[Qemu-devel] [PATCH] virtio-blk: Cancel the pending BH when the dataplane is reset

[Qemu-devel] [PATCH] virtio-blk: Cancel the pending BH when the dataplane is reset

[Philippe Mathieu-Daudé]

When 'system_reset' is called, the main loop clear the memory
region cache before the BH has a chance to execute. Later when
the deferred function is called, some assumptions that were
made when scheduling them are no longer true when they actually
execute.

This is what happens using a virtio-blk device (fresh RHEL7.8 install):

 $ (sleep 12.3; echo system_reset; sleep 12.3; echo system_reset; sleep 1; echo q) \
   | qemu-system-x86_64 -m 4G -smp 8 -boot menu=on \
     -device virtio-blk-pci,id=image1,drive=drive_image1 \
     -drive file=/var/lib/libvirt/images/rhel78.qcow2,if=none,id=drive_image1,format=qcow2,cache=none \
     -device virtio-net-pci,netdev=net0,id=nic0,mac=52:54:00:c4:e7:84 \
     -netdev tap,id=net0,script=/bin/true,downscript=/bin/true,vhost=on \
     -monitor stdio -serial null -nographic
  (qemu) system_reset
  (qemu) system_reset
  (qemu) qemu-system-x86_64: hw/virtio/virtio.c:225: vring_get_region_caches: Assertion `caches != NULL' failed.
  Aborted

  (gdb) bt
  Thread 1 (Thread 0x7f109c17b680 (LWP 10939)):
  #0  0x00005604083296d1 in vring_get_region_caches (vq=0x56040a24bdd0) at hw/virtio/virtio.c:227
  #1  0x000056040832972b in vring_avail_flags (vq=0x56040a24bdd0) at hw/virtio/virtio.c:235
  #2  0x000056040832d13d in virtio_should_notify (vdev=0x56040a240630, vq=0x56040a24bdd0) at hw/virtio/virtio.c:1648
  #3  0x000056040832d1f8 in virtio_notify_irqfd (vdev=0x56040a240630, vq=0x56040a24bdd0) at hw/virtio/virtio.c:1662
  #4  0x00005604082d213d in notify_guest_bh (opaque=0x56040a243ec0) at hw/block/dataplane/virtio-blk.c:75
  #5  0x000056040883dc35 in aio_bh_call (bh=0x56040a243f10) at util/async.c:90
  #6  0x000056040883dccd in aio_bh_poll (ctx=0x560409161980) at util/async.c:118
  #7  0x0000560408842af7 in aio_dispatch (ctx=0x560409161980) at util/aio-posix.c:460
  #8  0x000056040883e068 in aio_ctx_dispatch (source=0x560409161980, callback=0x0, user_data=0x0) at util/async.c:261
  #9  0x00007f10a8fca06d in g_main_context_dispatch () at /lib64/libglib-2.0.so.0
  #10 0x0000560408841445 in glib_pollfds_poll () at util/main-loop.c:215
  #11 0x00005604088414bf in os_host_main_loop_wait (timeout=0) at util/main-loop.c:238
  #12 0x00005604088415c4 in main_loop_wait (nonblocking=0) at util/main-loop.c:514
  #13 0x0000560408416b1e in main_loop () at vl.c:1923
  #14 0x000056040841e0e8 in main (argc=20, argv=0x7ffc2c3f9c58, envp=0x7ffc2c3f9d00) at vl.c:4578

Fix this by cancelling the BH when the virtio dataplane is stopped.

Reported-by: Yihuang Yu <yihyu@redhat.com>
Suggested-by: Stefan Hajnoczi <stefanha@redhat.com>
Fixes: https://bugs.launchpad.net/qemu/+bug/1839428
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
 hw/block/dataplane/virtio-blk.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
index 9299a1a7c2..4030faa21d 100644
--- a/hw/block/dataplane/virtio-blk.c
+++ b/hw/block/dataplane/virtio-blk.c
@@ -301,6 +301,8 @@ void virtio_blk_data_plane_stop(VirtIODevice *vdev)
     /* Clean up guest notifier (irq) */
     k->set_guest_notifiers(qbus->parent, nvqs, false);
 
