[Qemu-devel] [PATCH 0/2] dataplane: fix start/stop races

[Qemu-devel] [PATCH 0/2] dataplane: fix start/stop races

[Michael S. Tsirkin]

This works around races that data plane introduces
simply by exiting immediately if we detect
that dataplane is active.

It's a small but ugly patch, it's only justification
is that it's minimally intrusive, and that it clearly
has no chance to break non data plane users.

The idea is to rework it all post 2.6.

Michael S. Tsirkin (2):
  virtio: add aio handler
  virtio-blk: use aio handler for data plane

 include/hw/virtio/virtio-blk.h  |  2 ++
 include/hw/virtio/virtio.h      |  4 ++++
 hw/block/dataplane/virtio-blk.c | 13 +++++++++++++
 hw/block/virtio-blk.c           | 28 ++++++++++++++++++----------
 hw/virtio/virtio.c              | 36 ++++++++++++++++++++++++++++++++----
 5 files changed, 69 insertions(+), 14 deletions(-)

-- 
MST

[Qemu-devel] [PATCH 1/2] virtio: add aio handler

[Michael S. Tsirkin]

In addition to handling IO in vcpu thread and
in io thread, blk dataplane introduces yet another mode:
handling it by aio.

This reuses the same handler as previous modes,
which triggers races as these were not designed to be reentrant.

As a temporary fix, add a separate handler just for aio, this will make
it possible to disable regular handlers when dataplane is active.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 include/hw/virtio/virtio.h |  4 ++++
 hw/virtio/virtio.c         | 36 ++++++++++++++++++++++++++++++++----
 2 files changed, 36 insertions(+), 4 deletions(-)

diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
index 2b5b248..c032067 100644
--- a/include/hw/virtio/virtio.h
+++ b/include/hw/virtio/virtio.h
@@ -142,6 +142,9 @@ VirtQueue *virtio_add_queue(VirtIODevice *vdev, int queue_size,
                             void (*handle_output)(VirtIODevice *,
                                                   VirtQueue *));
 
+void virtio_set_queue_aio(VirtQueue *vq,
+                          void (*handle_output)(VirtIODevice *, VirtQueue *));
+
 void virtio_del_queue(VirtIODevice *vdev, int n);
 
 void *virtqueue_alloc_element(size_t sz, unsigned out_num, unsigned in_num);
@@ -253,6 +256,7 @@ void virtio_queue_set_host_notifier_fd_handler(VirtQueue *vq, bool assign,
 void virtio_queue_aio_set_host_notifier_handler(VirtQueue *vq, AioContext *ctx,
                                                 bool assign, bool set_handler);
 void virtio_queue_notify_vq(VirtQueue *vq);
+void virtio_queue_notify_aio_vq(VirtQueue *vq);
 void virtio_irq(VirtQueue *vq);
 VirtQueue *virtio_vector_first_queue(VirtIODevice *vdev, uint16_t vector);
 VirtQueue *virtio_vector_next_queue(VirtQueue *vq);
diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index 08275a9..182bc56 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -94,6 +94,7 @@ struct VirtQueue
 
     uint16_t vector;
     void (*handle_output)(VirtIODevice *vdev, VirtQueue *vq);
+    void (*handle_aio_output)(VirtIODevice *vdev, VirtQueue *vq);
     VirtIODevice *vdev;
     EventNotifier guest_notifier;
     EventNotifier host_notifier;
@@ -1086,6 +1087,16 @@ void virtio_queue_set_align(VirtIODevice *vdev, int n, int align)
     virtio_queue_update_rings(vdev, n);
 }
 
+void virtio_queue_notify_aio_vq(VirtQueue *vq)
+{
+    if (vq->vring.desc && vq->handle_aio_output) {
+        VirtIODevice *vdev = vq->vdev;
+
+        trace_virtio_queue_notify(vdev, vq - vdev->vq, vq);
+        vq->handle_aio_output(vdev, vq);
+    }
+}
+
 void virtio_queue_notify_vq(VirtQueue *vq)
 {
     if (vq->vring.desc && vq->handle_output) {
@@ -1141,10 +1152,19 @@ VirtQueue *virtio_add_queue(VirtIODevice *vdev, int queue_size,
     vdev->vq[i].vring.num_default = queue_size;
     vdev->vq[i].vring.align = VIRTIO_PCI_VRING_ALIGN;
     vdev->vq[i].handle_output = handle_output;
+    vdev->vq[i].handle_aio_output = NULL;
 
