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List:       git-commits-head
Subject:    virtio: add api for delayed callbacks
From:       Linux Kernel Mailing List <linux-kernel () vger ! kernel ! org>
Date:       2011-05-31 21:59:10
Message-ID: 201105312159.p4VLxAxF013187 () hera ! kernel ! org
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Gitweb:     http://git.kernel.org/linus/7ab358c23cbf15cea08129cd722d1ce77433a94d
Commit:     7ab358c23cbf15cea08129cd722d1ce77433a94d
Parent:     4423fe40b03f32b11e72ecfa03077e702e55d5a9
Author:     Michael S. Tsirkin <mst@redhat.com>
AuthorDate: Fri May 20 02:11:14 2011 +0300
Committer:  Rusty Russell <rusty@rustcorp.com.au>
CommitDate: Mon May 30 11:14:16 2011 +0930

    virtio: add api for delayed callbacks
    
    Add an API that tells the other side that callbacks
    should be delayed until a lot of work has been done.
    Implement using the new event_idx feature.
    
    Note: it might seem advantageous to let the drivers
    ask for a callback after a specific capacity has
    been reached. However, as a single head can
    free many entries in the descriptor table,
    we don't really have a clue about capacity
    until get_buf is called. The API is the simplest
    to implement at the moment, we'll see what kind of
    hints drivers can pass when there's more than one
    user of the feature.
    
    Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
    Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
---
 drivers/virtio/virtio_ring.c |   27 +++++++++++++++++++++++++++
 include/linux/virtio.h       |    9 +++++++++
 2 files changed, 36 insertions(+), 0 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index a5fadc4..68b9136 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -376,6 +376,33 @@ bool virtqueue_enable_cb(struct virtqueue *_vq)
 }
 EXPORT_SYMBOL_GPL(virtqueue_enable_cb);
 
+bool virtqueue_enable_cb_delayed(struct virtqueue *_vq)
+{
+	struct vring_virtqueue *vq = to_vvq(_vq);
+	u16 bufs;
+
+	START_USE(vq);
+
+	/* We optimistically turn back on interrupts, then check if there was
+	 * more to do. */
+	/* Depending on the VIRTIO_RING_F_USED_EVENT_IDX feature, we need to
+	 * 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;
+	/* TODO: tune this threshold */
+	bufs = (u16)(vq->vring.avail->idx - vq->last_used_idx) * 3 / 4;
+	vring_used_event(&vq->vring) = vq->last_used_idx + bufs;
+	virtio_mb();
+	if (unlikely((u16)(vq->vring.used->idx - vq->last_used_idx) > bufs)) {
+		END_USE(vq);
+		return false;
+	}
+
+	END_USE(vq);
+	return true;
+}
+EXPORT_SYMBOL_GPL(virtqueue_enable_cb_delayed);
+
 void *virtqueue_detach_unused_buf(struct virtqueue *_vq)
 {
 	struct vring_virtqueue *vq = to_vvq(_vq);
diff --git a/include/linux/virtio.h b/include/linux/virtio.h
index aff5b4f..7108857 100644
--- a/include/linux/virtio.h
+++ b/include/linux/virtio.h
@@ -51,6 +51,13 @@ struct virtqueue {
  *	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.
+ * virtqueue_enable_cb_delayed: restart callbacks after disable_cb.
+ *	vq: the struct virtqueue we're talking about.
+ *	This re-enables callbacks but hints to the other side to delay
+ *	interrupts until most of the available buffers have been processed;
+ *	it returns "false" if there are many pending buffers in the queue,
+ *	to detect a possible race between the driver checking for more work,
+ *	and enabling callbacks.
  * virtqueue_detach_unused_buf: detach first unused buffer
  * 	vq: the struct virtqueue we're talking about.
  * 	Returns NULL or the "data" token handed to add_buf
@@ -86,6 +93,8 @@ void virtqueue_disable_cb(struct virtqueue *vq);
 
 bool virtqueue_enable_cb(struct virtqueue *vq);
 
+bool virtqueue_enable_cb_delayed(struct virtqueue *vq);
+
 void *virtqueue_detach_unused_buf(struct virtqueue *vq);
 
 /**
--
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