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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 26 Dec 2022 18:34:39 -0500

On Mon, Dec 26, 2022 at 03:49:07PM +0800, Jason Wang wrote:
> 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 <[email-masked]>
> ---
> Changes since V1:
> - check virtqueue_is_broken() as well
> - use more_used() instead of virtqueue_get_buf() to allow caller to
>   get buffers afterwards
> ---
>  drivers/virtio/virtio_ring.c | 29 +++++++++++++++++++++++++++++
>  include/linux/virtio.h       |  3 +++
>  2 files changed, 32 insertions(+)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 5cfb2fa8abee..9c83eb945493 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
> @@ -60,6 +61,7 @@
>  			"%s:"fmt, (_vq)->vq.name, ##args);	\
>  		/* Pairs with READ_ONCE() in virtqueue_is_broken(). */ \
>  		WRITE_ONCE((_vq)->broken, true);		       \
> +		wake_up_interruptible(&(_vq)->wq);		       \
>  	} while (0)
>  #define START_USE(vq)
>  #define END_USE(vq)
> @@ -203,6 +205,9 @@ 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;
> @@ -2024,6 +2029,8 @@ 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;
> @@ -2517,6 +2524,8 @@ 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);
> @@ -2654,6 +2663,8 @@ 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 +2874,22 @@ const struct vring *virtqueue_get_vring(struct virtqueue *vq)
>  }
>  EXPORT_SYMBOL_GPL(virtqueue_get_vring);
>  
> +int virtqueue_wait_for_used(struct virtqueue *_vq)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +
> +	/* TODO: Tweak the timeout. */
> +	return wait_event_interruptible_timeout(vq->wq,
> +	       virtqueue_is_broken(_vq) || more_used(vq), HZ);

There's no good timeout. Let's not even go there, if device goes
bad it should set the need reset bit.

> +}
> +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..2eb62c774895 100644
> --- a/include/linux/virtio.h
> +++ b/include/linux/virtio.h
> @@ -72,6 +72,9 @@ 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);
> +void virtqueue_wake_up(struct virtqueue *vq);
> +
>  void virtqueue_disable_cb(struct virtqueue *vq);
>  
>  bool virtqueue_enable_cb(struct virtqueue *vq);
> -- 
> 2.25.1