     return &vdev->vq[i];
 }
 
+void virtio_set_queue_aio(VirtQueue *vq,
+                          void (*handle_output)(VirtIODevice *, VirtQueue *))
+{
+    assert(vq->handle_output);
+
+    vq->handle_aio_output = handle_output;
+}
+
 void virtio_del_queue(VirtIODevice *vdev, int n)
 {
     if (n < 0 || n >= VIRTIO_QUEUE_MAX) {
@@ -1778,11 +1798,11 @@ EventNotifier *virtio_queue_get_guest_notifier(VirtQueue *vq)
     return &vq->guest_notifier;
 }
 
-static void virtio_queue_host_notifier_read(EventNotifier *n)
+static void virtio_queue_host_notifier_aio_read(EventNotifier *n)
 {
     VirtQueue *vq = container_of(n, VirtQueue, host_notifier);
     if (event_notifier_test_and_clear(n)) {
-        virtio_queue_notify_vq(vq);
+        virtio_queue_notify_aio_vq(vq);
     }
 }
 
@@ -1791,14 +1811,22 @@ void virtio_queue_aio_set_host_notifier_handler(VirtQueue *vq, AioContext *ctx,
 {
     if (assign && set_handler) {
         aio_set_event_notifier(ctx, &vq->host_notifier, true,
-                               virtio_queue_host_notifier_read);
+                               virtio_queue_host_notifier_aio_read);
     } else {
         aio_set_event_notifier(ctx, &vq->host_notifier, true, NULL);
     }
     if (!assign) {
         /* Test and clear notifier before after disabling event,
          * in case poll callback didn't have time to run. */
-        virtio_queue_host_notifier_read(&vq->host_notifier);
+        virtio_queue_host_notifier_aio_read(&vq->host_notifier);
+    }
+}
+
+static void virtio_queue_host_notifier_read(EventNotifier *n)
+{
+    VirtQueue *vq = container_of(n, VirtQueue, host_notifier);
+    if (event_notifier_test_and_clear(n)) {
+        virtio_queue_notify_vq(vq);
     }
 }
 
-- 
MST

Re: [Qemu-devel] [PATCH 1/2] virtio: add aio handler

[Paolo Bonzini]

On 29/03/2016 15:42, Michael S. Tsirkin wrote:
> +void virtio_queue_notify_aio_vq(VirtQueue *vq)
> +{
> +    if (vq->vring.desc && vq->handle_aio_output) {
> +        VirtIODevice *vdev = vq->vdev;
> +
> +        trace_virtio_queue_notify(vdev, vq - vdev->vq, vq);
> +        vq->handle_aio_output(vdev, vq);
> +    }
> +}
> +

This can be static, and virtio_queue_notify_vq probably should too.

Paolo

[Qemu-devel] [PATCH 2/2] virtio-blk: use aio handler for data plane

[Michael S. Tsirkin]

In addition to handling IO in vcpu thread and
in io thread, blk dataplane introduces yet another mode:
handling it by aio.

This reuses the same handler as previous modes,
which triggers races as these were not designed to be reentrant.

As a temporary fix, use a separate handler just for aio, and
disable regular handlers when dataplane is active.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 include/hw/virtio/virtio-blk.h  |  2 ++
 hw/block/dataplane/virtio-blk.c | 13 +++++++++++++
 hw/block/virtio-blk.c           | 28 ++++++++++++++++++----------
 3 files changed, 33 insertions(+), 10 deletions(-)

diff --git a/include/hw/virtio/virtio-blk.h b/include/hw/virtio/virtio-blk.h
index ae84d92..df517ff 100644
--- a/include/hw/virtio/virtio-blk.h
+++ b/include/hw/virtio/virtio-blk.h
@@ -85,4 +85,6 @@ void virtio_blk_handle_request(VirtIOBlockReq *req, MultiReqBuffer *mrb);
 
 void virtio_blk_submit_multireq(BlockBackend *blk, MultiReqBuffer *mrb);
 
+void virtio_blk_handle_vq(VirtIOBlock *s, VirtQueue *vq);
+
 #endif
diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
index 36f3d2b..7d1f3b2 100644
--- a/hw/block/dataplane/virtio-blk.c
+++ b/hw/block/dataplane/virtio-blk.c
@@ -184,6 +184,17 @@ void virtio_blk_data_plane_destroy(VirtIOBlockDataPlane *s)
     g_free(s);
 }
 
