Re: [PATCH 13/19] virtio_ring: introduce virtqueue ops

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Mar 24, 2025 at 02:01:21PM +0800, Jason Wang wrote:
> This patch introduces virtqueue ops which is a set of the callbacks
> that will be called for different queue layout or features. This would
> help to avoid branches for split/packed and will ease the future
> implementation like in order.
> 
> Signed-off-by: Jason Wang <jasowang@redhat.com>




> ---
>  drivers/virtio/virtio_ring.c | 96 +++++++++++++++++++++++++-----------
>  1 file changed, 67 insertions(+), 29 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index a2884eae14d9..ce1dc90ee89d 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -159,9 +159,30 @@ struct vring_virtqueue_packed {
>  	size_t event_size_in_bytes;
>  };
>  
> +struct vring_virtqueue;
> +
> +struct virtqueue_ops {
> +	int (*add)(struct vring_virtqueue *_vq, struct scatterlist *sgs[],
> +		   unsigned int total_sg, unsigned int out_sgs,
> +		   unsigned int in_sgs,	void *data,
> +		   void *ctx, bool premapped, gfp_t gfp);
> +	void *(*get)(struct vring_virtqueue *vq, unsigned int *len, void **ctx);
> +	bool (*kick_prepare)(struct vring_virtqueue *vq);
> +	void (*disable_cb)(struct vring_virtqueue *vq);
> +	bool (*enable_cb_delayed)(struct vring_virtqueue *vq);
> +	unsigned int (*enable_cb_prepare)(struct vring_virtqueue *vq);
> +	bool (*poll)(const struct vring_virtqueue *vq, u16 last_used_idx);
> +	void *(*detach_unused_buf)(struct vring_virtqueue *vq);
> +	bool (*more_used)(const struct vring_virtqueue *vq);
> +	int (*resize)(struct vring_virtqueue *vq, u32 num);
> +	void (*reset)(struct vring_virtqueue *vq);
> +};

I like it that it's organized but
I worry about the overhead of indirect calls here.
How about a switch statement instead?

struct vring_virtqueue {
	enum vring_virtqueue_ops ops;

}


@@ -2248,10 +2303,8 @@ static inline int virtqueue_add(struct virtqueue *_vq,
 {
      struct vring_virtqueue *vq = to_vvq(_vq);

	switch (vq->ops) {
	 VQ_PACKED:
	 VQ_SPLIT:
	 VQ_IN_ORDER:
	}


}


What do you think?



