> >>   net: notify about link status only if it changed.
> >>
> >>  hw/net/vhost_net.c             | 48 ++++++++++++++++++++++++++++++++++++++----
> >>  hw/net/virtio-net.c            | 33 ++++++++++++++++++++++-------
> >>  include/hw/virtio/virtio-net.h |  1 +
> >>  include/net/vhost-user.h       |  1 +
> >>  include/net/vhost_net.h        |  1 +
> >>  net/filter.c                   |  1 +
> >>  net/net.c                      |  7 +++---
> >>  net/vhost-user.c               | 43 +++++++++++++++++++++++++++++--------
> >>  8 files changed, 111 insertions(+), 24 deletions(-)
> >>
> >> -- 
> >> 2.5.0
> > 
> >