[RFC PATCH 3/4] virtio_ring: introduce a per virtqueue waitqueue

[Jason Wang]

This patch introduces a per virtqueue waitqueue to allow driver to
sleep and wait for more used. Two new helpers are introduced to allow
driver to sleep and wake up.

Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
---
 drivers/virtio/virtio_ring.c | 31 +++++++++++++++++++++++++++++++
 include/linux/virtio.h       |  4 ++++
 2 files changed, 35 insertions(+)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 90c2034a77f3..4a2d5ac30b0f 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -13,6 +13,7 @@
 #include <linux/dma-mapping.h>
 #include <linux/kmsan.h>
 #include <linux/spinlock.h>
+#include <linux/wait.h>
 #include <xen/xen.h>
 
 #ifdef DEBUG
@@ -59,6 +60,7 @@
 		dev_err(&_vq->vq.vdev->dev,			\
 			"%s:"fmt, (_vq)->vq.name, ##args);	\
 		(_vq)->broken = true;				\
+		wake_up_interruptible(&(_vq)->wq);		\
 	} while (0)
 #define START_USE(vq)
 #define END_USE(vq)
@@ -202,6 +204,9 @@ struct vring_virtqueue {
 	/* DMA, allocation, and size information */
 	bool we_own_ring;
 
+	/* Wait for buffer to be used */
+	wait_queue_head_t wq;
+
 #ifdef DEBUG
 	/* They're supposed to lock for us. */
 	unsigned int in_use;
@@ -2023,6 +2028,8 @@ static struct virtqueue *vring_create_virtqueue_packed(
 	if (virtio_has_feature(vdev, VIRTIO_F_ORDER_PLATFORM))
 		vq->weak_barriers = false;
 
+	init_waitqueue_head(&vq->wq);
+
 	err = vring_alloc_state_extra_packed(&vring_packed);
 	if (err)
 		goto err_state_extra;
@@ -2516,6 +2523,8 @@ static struct virtqueue *__vring_new_virtqueue(unsigned int index,
 	if (virtio_has_feature(vdev, VIRTIO_F_ORDER_PLATFORM))
 		vq->weak_barriers = false;
 
+	init_waitqueue_head(&vq->wq);
+
 	err = vring_alloc_state_extra_split(vring_split);
 	if (err) {
 		kfree(vq);
@@ -2653,6 +2662,8 @@ static void vring_free(struct virtqueue *_vq)
 {
 	struct vring_virtqueue *vq = to_vvq(_vq);
 
+	wake_up_interruptible(&vq->wq);
+
 	if (vq->we_own_ring) {
 		if (vq->packed_ring) {
 			vring_free_queue(vq->vq.vdev,
@@ -2863,4 +2874,24 @@ const struct vring *virtqueue_get_vring(struct virtqueue *vq)
 }
 EXPORT_SYMBOL_GPL(virtqueue_get_vring);
 
+int virtqueue_wait_for_used(struct virtqueue *_vq,
+			    unsigned int *len)
+{
+	struct vring_virtqueue *vq = to_vvq(_vq);
+
+	/* Use a better timeout or simply start from no timeout */
+	return wait_event_interruptible_timeout(vq->wq,
+						virtqueue_get_buf(_vq, len),
+						HZ);
+}
+EXPORT_SYMBOL_GPL(virtqueue_wait_for_used);
+
+void virtqueue_wake_up(struct virtqueue *_vq)
+{
+	struct vring_virtqueue *vq = to_vvq(_vq);
+
+	wake_up_interruptible(&vq->wq);
+}
+EXPORT_SYMBOL_GPL(virtqueue_wake_up);
+
 MODULE_LICENSE("GPL");
diff --git a/include/linux/virtio.h b/include/linux/virtio.h
index dcab9c7e8784..4df098b8f1ca 100644
--- a/include/linux/virtio.h
+++ b/include/linux/virtio.h
@@ -72,6 +72,10 @@ void *virtqueue_get_buf(struct virtqueue *vq, unsigned int *len);
 void *virtqueue_get_buf_ctx(struct virtqueue *vq, unsigned int *len,
 			    void **ctx);
 
+int virtqueue_wait_for_used(struct virtqueue *vq,
+			    unsigned int *len);
+void virtqueue_wake_up(struct virtqueue *vq);
+
 void virtqueue_disable_cb(struct virtqueue *vq);
 
 bool virtqueue_enable_cb(struct virtqueue *vq);
-- 
2.25.1