> +
>  struct vring_virtqueue {
>  	struct virtqueue vq;
>  
> +	struct virtqueue_ops *ops;
> +
>  	/* Is this a packed ring? */
>  	bool packed_ring;
>  
> @@ -1116,6 +1137,8 @@ static int vring_alloc_queue_split(struct vring_virtqueue_split *vring_split,
>  	return 0;
>  }
>  
> +struct virtqueue_ops split_ops;
> +
>  static struct virtqueue *__vring_new_virtqueue_split(unsigned int index,
>  					       struct vring_virtqueue_split *vring_split,
>  					       struct virtio_device *vdev,
> @@ -1134,6 +1157,7 @@ static struct virtqueue *__vring_new_virtqueue_split(unsigned int index,
>  		return NULL;
>  
>  	vq->packed_ring = false;
> +	vq->ops = &split_ops;
>  	vq->vq.callback = callback;
>  	vq->vq.vdev = vdev;
>  	vq->vq.name = name;
> @@ -2076,6 +2100,8 @@ static void virtqueue_reset_packed(struct vring_virtqueue *vq)
>  	virtqueue_vring_init_packed(&vq->packed, !!vq->vq.callback);
>  }
>  
> +struct virtqueue_ops packed_ops;
> +
>  static struct virtqueue *__vring_new_virtqueue_packed(unsigned int index,
>  					       struct vring_virtqueue_packed *vring_packed,
>  					       struct virtio_device *vdev,
> @@ -2107,6 +2133,7 @@ static struct virtqueue *__vring_new_virtqueue_packed(unsigned int index,
>  	vq->broken = false;
>  #endif
>  	vq->packed_ring = true;
> +	vq->ops = &packed_ops;
>  	vq->dma_dev = dma_dev;
>  	vq->use_dma_api = vring_use_dma_api(vdev);
>  
> @@ -2194,6 +2221,34 @@ static int virtqueue_resize_packed(struct vring_virtqueue *vq, u32 num)
>  	return -ENOMEM;
>  }
>  
> +struct virtqueue_ops split_ops = {
> +	.add = virtqueue_add_split,
> +	.get = virtqueue_get_buf_ctx_split,
> +	.kick_prepare = virtqueue_kick_prepare_split,
> +	.disable_cb = virtqueue_disable_cb_split,
> +	.enable_cb_delayed = virtqueue_enable_cb_delayed_split,
> +	.enable_cb_prepare = virtqueue_enable_cb_prepare_split,
> +	.poll = virtqueue_poll_split,
> +	.detach_unused_buf = virtqueue_detach_unused_buf_split,
> +	.more_used = more_used_split,
> +	.resize = virtqueue_resize_split,
> +	.reset = virtqueue_reset_split,
> +};
> +
> +struct virtqueue_ops packed_ops = {
> +	.add = virtqueue_add_packed,
> +	.get = virtqueue_get_buf_ctx_packed,
> +	.kick_prepare = virtqueue_kick_prepare_packed,
> +	.disable_cb = virtqueue_disable_cb_packed,
> +	.enable_cb_delayed = virtqueue_enable_cb_delayed_packed,
> +	.enable_cb_prepare = virtqueue_enable_cb_prepare_packed,
> +	.poll = virtqueue_poll_packed,
> +	.detach_unused_buf = virtqueue_detach_unused_buf_packed,
> +	.more_used = more_used_packed,
> +	.resize = virtqueue_resize_packed,
> +	.reset = virtqueue_reset_packed,
> +};
> +
>  static int virtqueue_disable_and_recycle(struct virtqueue *_vq,
>  					 void (*recycle)(struct virtqueue *vq, void *buf))
>  {
> @@ -2248,10 +2303,8 @@ static inline int virtqueue_add(struct virtqueue *_vq,
>  {
>  	struct vring_virtqueue *vq = to_vvq(_vq);
>  
> -	return vq->packed_ring ? virtqueue_add_packed(vq, sgs, total_sg,
> -					out_sgs, in_sgs, data, ctx, premapped, gfp) :
> -				 virtqueue_add_split(vq, sgs, total_sg,
> -					out_sgs, in_sgs, data, ctx, premapped, gfp);
> +	return vq->ops->add(vq, sgs, total_sg,
> +			    out_sgs, in_sgs, data, ctx, premapped, gfp);
>  }
>  
>  /**
> @@ -2437,8 +2490,7 @@ bool virtqueue_kick_prepare(struct virtqueue *_vq)