+    qemu_bh_cancel(s->bh);
+
     vblk->dataplane_started = false;
     s->stopping = false;
 }
-- 
2.20.1

Re: [Qemu-devel] [Qemu-block] [PATCH] virtio-blk: Cancel the pending BH when the dataplane is reset

[John Snow]

On 8/16/19 1:15 PM, Philippe Mathieu-Daudé wrote:
> When 'system_reset' is called, the main loop clear the memory
> region cache before the BH has a chance to execute. Later when
> the deferred function is called, some assumptions that were
> made when scheduling them are no longer true when they actually
> execute.
> 
> This is what happens using a virtio-blk device (fresh RHEL7.8 install):
> 
>  $ (sleep 12.3; echo system_reset; sleep 12.3; echo system_reset; sleep 1; echo q) \
>    | qemu-system-x86_64 -m 4G -smp 8 -boot menu=on \
>      -device virtio-blk-pci,id=image1,drive=drive_image1 \
>      -drive file=/var/lib/libvirt/images/rhel78.qcow2,if=none,id=drive_image1,format=qcow2,cache=none \
>      -device virtio-net-pci,netdev=net0,id=nic0,mac=52:54:00:c4:e7:84 \
>      -netdev tap,id=net0,script=/bin/true,downscript=/bin/true,vhost=on \
>      -monitor stdio -serial null -nographic
>   (qemu) system_reset
>   (qemu) system_reset
>   (qemu) qemu-system-x86_64: hw/virtio/virtio.c:225: vring_get_region_caches: Assertion `caches != NULL' failed.
>   Aborted
> 
>   (gdb) bt
>   Thread 1 (Thread 0x7f109c17b680 (LWP 10939)):
>   #0  0x00005604083296d1 in vring_get_region_caches (vq=0x56040a24bdd0) at hw/virtio/virtio.c:227
>   #1  0x000056040832972b in vring_avail_flags (vq=0x56040a24bdd0) at hw/virtio/virtio.c:235
>   #2  0x000056040832d13d in virtio_should_notify (vdev=0x56040a240630, vq=0x56040a24bdd0) at hw/virtio/virtio.c:1648
>   #3  0x000056040832d1f8 in virtio_notify_irqfd (vdev=0x56040a240630, vq=0x56040a24bdd0) at hw/virtio/virtio.c:1662
>   #4  0x00005604082d213d in notify_guest_bh (opaque=0x56040a243ec0) at hw/block/dataplane/virtio-blk.c:75
>   #5  0x000056040883dc35 in aio_bh_call (bh=0x56040a243f10) at util/async.c:90
>   #6  0x000056040883dccd in aio_bh_poll (ctx=0x560409161980) at util/async.c:118
>   #7  0x0000560408842af7 in aio_dispatch (ctx=0x560409161980) at util/aio-posix.c:460
>   #8  0x000056040883e068 in aio_ctx_dispatch (source=0x560409161980, callback=0x0, user_data=0x0) at util/async.c:261
>   #9  0x00007f10a8fca06d in g_main_context_dispatch () at /lib64/libglib-2.0.so.0
>   #10 0x0000560408841445 in glib_pollfds_poll () at util/main-loop.c:215
>   #11 0x00005604088414bf in os_host_main_loop_wait (timeout=0) at util/main-loop.c:238
>   #12 0x00005604088415c4 in main_loop_wait (nonblocking=0) at util/main-loop.c:514
>   #13 0x0000560408416b1e in main_loop () at vl.c:1923
>   #14 0x000056040841e0e8 in main (argc=20, argv=0x7ffc2c3f9c58, envp=0x7ffc2c3f9d00) at vl.c:4578
> 
> Fix this by cancelling the BH when the virtio dataplane is stopped.
> 
> Reported-by: Yihuang Yu <yihyu@redhat.com>
> Suggested-by: Stefan Hajnoczi <stefanha@redhat.com>
> Fixes: https://bugs.launchpad.net/qemu/+bug/1839428
> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> ---
>  hw/block/dataplane/virtio-blk.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
> index 9299a1a7c2..4030faa21d 100644
> --- a/hw/block/dataplane/virtio-blk.c
> +++ b/hw/block/dataplane/virtio-blk.c
> @@ -301,6 +301,8 @@ void virtio_blk_data_plane_stop(VirtIODevice *vdev)
>      /* Clean up guest notifier (irq) */
>      k->set_guest_notifiers(qbus->parent, nvqs, false);
>  
> +    qemu_bh_cancel(s->bh);
> +
>      vblk->dataplane_started = false;
>      s->stopping = false;
>  }
> 

