On 07/08/2013 05:04 PM, Michael S. Tsirkin wrote:
> This adds a way to check ring empty state after enable_cb outside any
> locks. Will be used by virtio_net.
>
> Note: there's room for more optimization: caller is likely to have a
> memory barrier already, which means we might be able to get rid of a
> barrier here. Deferring this optimization until we do some
> benchmarking.
>
> Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> ---
Tested-by: Jason Wang <jasowang@xxxxxxxxxx>
Acked-by: Jason Wang <jasowang@xxxxxxxxxx>
> drivers/virtio/virtio_ring.c | 56 ++++++++++++++++++++++++++++++++++----------
> include/linux/virtio.h | 4 ++++
> 2 files changed, 48 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 5217baf..37d58f8 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -607,19 +607,21 @@ void virtqueue_disable_cb(struct virtqueue *_vq)
> EXPORT_SYMBOL_GPL(virtqueue_disable_cb);
>
> /**
> - * virtqueue_enable_cb - restart callbacks after disable_cb.
> + * virtqueue_enable_cb_prepare - restart callbacks after disable_cb
> * @vq: the struct virtqueue we're talking about.
> *
> - * This re-enables callbacks; it returns "false" if there are pending
> - * buffers in the queue, to detect a possible race between the driver
> - * checking for more work, and enabling callbacks.
> + * This re-enables callbacks; it returns current queue state
> + * in an opaque unsigned value. This value should be later tested by
> + * virtqueue_poll, to detect a possible race between the driver checking for
> + * more work, and enabling callbacks.
> *
> * Caller must ensure we don't call this with other virtqueue
> * operations at the same time (except where noted).
> */
> -bool virtqueue_enable_cb(struct virtqueue *_vq)
> +unsigned virtqueue_enable_cb_prepare(struct virtqueue *_vq)
> {
> struct vring_virtqueue *vq = to_vvq(_vq);
> + u16 last_used_idx;
>
> START_USE(vq);
>
> @@ -629,15 +631,45 @@ bool virtqueue_enable_cb(struct virtqueue *_vq)
> * either clear the flags bit or point the event index at the next
> * entry. Always do both to keep code simple. */
> vq->vring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT;
> - vring_used_event(&vq->vring) = vq->last_used_idx;
> + vring_used_event(&vq->vring) = last_used_idx = vq->last_used_idx;
> + END_USE(vq);
> + return last_used_idx;
> +}
> +EXPORT_SYMBOL_GPL(virtqueue_enable_cb_prepare);
> +
> +/**
> + * virtqueue_poll - query pending used buffers
> + * @vq: the struct virtqueue we're talking about.
> + * @last_used_idx: virtqueue state (from call to virtqueue_enable_cb_prepare).
> + *
> + * Returns "true" if there are pending used buffers in the queue.
> + *
> + * This does not need to be serialized.
> + */
> +bool virtqueue_poll(struct virtqueue *_vq, unsigned last_used_idx)
> +{
> + struct vring_virtqueue *vq = to_vvq(_vq);
> +
> virtio_mb(vq->weak_barriers);
> - if (unlikely(more_used(vq))) {
> - END_USE(vq);
> - return false;
> - }
> + return (u16)last_used_idx != vq->vring.used->idx;
> +}
> +EXPORT_SYMBOL_GPL(virtqueue_poll);
>
> - END_USE(vq);
> - return true;
> +/**
> + * virtqueue_enable_cb - restart callbacks after disable_cb.
> + * @vq: the struct virtqueue we're talking about.
> + *
> + * This re-enables callbacks; it returns "false" if there are pending
> + * buffers in the queue, to detect a possible race between the driver
> + * checking for more work, and enabling callbacks.
> + *
> + * Caller must ensure we don't call this with other virtqueue
> + * operations at the same time (except where noted).
> + */
> +bool virtqueue_enable_cb(struct virtqueue *_vq)
> +{
> + unsigned last_used_idx = virtqueue_enable_cb_prepare(_vq);
> + return !virtqueue_poll(_vq, last_used_idx);
> }
> EXPORT_SYMBOL_GPL(virtqueue_enable_cb);
>
> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> index 9ff8645..72398ee 100644
> --- a/include/linux/virtio.h
> +++ b/include/linux/virtio.h
> @@ -70,6 +70,10 @@ void virtqueue_disable_cb(struct virtqueue *vq);
>
> bool virtqueue_enable_cb(struct virtqueue *vq);
>
> +unsigned virtqueue_enable_cb_prepare(struct virtqueue *vq);
> +
> +bool virtqueue_poll(struct virtqueue *vq, unsigned);
> +
> bool virtqueue_enable_cb_delayed(struct virtqueue *vq);
>
> void *virtqueue_detach_unused_buf(struct virtqueue *vq);
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