+static void virtio_blk_data_plane_handle_output(VirtIODevice *vdev,
+                                                VirtQueue *vq)
+{
+    VirtIOBlock *s = VIRTIO_BLK(vdev);
+
+    assert(s->dataplane);
+    assert(s->dataplane_started);
+
+    virtio_blk_handle_vq(s, vq);
+}
+
 /* Context: QEMU global mutex held */
 void virtio_blk_data_plane_start(VirtIOBlockDataPlane *s)
 {
@@ -226,6 +237,7 @@ void virtio_blk_data_plane_start(VirtIOBlockDataPlane *s)
 
     /* Get this show started by hooking up our callbacks */
     aio_context_acquire(s->ctx);
+    virtio_set_queue_aio(s->vq, virtio_blk_data_plane_handle_output);
     virtio_queue_aio_set_host_notifier_handler(s->vq, s->ctx, true, true);
     aio_context_release(s->ctx);
     return;
@@ -262,6 +274,7 @@ void virtio_blk_data_plane_stop(VirtIOBlockDataPlane *s)
 
     /* Stop notifications for new requests from guest */
     virtio_queue_aio_set_host_notifier_handler(s->vq, s->ctx, false, false);
+    virtio_set_queue_aio(s->vq, NULL);
 
     /* Drain and switch bs back to the QEMU main loop */
     blk_set_aio_context(s->conf->conf.blk, qemu_get_aio_context());
diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
index cb710f1..5717f09 100644
--- a/hw/block/virtio-blk.c
+++ b/hw/block/virtio-blk.c
@@ -577,20 +577,11 @@ void virtio_blk_handle_request(VirtIOBlockReq *req, MultiReqBuffer *mrb)
     }
 }
 
-static void virtio_blk_handle_output(VirtIODevice *vdev, VirtQueue *vq)
+void virtio_blk_handle_vq(VirtIOBlock *s, VirtQueue *vq)
 {
-    VirtIOBlock *s = VIRTIO_BLK(vdev);
     VirtIOBlockReq *req;
     MultiReqBuffer mrb = {};
 
-    /* Some guests kick before setting VIRTIO_CONFIG_S_DRIVER_OK so start
-     * dataplane here instead of waiting for .set_status().
-     */
-    if (s->dataplane && !s->dataplane_started) {
-        virtio_blk_data_plane_start(s->dataplane);
-        return;
-    }
-
     blk_io_plug(s->blk);
 
     while ((req = virtio_blk_get_request(s))) {
@@ -604,6 +595,23 @@ static void virtio_blk_handle_output(VirtIODevice *vdev, VirtQueue *vq)
     blk_io_unplug(s->blk);
 }
 
+static void virtio_blk_handle_output(VirtIODevice *vdev, VirtQueue *vq)
+{
+    VirtIOBlock *s = VIRTIO_BLK(vdev);
+
+    if (s->dataplane) {
+        /* Some guests kick before setting VIRTIO_CONFIG_S_DRIVER_OK so start
+         * dataplane here instead of waiting for .set_status().
+         */
+        if (!s->dataplane_started) {
+            virtio_blk_data_plane_start(s->dataplane);
+        }
+        return;
+    }
+
+    virtio_blk_handle_vq(s, vq);
+}
+
 static void virtio_blk_dma_restart_bh(void *opaque)
 {
     VirtIOBlock *s = opaque;
-- 
MST

Re: [Qemu-devel] [PATCH 2/2] virtio-blk: use aio handler for data plane

[Paolo Bonzini]

On 29/03/2016 15:42, Michael S. Tsirkin wrote:
> @@ -262,6 +274,7 @@ void virtio_blk_data_plane_stop(VirtIOBlockDataPlane *s)
>  
>      /* Stop notifications for new requests from guest */
>      virtio_queue_aio_set_host_notifier_handler(s->vq, s->ctx, false, false);

I think that this should have been ", true, false" even before your
patch; I'd prefer to fix it even if the problem is latent.

The patch looks good, and it might even be an API improvement
independent of Conny's patches.  The reentrancy _is_ hard to understand,
and eliminating it makes the new API not just a hack.

