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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 26 Dec 2022 18:38:24 -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);

BTW undocumented that you also make it interruptible.
So if we get an interrupt then this will fail.
But device is still going and will later use the buffers.

Same for timeout really.

> +}
> +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