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Michael S. Tsirkin

2013-Jul-08 09:04 UTC

[PATCH 1/2] virtio: support unlocked queue poll

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 at redhat.com>
---
 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);
-- 
MST

Jason Wang

2013-Jul-09 03:26 UTC

head link

[PATCH 1/2] virtio: support unlocked queue poll

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 at redhat.com>
> ---
Tested-by: Jason Wang <jasowang at redhat.com>
Acked-by: Jason Wang <jasowang at redhat.com>>  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);

Rusty Russell

2013-Jul-10 03:09 UTC

head link

[PATCH 1/2] virtio: support unlocked queue poll

"Michael S. Tsirkin" <mst at redhat.com>
writes:> 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 at redhat.com>
Acked-by: Rusty Russell <rusty at rustcorp.com.au>

Thanks,
Rusty.

Asias He

2013-Jul-10 04:38 UTC

head link

[PATCH 1/2] virtio: support unlocked queue poll

On Mon, Jul 08, 2013 at 12:04:36PM +0300, 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 at redhat.com>
Acked-by: Asias He <asias at redhat.com>
> ---
>  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);
> -- 
> MST
> 
> _______________________________________________
> Virtualization mailing list
> Virtualization at lists.linux-foundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/virtualization
-- 
Asias

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