>  {
>  	struct vring_virtqueue *vq = to_vvq(_vq);
>  
> -	return vq->packed_ring ? virtqueue_kick_prepare_packed(vq) :
> -				 virtqueue_kick_prepare_split(vq);
> +	return vq->ops->kick_prepare(vq);
>  }
>  EXPORT_SYMBOL_GPL(virtqueue_kick_prepare);
>  
> @@ -2508,8 +2560,7 @@ void *virtqueue_get_buf_ctx(struct virtqueue *_vq, unsigned int *len,
>  {
>  	struct vring_virtqueue *vq = to_vvq(_vq);
>  
> -	return vq->packed_ring ? virtqueue_get_buf_ctx_packed(vq, len, ctx) :
> -				 virtqueue_get_buf_ctx_split(vq, len, ctx);
> +	return vq->ops->get(vq, len, ctx);
>  }
>  EXPORT_SYMBOL_GPL(virtqueue_get_buf_ctx);
>  
> @@ -2531,10 +2582,7 @@ void virtqueue_disable_cb(struct virtqueue *_vq)
>  {
>  	struct vring_virtqueue *vq = to_vvq(_vq);
>  
> -	if (vq->packed_ring)
> -		virtqueue_disable_cb_packed(vq);
> -	else
> -		virtqueue_disable_cb_split(vq);
> +	return vq->ops->disable_cb(vq);
>  }
>  EXPORT_SYMBOL_GPL(virtqueue_disable_cb);
>  
> @@ -2557,8 +2605,7 @@ unsigned int virtqueue_enable_cb_prepare(struct virtqueue *_vq)
>  	if (vq->event_triggered)
>  		vq->event_triggered = false;
>  
> -	return vq->packed_ring ? virtqueue_enable_cb_prepare_packed(vq) :
> -				 virtqueue_enable_cb_prepare_split(vq);
> +	return vq->ops->enable_cb_prepare(vq);
>  }
>  EXPORT_SYMBOL_GPL(virtqueue_enable_cb_prepare);
>  
> @@ -2579,8 +2626,7 @@ bool virtqueue_poll(struct virtqueue *_vq, unsigned int last_used_idx)
>  		return false;
>  
>  	virtio_mb(vq->weak_barriers);
> -	return vq->packed_ring ? virtqueue_poll_packed(vq, last_used_idx) :
> -				 virtqueue_poll_split(vq, last_used_idx);
> +	return vq->ops->poll(vq, last_used_idx);
>  }
>  EXPORT_SYMBOL_GPL(virtqueue_poll);
>  
> @@ -2623,8 +2669,7 @@ bool virtqueue_enable_cb_delayed(struct virtqueue *_vq)
>  	if (vq->event_triggered)
>  		vq->event_triggered = false;
>  
> -	return vq->packed_ring ? virtqueue_enable_cb_delayed_packed(vq) :
> -				 virtqueue_enable_cb_delayed_split(vq);
> +	return vq->ops->enable_cb_delayed(vq);
>  }
>  EXPORT_SYMBOL_GPL(virtqueue_enable_cb_delayed);
>  
> @@ -2640,14 +2685,13 @@ void *virtqueue_detach_unused_buf(struct virtqueue *_vq)
>  {
>  	struct vring_virtqueue *vq = to_vvq(_vq);
>  
> -	return vq->packed_ring ? virtqueue_detach_unused_buf_packed(vq) :
> -				 virtqueue_detach_unused_buf_split(vq);
> +	return vq->ops->detach_unused_buf(vq);
>  }
>  EXPORT_SYMBOL_GPL(virtqueue_detach_unused_buf);
>  
>  static inline bool more_used(const struct vring_virtqueue *vq)
>  {
> -	return vq->packed_ring ? more_used_packed(vq) : more_used_split(vq);
> +	return vq->ops->more_used(vq);
>  }
>  
>  /**
> @@ -2785,10 +2829,7 @@ int virtqueue_resize(struct virtqueue *_vq, u32 num,
>  	if (recycle_done)
>  		recycle_done(_vq);
>  
> -	if (vq->packed_ring)
> -		err = virtqueue_resize_packed(vq, num);
> -	else
> -		err = virtqueue_resize_split(vq, num);
> +	err = vq->ops->resize(vq, num);
>  
>  	return virtqueue_enable_after_reset(_vq);
>  }
> @@ -2822,10 +2863,7 @@ int virtqueue_reset(struct virtqueue *_vq,
>  	if (recycle_done)
>  		recycle_done(_vq);
>  
> -	if (vq->packed_ring)
> -		virtqueue_reset_packed(vq);
> -	else
> -		virtqueue_reset_split(vq);
> +	vq->ops->reset(vq);
>  
>  	return virtqueue_enable_after_reset(_vq);
>  }
> -- 
> 2.42.0