[Qemu-devel] [RFC PATCH] vhost: fix lost interrupts from slow reacting back-end

[Victor Kaplansky]

This RFC PATCH tries to solve the problem of lost interrupts
from a slow back-end. Didier could you test it?

Thanks, Victor

When interrupts are unmasked, it could take some undefined time
to the back-end to start routing events to guest_notifier. Till
that the events will continue flow to masked_notifier, and some
interrupts could be lost.

This patch tries to handle the above situation by testing and
cleaning both masked_notifier and guest_notifier in
guest_notifier read handler.

Signed-off-by: Victor Kaplansky <address@hidden>
---
 include/hw/virtio/virtio.h |  1 +
 hw/virtio/vhost.c          |  3 +++
 hw/virtio/virtio.c         | 14 ++++++++++++++
 3 files changed, 18 insertions(+)

diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
index 205fadf2..f52b0b6a 100644
--- a/include/hw/virtio/virtio.h
+++ b/include/hw/virtio/virtio.h
@@ -240,6 +240,7 @@ VirtQueue *virtio_get_queue(VirtIODevice *vdev, int n);
 uint16_t virtio_get_queue_index(VirtQueue *vq);
 int virtio_queue_get_id(VirtQueue *vq);
 EventNotifier *virtio_queue_get_guest_notifier(VirtQueue *vq);
+void virtio_queue_set_masked_guest_notifier(VirtQueue *vq, EventNotifier *n);
 void virtio_queue_set_guest_notifier_fd_handler(VirtQueue *vq, bool assign,
                                                 bool with_irqfd);
 EventNotifier *virtio_queue_get_host_notifier(VirtQueue *vq);
diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
index de29968a..51ce1532 100644
--- a/hw/virtio/vhost.c
+++ b/hw/virtio/vhost.c
@@ -854,6 +854,9 @@ static int vhost_virtqueue_start(struct vhost_dev *dev,
     /* Clear and discard previous events if any. */
     event_notifier_test_and_clear(&vq->masked_notifier);
 
+    /* Set masked guest_notifier. */
+    virtio_queue_set_masked_guest_notifier(vvq, &vq->masked_notifier);
+
     return 0;
 
 fail_kick:
diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index bd6b4df9..d9095c51 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -89,6 +89,7 @@ struct VirtQueue
     VirtIODevice *vdev;
     EventNotifier guest_notifier;
     EventNotifier host_notifier;
+    EventNotifier *masked_guest_notifier;
     QLIST_ENTRY(VirtQueue) node;
 };
 
@@ -1622,6 +1623,14 @@ static void 
virtio_queue_guest_notifier_read(EventNotifier *n)
     if (event_notifier_test_and_clear(n)) {
         virtio_irq(vq);
     }
+    /* It could take some time to the backend to switch to
+     * sending to unmasked evenfd, so we have to test masked
+     * notifier too. */
+    if (vq->masked_guest_notifier) {
+        if (event_notifier_test_and_clear(vq->masked_guest_notifier)) {
+            virtio_irq(vq);
+        }
+    }
 }
 
 void virtio_queue_set_guest_notifier_fd_handler(VirtQueue *vq, bool assign,
@@ -1645,6 +1654,11 @@ EventNotifier *virtio_queue_get_guest_notifier(VirtQueue 
*vq)
     return &vq->guest_notifier;
 }
 
+void virtio_queue_set_masked_guest_notifier(VirtQueue *vq, EventNotifier *n)
+{
+    vq->masked_guest_notifier = n;
+}
+
 static void virtio_queue_host_notifier_read(EventNotifier *n)
 {
     VirtQueue *vq = container_of(n, VirtQueue, host_notifier);
-- 
--Victor