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    SubjectRe: [PATCH 09/18] virtio: use avail_event index
    Date
    On Wed, 4 May 2011 23:51:47 +0300, "Michael S. Tsirkin" <mst@redhat.com> wrote:
    > Use the new avail_event feature to reduce the number
    > of exits from the guest.

    Figures here would be nice :)

    > @@ -228,6 +237,12 @@ add_head:
    > * new available array entries. */
    > virtio_wmb();
    > vq->vring.avail->idx++;
    > + /* If the driver never bothers to kick in a very long while,
    > + * avail index might wrap around. If that happens, invalidate
    > + * kicked_avail index we stored. TODO: make sure all drivers
    > + * kick at least once in 2^16 and remove this. */
    > + if (unlikely(vq->vring.avail->idx == vq->kicked_avail))
    > + vq->kicked_avail_valid = true;

    If they don't, they're already buggy. Simply do:
    WARN_ON(vq->vring.avail->idx == vq->kicked_avail);

    > +static bool vring_notify(struct vring_virtqueue *vq)
    > +{
    > + u16 old, new;
    > + bool v;
    > + if (!vq->event)
    > + return !(vq->vring.used->flags & VRING_USED_F_NO_NOTIFY);
    > +
    > + v = vq->kicked_avail_valid;
    > + old = vq->kicked_avail;
    > + new = vq->kicked_avail = vq->vring.avail->idx;
    > + vq->kicked_avail_valid = true;
    > + if (unlikely(!v))
    > + return true;

    This is the only place you actually used kicked_avail_valid. Is it
    possible to initialize it in such a way that you can remove this?

    > @@ -482,6 +517,8 @@ void vring_transport_features(struct virtio_device *vdev)
    > break;
    > case VIRTIO_RING_F_USED_EVENT_IDX:
    > break;
    > + case VIRTIO_RING_F_AVAIL_EVENT_IDX:
    > + break;
    > default:
    > /* We don't understand this bit. */
    > clear_bit(i, vdev->features);

    Does this belong in a prior patch?

    Thanks,
    Rusty.


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