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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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