Re: [RFC PATCH v4 11/20] vhost: Route host->guest notification through shadow virtqueue

[Jason Wang]

在 2021/10/1 下午3:05, Eugenio Pérez 写道:
> This will make qemu aware of the device used buffers, allowing it to
> write the guest memory with its contents if needed.
>
> Since the use of vhost_virtqueue_start can unmasks and discard call
> events, vhost_virtqueue_start should be modified in one of these ways:
> * Split in two: One of them uses all logic to start a queue with no
>    side effects for the guest, and another one tha actually assumes that
>    the guest has just started the device. Vdpa should use just the
>    former.
> * Actually store and check if the guest notifier is masked, and do it
>    conditionally.
> * Left as it is, and duplicate all the logic in vhost-vdpa.
>
> Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> ---
>   hw/virtio/vhost-shadow-virtqueue.c | 19 +++++++++++++++
>   hw/virtio/vhost-vdpa.c             | 38 +++++++++++++++++++++++++++++-
>   2 files changed, 56 insertions(+), 1 deletion(-)
>
> diff --git a/hw/virtio/vhost-shadow-virtqueue.c 
> b/hw/virtio/vhost-shadow-virtqueue.c
> index 21dc99ab5d..3fe129cf63 100644
> --- a/hw/virtio/vhost-shadow-virtqueue.c
> +++ b/hw/virtio/vhost-shadow-virtqueue.c
> @@ -53,6 +53,22 @@ static void vhost_handle_guest_kick(EventNotifier *n)
>       event_notifier_set(&svq->kick_notifier);
>   }
>   
> +/* Forward vhost notifications */
> +static void vhost_svq_handle_call_no_test(EventNotifier *n)
> +{
> +    VhostShadowVirtqueue *svq = container_of(n, VhostShadowVirtqueue,
> +                                             call_notifier);
> +
> +    event_notifier_set(&svq->guest_call_notifier);
> +}
> +
> +static void vhost_svq_handle_call(EventNotifier *n)
> +{
> +    if (likely(event_notifier_test_and_clear(n))) {
> +        vhost_svq_handle_call_no_test(n);
> +    }
> +}
> +
>   /*
>    * Obtain the SVQ call notifier, where vhost device notifies SVQ that there
>    * exists pending used buffers.
> @@ -180,6 +196,8 @@ VhostShadowVirtqueue *vhost_svq_new(struct vhost_dev *dev, 
> int idx)
>       }
>   
>       svq->vq = virtio_get_queue(dev->vdev, vq_idx);
> +    event_notifier_set_handler(&svq->call_notifier,
> +                               vhost_svq_handle_call);
>       return g_steal_pointer(&svq);
>   
>   err_init_call_notifier:
> @@ -195,6 +213,7 @@ err_init_kick_notifier:
>   void vhost_svq_free(VhostShadowVirtqueue *vq)
>   {
>       event_notifier_cleanup(&vq->kick_notifier);
> +    event_notifier_set_handler(&vq->call_notifier, NULL);
>       event_notifier_cleanup(&vq->call_notifier);
>       g_free(vq);
>   }
> diff --git a/hw/virtio/vhost-vdpa.c b/hw/virtio/vhost-vdpa.c
> index bc34de2439..6c5f4c98b8 100644
> --- a/hw/virtio/vhost-vdpa.c
> +++ b/hw/virtio/vhost-vdpa.c
> @@ -712,13 +712,40 @@ static bool vhost_vdpa_svq_start_vq(struct vhost_dev 
> *dev, unsigned idx)
>   {
>       struct vhost_vdpa *v = dev->opaque;
>       VhostShadowVirtqueue *svq = g_ptr_array_index(v->shadow_vqs, idx);
> -    return vhost_svq_start(dev, idx, svq);
> +    EventNotifier *vhost_call_notifier = vhost_svq_get_svq_call_notifier(svq);
> +    struct vhost_vring_file vhost_call_file = {
> +        .index = idx + dev->vq_index,
> +        .fd = event_notifier_get_fd(vhost_call_notifier),
> +    };
> +    int r;
> +    bool b;
> +
> +    /* Set shadow vq -> guest notifier */
> +    assert(v->call_fd[idx]);


We need aovid the asser() here. On which case we can hit this?


> +    vhost_svq_set_guest_call_notifier(svq, v->call_fd[idx]);
> +
> +    b = vhost_svq_start(dev, idx, svq);
> +    if (unlikely(!b)) {
> +        return false;
> +    }
> +
> +    /* Set device -> SVQ notifier */
> +    r = vhost_vdpa_set_vring_dev_call(dev, &vhost_call_file);
> +    if (unlikely(r)) {
> +        error_report("vhost_vdpa_set_vring_call for shadow vq failed");
> +        return false;
> +    }


Similar to kick, do we need to set_vring_call() before vhost_svq_start()?


> +
> +    /* Check for pending calls */
> +    event_notifier_set(vhost_call_notifier);


Interesting, can this result spurious interrupt?


> +    return true;
>   }
>   
>   static unsigned vhost_vdpa_enable_svq(struct vhost_vdpa *v, bool enable)
>   {
>       struct vhost_dev *hdev = v->dev;
>       unsigned n;
> +    int r;
>   
>       if (enable == v->shadow_vqs_enabled) {
>           return hdev->nvqs;
> @@ -752,9 +779,18 @@ static unsigned vhost_vdpa_enable_svq(struct vhost_vdpa 
> *v, bool enable)
>       if (!enable) {
>           /* Disable all queues or clean up failed start */
>           for (n = 0; n < v->shadow_vqs->len; ++n) {
> +            struct vhost_vring_file file = {
> +                .index = vhost_vdpa_get_vq_index(hdev, n),
> +                .fd = v->call_fd[n],
> +            };
> +
> +            r = vhost_vdpa_set_vring_call(hdev, &file);
> +            assert(r == 0);
> +
>               unsigned vq_idx = vhost_vdpa_get_vq_index(hdev, n);
>               VhostShadowVirtqueue *svq = g_ptr_array_index(v->shadow_vqs, n);
>               vhost_svq_stop(hdev, n, svq);
> +            /* TODO: This can unmask or override call fd! */


I don't get this comment. Does this mean the current code can't work 
with mask_notifiers? If yes, this is something we need to fix.

Thanks


>               vhost_virtqueue_start(hdev, hdev->vdev, &hdev->vqs[n], vq_idx);
>           }
>