Naive question:

Since we're canceling the BH here and we're stopping the device; do we
need to do anything like clear out batch_notify_vqs? I assume in
system_reset contexts that's going to be handled anyway, are there
non-reset contexts where it matters?

--js

Re: [Qemu-devel] [PATCH] virtio-blk: Cancel the pending BH when the dataplane is reset

[Stefan Hajnoczi]

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On Fri, Aug 16, 2019 at 07:15:03PM +0200, Philippe Mathieu-Daudé wrote:
> When 'system_reset' is called, the main loop clear the memory
> region cache before the BH has a chance to execute. Later when
> the deferred function is called, some assumptions that were
> made when scheduling them are no longer true when they actually
> execute.
> 
> This is what happens using a virtio-blk device (fresh RHEL7.8 install):
> 
>  $ (sleep 12.3; echo system_reset; sleep 12.3; echo system_reset; sleep 1; echo q) \
>    | qemu-system-x86_64 -m 4G -smp 8 -boot menu=on \
>      -device virtio-blk-pci,id=image1,drive=drive_image1 \
>      -drive file=/var/lib/libvirt/images/rhel78.qcow2,if=none,id=drive_image1,format=qcow2,cache=none \
>      -device virtio-net-pci,netdev=net0,id=nic0,mac=52:54:00:c4:e7:84 \
>      -netdev tap,id=net0,script=/bin/true,downscript=/bin/true,vhost=on \
>      -monitor stdio -serial null -nographic
>   (qemu) system_reset
>   (qemu) system_reset
>   (qemu) qemu-system-x86_64: hw/virtio/virtio.c:225: vring_get_region_caches: Assertion `caches != NULL' failed.
>   Aborted
> 
>   (gdb) bt
>   Thread 1 (Thread 0x7f109c17b680 (LWP 10939)):
>   #0  0x00005604083296d1 in vring_get_region_caches (vq=0x56040a24bdd0) at hw/virtio/virtio.c:227
>   #1  0x000056040832972b in vring_avail_flags (vq=0x56040a24bdd0) at hw/virtio/virtio.c:235
>   #2  0x000056040832d13d in virtio_should_notify (vdev=0x56040a240630, vq=0x56040a24bdd0) at hw/virtio/virtio.c:1648
>   #3  0x000056040832d1f8 in virtio_notify_irqfd (vdev=0x56040a240630, vq=0x56040a24bdd0) at hw/virtio/virtio.c:1662
>   #4  0x00005604082d213d in notify_guest_bh (opaque=0x56040a243ec0) at hw/block/dataplane/virtio-blk.c:75
>   #5  0x000056040883dc35 in aio_bh_call (bh=0x56040a243f10) at util/async.c:90
>   #6  0x000056040883dccd in aio_bh_poll (ctx=0x560409161980) at util/async.c:118
>   #7  0x0000560408842af7 in aio_dispatch (ctx=0x560409161980) at util/aio-posix.c:460
>   #8  0x000056040883e068 in aio_ctx_dispatch (source=0x560409161980, callback=0x0, user_data=0x0) at util/async.c:261
>   #9  0x00007f10a8fca06d in g_main_context_dispatch () at /lib64/libglib-2.0.so.0
>   #10 0x0000560408841445 in glib_pollfds_poll () at util/main-loop.c:215
>   #11 0x00005604088414bf in os_host_main_loop_wait (timeout=0) at util/main-loop.c:238
>   #12 0x00005604088415c4 in main_loop_wait (nonblocking=0) at util/main-loop.c:514
>   #13 0x0000560408416b1e in main_loop () at vl.c:1923
>   #14 0x000056040841e0e8 in main (argc=20, argv=0x7ffc2c3f9c58, envp=0x7ffc2c3f9d00) at vl.c:4578
> 
> Fix this by cancelling the BH when the virtio dataplane is stopped.
> 
> Reported-by: Yihuang Yu <yihyu@redhat.com>
> Suggested-by: Stefan Hajnoczi <stefanha@redhat.com>
> Fixes: https://bugs.launchpad.net/qemu/+bug/1839428
> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> ---
>  hw/block/dataplane/virtio-blk.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
> index 9299a1a7c2..4030faa21d 100644
> --- a/hw/block/dataplane/virtio-blk.c
> +++ b/hw/block/dataplane/virtio-blk.c
> @@ -301,6 +301,8 @@ void virtio_blk_data_plane_stop(VirtIODevice *vdev)
>      /* Clean up guest notifier (irq) */
>      k->set_guest_notifiers(qbus->parent, nvqs, false);
>  
> +    qemu_bh_cancel(s->bh);
> +
>      vblk->dataplane_started = false;
>      s->stopping = false;
>  }
> -- 
> 2.20.1
> 