In that case I would unify the new function with
virtio_queue_aio_set_host_notifier_handler.  In other words

- virtio_queue_aio_set_host_notifier_handler(vq, ctx, NULL) is
the same as

     virtio_set_queue_aio(s->vq, NULL);
     virtio_queue_aio_set_host_notifier_handler(vq, ctx, true, false);

- virtio_queue_aio_set_host_notifier_handler(vq, ctx, fn) is the same as

     virtio_queue_aio_set_host_notifier_handler(vq, ctx, true, true);
     virtio_set_queue_aio(vq, fn);

Thanks,

Paolo

> +    virtio_set_queue_aio(s->vq, NULL);
>  
>      /* Drain and switch bs back to the QEMU main loop */
>      blk_set_aio_context(s->conf->conf.blk, qemu_get_aio_context());

Re: [Qemu-devel] [PATCH 2/2] virtio-blk: use aio handler for data plane

[Michael S. Tsirkin]

On Tue, Mar 29, 2016 at 03:56:18PM +0200, Paolo Bonzini wrote:
> 
> 
> On 29/03/2016 15:42, Michael S. Tsirkin wrote:
> > @@ -262,6 +274,7 @@ void virtio_blk_data_plane_stop(VirtIOBlockDataPlane *s)
> >  
> >      /* Stop notifications for new requests from guest */
> >      virtio_queue_aio_set_host_notifier_handler(s->vq, s->ctx, false, false);
> 
> I think that this should have been ", true, false" even before your
> patch; I'd prefer to fix it even if the problem is latent.

Makes sense - post a patch?

> The patch looks good, and it might even be an API improvement
> independent of Conny's patches.  The reentrancy _is_ hard to understand,
> and eliminating it makes the new API not just a hack.
> 
> In that case I would unify the new function with
> virtio_queue_aio_set_host_notifier_handler.  In other words
> 
> - virtio_queue_aio_set_host_notifier_handler(vq, ctx, NULL) is
> the same as
> 
>      virtio_set_queue_aio(s->vq, NULL);
>      virtio_queue_aio_set_host_notifier_handler(vq, ctx, true, false);
> 
> - virtio_queue_aio_set_host_notifier_handler(vq, ctx, fn) is the same as
> 
>      virtio_queue_aio_set_host_notifier_handler(vq, ctx, true, true);
>      virtio_set_queue_aio(vq, fn);
> 
> Thanks,
> 
> Paolo

In that case, we'll have to also change scsi to use the new API.
A bit more work, to be sure.
Does scsi have the same problem as blk?

> > +    virtio_set_queue_aio(s->vq, NULL);
> >  
> >      /* Drain and switch bs back to the QEMU main loop */
> >      blk_set_aio_context(s->conf->conf.blk, qemu_get_aio_context());

Re: [Qemu-devel] [PATCH 2/2] virtio-blk: use aio handler for data plane

[Paolo Bonzini]

On 29/03/2016 15:58, Michael S. Tsirkin wrote:
> In that case, we'll have to also change scsi to use the new API.
> A bit more work, to be sure.
> Does scsi have the same problem as blk?

Yes.  The bug is in the virtio core.

Paolo

Re: [Qemu-devel] [PATCH 2/2] virtio-blk: use aio handler for data plane

[Paolo Bonzini]

On 29/03/2016 15:42, Michael S. Tsirkin wrote:
> +    if (s->dataplane) {
> +        /* Some guests kick before setting VIRTIO_CONFIG_S_DRIVER_OK so start
> +         * dataplane here instead of waiting for .set_status().
> +         */
> +        if (!s->dataplane_started) {
> +            virtio_blk_data_plane_start(s->dataplane);
> +        }
> +        return;
> +    }
> +
> +    virtio_blk_handle_vq(s, vq);

Another small comment, this can be written simply as

    if (s->dataplane) {
        virtio_blk_data_plane_start(s->dataplane);
    } else {
        virtio_blk_handle_vq(s, vq);
    }

Paolo

Re: [Qemu-devel] [PATCH 2/2] virtio-blk: use aio handler for data plane

[Michael S. Tsirkin]

On Tue, Mar 29, 2016 at 04:05:46PM +0200, Paolo Bonzini wrote:
> 
> 
> On 29/03/2016 15:42, Michael S. Tsirkin wrote:
> > +    if (s->dataplane) {
> > +        /* Some guests kick before setting VIRTIO_CONFIG_S_DRIVER_OK so start
> > +         * dataplane here instead of waiting for .set_status().
> > +         */
> > +        if (!s->dataplane_started) {
> > +            virtio_blk_data_plane_start(s->dataplane);
> > +        }
> > +        return;
> > +    }
> > +
> > +    virtio_blk_handle_vq(s, vq);
> 
> Another small comment, this can be written simply as
> 
>     if (s->dataplane) {
>         virtio_blk_data_plane_start(s->dataplane);

True, it's best not to poke at dataplane_started.

