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

[Michael S. Tsirkin]

On Thu, Apr 07, 2016 at 11:09:48AM +, Ilya Maximets wrote:
> > --- Original Message ---
> > Sender : Michael S. Tsirkin
> > 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 +, 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.

In cover letter you describe killing vhost user client.
So don't do it then.

> > 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.

I agree the current handling isn't good. I am just reluctant to
stick even more stuff in there before 2.6. We need a plan going
forward. I'll think it over during the weekend.

-- 
MST

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

[Ilya Maximets]

> --- Original Message ---
> Sender : Michael S. Tsirkin
> 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 +, 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.

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

[Michael S. Tsirkin]

On Wed, Apr 06, 2016 at 11:52:56PM +, 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

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.

-- 
MST

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

[Ilya Maximets]

--- 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.

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

[Michael S. Tsirkin]

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:  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:  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 ':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 ':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 ':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:  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
> > 
> > 

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

[Ilya Maximets]

On 31.03.2016 12:21, Michael S. Tsirkin wrote:
> 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. There may be still working vhost-kernel interfaces.
>> Even connection can still be established if vhost-user interface was in
>> bonding with kernel interface.
>> Anyway user should be able to save all his data before QEMU restart.
> 
> 
> Unfrotunately some guests are known to crash if we defer
> using available buffers indefinitely.

What exactly do you mean?

Anyway here we have 'crash of some guests' vs. '100% constant QEMU crash'.

> 
> If you see bugs let's fix them, but recovering would be
> a new feature, let's defer it until after 2.6.

The segmentation fault is a bug. This series *doesn't introduce any recovering*
features. Disconnected network interface in guest will never work again.
I just tried to avoid segmentation fault in this situation. There will
be no difference for guest between QEMU with this patches and without them.
There just will not be any crashes. 

> 
>>>
 1. # Check that link in guest is in a normal state.
[guest]# ip link show eth0
 2: eth0:  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:  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 ':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 ':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 ':00:01.0' > /sys/bus/pci/drivers/virtio-pci/bind

 6. # Check link in guest. No crashes here, link in DOWN state.
[guest]# ip link show eth0
 7: eth0:  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
>>>
>>>
> 
> 

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

[Michael S. Tsirkin]

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. There may be still working vhost-kernel interfaces.
> Even connection can still be established if vhost-user interface was in
> bonding with kernel interface.
> Anyway user should be able to save all his data before QEMU restart.


Unfrotunately some guests are known to crash if we defer
using available buffers indefinitely.

If you see bugs let's fix them, but recovering would be
a new feature, let's defer it until after 2.6.

> > 
> >> 1. # Check that link in guest is in a normal state.
> >>[guest]# ip link show eth0
> >> 2: eth0:  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:  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 ':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 ':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 ':00:01.0' > /sys/bus/pci/drivers/virtio-pci/bind
> >>
> >> 6. # Check link in guest. No crashes here, link in DOWN state.
> >>[guest]# ip link show eth0
> >> 7: eth0:  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
> > 
> > 

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

[Ilya Maximets]

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. There may be still working vhost-kernel interfaces.
Even connection can still be established if vhost-user interface was in
bonding with kernel interface.
Anyway user should be able to save all his data before QEMU restart.

> 
>> 1. # Check that link in guest is in a normal state.
>>[guest]# ip link show eth0
>> 2: eth0:  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:  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 ':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 ':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 ':00:01.0' > /sys/bus/pci/drivers/virtio-pci/bind
>>
>> 6. # Check link in guest. No crashes here, link in DOWN state.
>>[guest]# ip link show eth0
>> 7: eth0:  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
> 
> 

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

[Michael S. Tsirkin]

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?

> 1. # Check that link in guest is in a normal state.
>[guest]# ip link show eth0
> 2: eth0:  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:  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 ':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 ':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 ':00:01.0' > /sys/bus/pci/drivers/virtio-pci/bind
> 
> 6. # Check link in guest. No crashes here, link in DOWN state.
>[guest]# ip link show eth0
> 7: eth0:  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