Along the lines of what John said:

diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
index 9299a1a7c2..4030faa21d 100644
--- a/hw/block/dataplane/virtio-blk.c
+++ b/hw/block/dataplane/virtio-blk.c
@@ -301,6 +301,8 @@ void virtio_blk_data_plane_stop(VirtIODevice *vdev)
+    qemu_bh_cancel(s->bh);
+    notify_guest_bh(s); /* final chance to notify guest */
+
     /* Clean up guest notifier (irq) */
     k->set_guest_notifiers(qbus->parent, nvqs, false);
 
     vblk->dataplane_started = false;
     s->stopping = false;
 }

Let's notify the guest if any batched notifications are waiting.  This
ensures that no notifications are lost when dataplane is stopped.

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Re: [Qemu-devel] [Qemu-block] [PATCH] virtio-blk: Cancel the pending BH when the dataplane is reset

[Stefan Hajnoczi]

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On Fri, Aug 16, 2019 at 02:35:56PM -0400, John Snow wrote:
> 
> 
> On 8/16/19 1:15 PM, Philippe Mathieu-Daudé wrote:
> > When 'system_reset' is called, the main loop clear the memory
> > region cache before the BH has a chance to execute. Later when
> > the deferred function is called, some assumptions that were
> > made when scheduling them are no longer true when they actually
> > execute.
> > 
> > This is what happens using a virtio-blk device (fresh RHEL7.8 install):
> > 
> >  $ (sleep 12.3; echo system_reset; sleep 12.3; echo system_reset; sleep 1; echo q) \
> >    | qemu-system-x86_64 -m 4G -smp 8 -boot menu=on \
> >      -device virtio-blk-pci,id=image1,drive=drive_image1 \
> >      -drive file=/var/lib/libvirt/images/rhel78.qcow2,if=none,id=drive_image1,format=qcow2,cache=none \
> >      -device virtio-net-pci,netdev=net0,id=nic0,mac=52:54:00:c4:e7:84 \
> >      -netdev tap,id=net0,script=/bin/true,downscript=/bin/true,vhost=on \
> >      -monitor stdio -serial null -nographic
> >   (qemu) system_reset
> >   (qemu) system_reset
> >   (qemu) qemu-system-x86_64: hw/virtio/virtio.c:225: vring_get_region_caches: Assertion `caches != NULL' failed.
> >   Aborted
> > 
> >   (gdb) bt
> >   Thread 1 (Thread 0x7f109c17b680 (LWP 10939)):
> >   #0  0x00005604083296d1 in vring_get_region_caches (vq=0x56040a24bdd0) at hw/virtio/virtio.c:227
> >   #1  0x000056040832972b in vring_avail_flags (vq=0x56040a24bdd0) at hw/virtio/virtio.c:235
> >   #2  0x000056040832d13d in virtio_should_notify (vdev=0x56040a240630, vq=0x56040a24bdd0) at hw/virtio/virtio.c:1648
