Patchwork [3.5.y.z,extended,stable] Patch "virtio: support unlocked queue poll" has been added to staging queue

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Submitter Luis Henriques
Date Aug. 4, 2013, 9:49 a.m.
Message ID <1375609767-11199-1-git-send-email-luis.henriques@canonical.com>
Download mbox | patch
Permalink /patch/264475/
State New
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Luis Henriques - Aug. 4, 2013, 9:49 a.m.
This is a note to let you know that I have just added a patch titled

    virtio: support unlocked queue poll

to the linux-3.5.y-queue branch of the 3.5.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.5.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.5.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

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From 23531217da4e981f30e418de01f6c7c03a40b69d Mon Sep 17 00:00:00 2001
From: "Michael S. Tsirkin" <mst@redhat.com>
Date: Tue, 9 Jul 2013 13:19:18 +0300
Subject: [PATCH] virtio: support unlocked queue poll

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: Luis Henriques<luis.henriques@canonical.com>
---
 drivers/virtio/virtio_ring.c | 54 +++++++++++++++++++++++++++++++++++---------
 include/linux/virtio.h       |  4 ++++
 2 files changed, 47 insertions(+), 11 deletions(-)

--
1.8.3.2

Patch

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 52bfd07..c212de7 100644
--- 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 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);
-	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 8efd28a..2b779bf 100644
--- a/include/linux/virtio.h
+++ b/include/linux/virtio.h
@@ -44,6 +44,10 @@  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);