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

[Victor Kaplansky <victork@redhat.com>]

On Tue, Jan 12, 2016 at 01:05:52PM +0100, Didier Pallard wrote:
> On 01/12/2016 09:26 AM, Victor Kaplansky wrote:
> >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 <victork@redhat.com>
> >---
> >  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);
> 
> Hi viktor,
> 
> i'm wondering how this patch works.
> virtio_queue_guest_notifier_read is only used in
> virtio_queue_set_guest_notifier_fd_handler.
> and it is only used if with_irq is not set:
>     if (assign && !with_irqfd) {
>         event_notifier_set_handler(&vq->guest_notifier,
> virtio_queue_guest_notifier_read);
>     } else {
>         event_notifier_set_handler(&vq->guest_notifier, NULL);
>     }
> else null handler is set in guest_notifier.
> And from my understanding, virtio-pci in kvm mode uses irqfd, so when are
> we entering the virtio_queue_guest_notifier_read?
> did you also change the qemu configuration?
> 

Hmm, right. So, probably it would be better to take your version
of the fix as a temporarily solution which just disables the
ability to mask interrupts when virtio-net-pci is backed by
vhost-user.

> thanks
> didier
> 
> -- 
> Didier PALLARD
> 6WIND
> Software Engineer
> 
> Tel: +33 1 39 30 92 46
> Mob: +33 6 49 11 40 14
> Fax: +33 1 39 30 92 11
> didier.pallard@6wind.com
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> 
>