> >   #3  0x000056040832d1f8 in virtio_notify_irqfd (vdev=0x56040a240630, vq=0x56040a24bdd0) at hw/virtio/virtio.c:1662
> >   #4  0x00005604082d213d in notify_guest_bh (opaque=0x56040a243ec0) at hw/block/dataplane/virtio-blk.c:75
> >   #5  0x000056040883dc35 in aio_bh_call (bh=0x56040a243f10) at util/async.c:90
> >   #6  0x000056040883dccd in aio_bh_poll (ctx=0x560409161980) at util/async.c:118
> >   #7  0x0000560408842af7 in aio_dispatch (ctx=0x560409161980) at util/aio-posix.c:460
> >   #8  0x000056040883e068 in aio_ctx_dispatch (source=0x560409161980, callback=0x0, user_data=0x0) at util/async.c:261
> >   #9  0x00007f10a8fca06d in g_main_context_dispatch () at /lib64/libglib-2.0.so.0
> >   #10 0x0000560408841445 in glib_pollfds_poll () at util/main-loop.c:215
> >   #11 0x00005604088414bf in os_host_main_loop_wait (timeout=0) at util/main-loop.c:238
> >   #12 0x00005604088415c4 in main_loop_wait (nonblocking=0) at util/main-loop.c:514
> >   #13 0x0000560408416b1e in main_loop () at vl.c:1923
> >   #14 0x000056040841e0e8 in main (argc=20, argv=0x7ffc2c3f9c58, envp=0x7ffc2c3f9d00) at vl.c:4578
> > 
> > Fix this by cancelling the BH when the virtio dataplane is stopped.
> > 
> > Reported-by: Yihuang Yu <yihyu@redhat.com>
> > Suggested-by: Stefan Hajnoczi <stefanha@redhat.com>
> > Fixes: https://bugs.launchpad.net/qemu/+bug/1839428
> > Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> > ---
> >  hw/block/dataplane/virtio-blk.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
> > index 9299a1a7c2..4030faa21d 100644
> > --- a/hw/block/dataplane/virtio-blk.c
> > +++ b/hw/block/dataplane/virtio-blk.c
> > @@ -301,6 +301,8 @@ void virtio_blk_data_plane_stop(VirtIODevice *vdev)
> >      /* Clean up guest notifier (irq) */
> >      k->set_guest_notifiers(qbus->parent, nvqs, false);
> >  
> > +    qemu_bh_cancel(s->bh);
> > +
> >      vblk->dataplane_started = false;
> >      s->stopping = false;
> >  }
> > 
> 
> Naive question:
> 
> Since we're canceling the BH here and we're stopping the device; do we
> need to do anything like clear out batch_notify_vqs? I assume in
> system_reset contexts that's going to be handled anyway, are there
> non-reset contexts where it matters?

Spurious guest notifications aren't a problem but missing notifications
can hang the guest.  I have proposed a modified version of this code
that ensures pending batched notifications are sent.

Stefan

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Re: [Qemu-devel] [PATCH] virtio-blk: Cancel the pending BH when the dataplane is reset

[Philippe Mathieu-Daudé]

