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Subject[ 37/39] virtio: support unlocked queue poll
Date
3.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Michael S. Tsirkin <mst@redhat.com>

commit cc229884d3f77ec3b1240e467e0236c3e0647c0c upstream.

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@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[wg: Backported to 3.2]
Signed-off-by: Wolfram Gloger <wmglo@dent.med.uni-muenchen.de>
[bwh: Backported to 3.4: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
---
drivers/virtio/virtio_ring.c | 54 ++++++++++++++++++++++++++++++++++---------
include/linux/virtio.h | 4 +++
2 files changed, 47 insertions(+), 11 deletions(-)

--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -498,16 +498,18 @@ EXPORT_SYMBOL_GPL(virtqueue_disable_cb);
* 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.
+ * 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);

@@ -517,15 +519,45 @@ bool virtqueue_enable_cb(struct virtqueu
* 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);
- 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);

--- a/include/linux/virtio.h
+++ b/include/linux/virtio.h
@@ -44,6 +44,10 @@ void virtqueue_disable_cb(struct virtque

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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