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

[Didier Pallard]

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 <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);

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?

thanks
didier

--
Didier PALLARD
6WIND
Software Engineer

Tel: +33 1 39 30 92 46
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