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authorMichael S. Tsirkin <mst@redhat.com>2013-07-09 13:19:18 +0300
committerDavid S. Miller <davem@davemloft.net>2013-07-09 12:45:37 -0700
commitcc229884d3f77ec3b1240e467e0236c3e0647c0c (patch)
treea2617adb191288d049629f911b7fef7038557a8e /drivers/virtio
parent01276ed2424eb78c95461545410923d5da154d31 (diff)
downloadlinux-cc229884d3f77ec3b1240e467e0236c3e0647c0c.tar.bz2
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@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'drivers/virtio')
-rw-r--r--drivers/virtio/virtio_ring.c56
1 files changed, 44 insertions, 12 deletions
diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 5217baf5528c..37d58f84dc50 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);