>     } else {
>         virtio_blk_handle_vq(s, vq);
>     }
> 

I prefer the return style I think, to stress the
fact that this is an unusual, unexpected case.

> Paolo

Re: [Qemu-devel] [PATCH 2/2] virtio-blk: use aio handler for data plane

[Paolo Bonzini]

On 29/03/2016 16:09, Michael S. Tsirkin wrote:
>> > Another small comment, this can be written simply as
>> > 
>> >     if (s->dataplane) {
>> >         virtio_blk_data_plane_start(s->dataplane);
> 
> True, it's best not to poke at dataplane_started.
> 
> >     } else {
> >         virtio_blk_handle_vq(s, vq);
> >     }
> > 
> 
> I prefer the return style I think, to stress the
> fact that this is an unusual, unexpected case.

We're getting dangerously close to personal preference, but I've noticed
virtio-scsi that you get a very common pattern of:

void virtio_scsi_handle_ctrl_vq(VirtIOSCSI *s, VirtQueue *vq)
{
    ... virtqueue_pop ...
}

static void virtio_scsi_data_plane_handle_ctrl(VirtIODevice *vdev,
                                               VirtQueue *vq)
{
    VirtIOSCSI *s = VIRTIO_SCSI(vdev);

    assert(dataplane active and started);
    virtio_scsi_handle_ctrl_vq(s, vq);
}


static void virtio_scsi_handle_ctrl(VirtIODevice *vdev, VirtQueue *vq)
{
    VirtIOSCSI *s = VIRTIO_SCSI(vdev);

    if (dataplane active) {
        virtio_scsi_dataplane_start(s);
    } else {
        virtio_scsi_handle_ctrl_vq(s, vq);
    }
}

so it's not really an unusual, unexpected case but a complete
separation between the dataplane case (handle_output starts
dataplane) and the non-dataplane case (handle_output just does
a cast and calls the actual workhorse).

Paolo

Re: [Qemu-devel] [PATCH 0/2] dataplane: fix start/stop races

[Michael S. Tsirkin]

On Tue, Mar 29, 2016 at 04:42:09PM +0300, Michael S. Tsirkin wrote:
> This works around races that data plane introduces
> simply by exiting immediately if we detect
> that dataplane is active.
> 
> It's a small but ugly patch, it's only justification
> is that it's minimally intrusive, and that it clearly
> has no chance to break non data plane users.
> 
> The idea is to rework it all post 2.6.

Testing status: lightly tested.

> Michael S. Tsirkin (2):
>   virtio: add aio handler
>   virtio-blk: use aio handler for data plane
> 
>  include/hw/virtio/virtio-blk.h  |  2 ++
>  include/hw/virtio/virtio.h      |  4 ++++
>  hw/block/dataplane/virtio-blk.c | 13 +++++++++++++
>  hw/block/virtio-blk.c           | 28 ++++++++++++++++++----------
>  hw/virtio/virtio.c              | 36 ++++++++++++++++++++++++++++++++----
>  5 files changed, 69 insertions(+), 14 deletions(-)
> 
> -- 
> MST
> 

Re: [Qemu-devel] [PATCH 0/2] dataplane: fix start/stop races

[Cornelia Huck]

On Tue, 29 Mar 2016 16:42:09 +0300
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> This works around races that data plane introduces
> simply by exiting immediately if we detect
> that dataplane is active.
> 
> It's a small but ugly patch, it's only justification
> is that it's minimally intrusive, and that it clearly
> has no chance to break non data plane users.
> 
> The idea is to rework it all post 2.6.
> 
> Michael S. Tsirkin (2):
>   virtio: add aio handler
>   virtio-blk: use aio handler for data plane
> 
>  include/hw/virtio/virtio-blk.h  |  2 ++
>  include/hw/virtio/virtio.h      |  4 ++++
>  hw/block/dataplane/virtio-blk.c | 13 +++++++++++++
>  hw/block/virtio-blk.c           | 28 ++++++++++++++++++----------
>  hw/virtio/virtio.c              | 36 ++++++++++++++++++++++++++++++++----
>  5 files changed, 69 insertions(+), 14 deletions(-)
> 

FWIW, this seems to survive an hour or so of reboot loops for me, but
that's only a small setup.