On 8/22/19 3:01 PM, Stefan Hajnoczi wrote:
> On Fri, Aug 16, 2019 at 07:15:03PM +0200, Philippe Mathieu-Daudé wrote:
>> When 'system_reset' is called, the main loop clear the memory
>> region cache before the BH has a chance to execute. Later when
>> the deferred function is called, some assumptions that were
>> made when scheduling them are no longer true when they actually
>> execute.
>>
>> This is what happens using a virtio-blk device (fresh RHEL7.8 install):
>>
>>  $ (sleep 12.3; echo system_reset; sleep 12.3; echo system_reset; sleep 1; echo q) \
>>    | qemu-system-x86_64 -m 4G -smp 8 -boot menu=on \
>>      -device virtio-blk-pci,id=image1,drive=drive_image1 \
>>      -drive file=/var/lib/libvirt/images/rhel78.qcow2,if=none,id=drive_image1,format=qcow2,cache=none \
>>      -device virtio-net-pci,netdev=net0,id=nic0,mac=52:54:00:c4:e7:84 \
>>      -netdev tap,id=net0,script=/bin/true,downscript=/bin/true,vhost=on \
>>      -monitor stdio -serial null -nographic
>>   (qemu) system_reset
>>   (qemu) system_reset
>>   (qemu) qemu-system-x86_64: hw/virtio/virtio.c:225: vring_get_region_caches: Assertion `caches != NULL' failed.
>>   Aborted
>>
>>   (gdb) bt
>>   Thread 1 (Thread 0x7f109c17b680 (LWP 10939)):
>>   #0  0x00005604083296d1 in vring_get_region_caches (vq=0x56040a24bdd0) at hw/virtio/virtio.c:227
>>   #1  0x000056040832972b in vring_avail_flags (vq=0x56040a24bdd0) at hw/virtio/virtio.c:235
>>   #2  0x000056040832d13d in virtio_should_notify (vdev=0x56040a240630, vq=0x56040a24bdd0) at hw/virtio/virtio.c:1648
>>   #3  0x000056040832d1f8 in virtio_notify_irqfd (vdev=0x56040a240630, vq=0x56040a24bdd0) at hw/virtio/virtio.c:1662
>>   #4  0x00005604082d213d in notify_guest_bh (opaque=0x56040a243ec0) at hw/block/dataplane/virtio-blk.c:75
>>   #5  0x000056040883dc35 in aio_bh_call (bh=0x56040a243f10) at util/async.c:90
>>   #6  0x000056040883dccd in aio_bh_poll (ctx=0x560409161980) at util/async.c:118
>>   #7  0x0000560408842af7 in aio_dispatch (ctx=0x560409161980) at util/aio-posix.c:460
>>   #8  0x000056040883e068 in aio_ctx_dispatch (source=0x560409161980, callback=0x0, user_data=0x0) at util/async.c:261
>>   #9  0x00007f10a8fca06d in g_main_context_dispatch () at /lib64/libglib-2.0.so.0
>>   #10 0x0000560408841445 in glib_pollfds_poll () at util/main-loop.c:215
>>   #11 0x00005604088414bf in os_host_main_loop_wait (timeout=0) at util/main-loop.c:238
>>   #12 0x00005604088415c4 in main_loop_wait (nonblocking=0) at util/main-loop.c:514
>>   #13 0x0000560408416b1e in main_loop () at vl.c:1923
>>   #14 0x000056040841e0e8 in main (argc=20, argv=0x7ffc2c3f9c58, envp=0x7ffc2c3f9d00) at vl.c:4578
>>
>> Fix this by cancelling the BH when the virtio dataplane is stopped.
>>
>> Reported-by: Yihuang Yu <yihyu@redhat.com>
>> Suggested-by: Stefan Hajnoczi <stefanha@redhat.com>
>> Fixes: https://bugs.launchpad.net/qemu/+bug/1839428
>> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
>> ---
>>  hw/block/dataplane/virtio-blk.c | 2 ++
>>  1 file changed, 2 insertions(+)
>>
>> diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
>> index 9299a1a7c2..4030faa21d 100644
>> --- a/hw/block/dataplane/virtio-blk.c
>> +++ b/hw/block/dataplane/virtio-blk.c
>> @@ -301,6 +301,8 @@ void virtio_blk_data_plane_stop(VirtIODevice *vdev)
>>      /* Clean up guest notifier (irq) */
>>      k->set_guest_notifiers(qbus->parent, nvqs, false);
>>  
>> +    qemu_bh_cancel(s->bh);
>> +
>>      vblk->dataplane_started = false;
>>      s->stopping = false;
>>  }
>> -- 
>> 2.20.1
>>
> 
> Along the lines of what John said:
> 
> diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
> index 9299a1a7c2..4030faa21d 100644
> --- a/hw/block/dataplane/virtio-blk.c
> +++ b/hw/block/dataplane/virtio-blk.c
> @@ -301,6 +301,8 @@ void virtio_blk_data_plane_stop(VirtIODevice *vdev)
> +    qemu_bh_cancel(s->bh);
> +    notify_guest_bh(s); /* final chance to notify guest */
> +
>      /* Clean up guest notifier (irq) */
>      k->set_guest_notifiers(qbus->parent, nvqs, false);
>  
>      vblk->dataplane_started = false;
>      s->stopping = false;
>  }
> 
> Let's notify the guest if any batched notifications are waiting.  This
> ensures that no notifications are lost when dataplane is stopped.

OK, works for me, thanks!