[Qemu-devel] [PATCH 1/1] virtio: fallback from irqfd to non-irqfd notify

[Halil Pasic <pasic@linux.vnet.ibm.com>]

The commits 03de2f527 "virtio-blk: do not use vring in dataplane"  and
9ffe337c08 "virtio-blk: always use dataplane path if ioeventfd is active"
changed how notifications are done for virtio-blk substantially. Due to a
race condition interrupts are lost when irqfd is torn down after
notify_guest_bh was scheduled but before it actually runs.  Furthermore
virtio_notify_irqfd ignores the value returned by event_notifier_set
which correctly indicates that notification has failed due to bad file
descriptor.

Let's fix this by making virtio_notify_irqfd fall back to the non-irqfd
notification mechanism if event_notifier_set fails.

Signed-off-by: Halil Pasic <pasic@linux.vnet.ibm.com>
---

This is probably not the only way to fix this: suggestions welcome. I
did not use a fixes tag because I'm not sure yet where exactly things got
broken. Maybe guys more familiar with dataplane an coroutines can help
(Paolo, Stefan).
---
 hw/virtio/virtio.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index 23483c7..8e1c1e9 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -1581,7 +1581,9 @@ void virtio_notify_irqfd(VirtIODevice *vdev, VirtQueue *vq)
      * to an atomic operation.
      */
     virtio_set_isr(vq->vdev, 0x1);
-    event_notifier_set(&vq->guest_notifier);
+    if (event_notifier_set(&vq->guest_notifier)) {
+        virtio_notify_vector(vdev, vq->vector);
+    }
 }
 
 static void virtio_irq(VirtQueue *vq)
-- 
2.8.4