Re: [Qemu-devel] [PATCH 0/2] dataplane: fix start/stop races

[Christian Borntraeger]

On 03/29/2016 03:42 PM, Michael S. Tsirkin wrote:
> This works around races that data plane introduces
> simply by exiting immediately if we detect
> that dataplane is active.
> 
> It's a small but ugly patch, it's only justification
> is that it's minimally intrusive, and that it clearly
> has no chance to break non data plane users.
> 
> The idea is to rework it all post 2.6.
> 
> Michael S. Tsirkin (2):
>   virtio: add aio handler
>   virtio-blk: use aio handler for data plane
>  
>  include/hw/virtio/virtio-blk.h  |  2 ++
>  include/hw/virtio/virtio.h      |  4 ++++
>  hw/block/dataplane/virtio-blk.c | 13 +++++++++++++
>  hw/block/virtio-blk.c           | 28 ++++++++++++++++++----------
>  hw/virtio/virtio.c              | 36 ++++++++++++++++++++++++++++++++----
>  5 files changed, 69 insertions(+), 14 deletions(-)
> 

This also seems to help on my setup.Tu Bo, would be good if you
can double check this patch set as well on your setup?

Christian

Re: [Qemu-devel] [PATCH 0/2] dataplane: fix start/stop races

[tu bo]

Hi Christian:

On 03/30/2016 12:31 AM, Christian Borntraeger wrote:
> On 03/29/2016 03:42 PM, Michael S. Tsirkin wrote:
>> This works around races that data plane introduces
>> simply by exiting immediately if we detect
>> that dataplane is active.
>>
>> It's a small but ugly patch, it's only justification
>> is that it's minimally intrusive, and that it clearly
>> has no chance to break non data plane users.
>>
>> The idea is to rework it all post 2.6.
>>
>> Michael S. Tsirkin (2):
>>    virtio: add aio handler
>>    virtio-blk: use aio handler for data plane
>>
>>   include/hw/virtio/virtio-blk.h  |  2 ++
>>   include/hw/virtio/virtio.h      |  4 ++++
>>   hw/block/dataplane/virtio-blk.c | 13 +++++++++++++
>>   hw/block/virtio-blk.c           | 28 ++++++++++++++++++----------
>>   hw/virtio/virtio.c              | 36 ++++++++++++++++++++++++++++++++----
>>   5 files changed, 69 insertions(+), 14 deletions(-)
>>
>
> This also seems to help on my setup.Tu Bo, would be good if you
> can double check this patch set as well on your setup?

With qemu master +  [PATCH 0/2] dataplane: fix start/stop races,

I did NOT see any crash, result is good in my box. thanks

>
> Christian
>
>

Re: [Qemu-devel] [PATCH 0/2] dataplane: fix start/stop races

[Paolo Bonzini]

On 30/03/2016 05:04, tu bo wrote:
> Hi Christian:
> 
> On 03/30/2016 12:31 AM, Christian Borntraeger wrote:
>> On 03/29/2016 03:42 PM, Michael S. Tsirkin wrote:
>>> This works around races that data plane introduces
>>> simply by exiting immediately if we detect
>>> that dataplane is active.
>>>
>>> It's a small but ugly patch, it's only justification
>>> is that it's minimally intrusive, and that it clearly
>>> has no chance to break non data plane users.
>>>
>>> The idea is to rework it all post 2.6.
>>>
>>> Michael S. Tsirkin (2):
>>>    virtio: add aio handler
>>>    virtio-blk: use aio handler for data plane
>>>
>>>   include/hw/virtio/virtio-blk.h  |  2 ++
>>>   include/hw/virtio/virtio.h      |  4 ++++
>>>   hw/block/dataplane/virtio-blk.c | 13 +++++++++++++
>>>   hw/block/virtio-blk.c           | 28 ++++++++++++++++++----------
>>>   hw/virtio/virtio.c              | 36
>>> ++++++++++++++++++++++++++++++++----
>>>   5 files changed, 69 insertions(+), 14 deletions(-)
>>>
>>
>> This also seems to help on my setup.Tu Bo, would be good if you
>> can double check this patch set as well on your setup?
> 
> With qemu master +  [PATCH 0/2] dataplane: fix start/stop races,
> 
> I did NOT see any crash, result is good in my box. thanks

Great, I will send a revised version of Michael's patches with
virtio-scsi too.

Paolo