Subject: Re: [PATCH 2/4] [RFC] virtio: Introduce new API to get free space

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 4 May 2011 17:50:19 +0300

On Wed, May 04, 2011 at 07:33:19PM +0530, Krishna Kumar wrote:
> Introduce virtqueue_get_capacity() to help bail out of transmit
> path early. Also remove notification when we run out of space (I
> am not sure if this should be under a feature bit).
> 
> Signed-off-by: Krishna Kumar <[email-masked]>
> ---
>  drivers/virtio/virtio_ring.c |   13 ++++++++-----
>  include/linux/virtio.h       |    5 +++++
>  2 files changed, 13 insertions(+), 5 deletions(-)
> 
> diff -ruNp org/include/linux/virtio.h new/include/linux/virtio.h
> --- org/include/linux/virtio.h	2011-05-04 18:57:06.000000000 +0530
> +++ new/include/linux/virtio.h	2011-05-04 18:57:09.000000000 +0530
> @@ -27,6 +27,9 @@ struct virtqueue {
>  
>  /**
>   * operations for virtqueue
> + * virtqueue_get_capacity: Get vq capacity
> + *	vq: the struct virtqueue we're talking about.
> + *	Returns remaining capacity of queue
>   * virtqueue_add_buf: expose buffer to other end
>   *	vq: the struct virtqueue we're talking about.
>   *	sg: the description of the buffer(s).
> @@ -62,6 +65,8 @@ struct virtqueue {
>   * All operations can be called in any context.
>   */
>  
> +int virtqueue_get_capacity(struct virtqueue *vq);
> +
>  int virtqueue_add_buf_gfp(struct virtqueue *vq,
>  			  struct scatterlist sg[],
>  			  unsigned int out_num,

This is same as Shirley sent?
Maybe split and attribute ...

> diff -ruNp org/drivers/virtio/virtio_ring.c new/drivers/virtio/virtio_ring.c
> --- org/drivers/virtio/virtio_ring.c	2011-05-04 18:57:06.000000000 +0530
> +++ new/drivers/virtio/virtio_ring.c	2011-05-04 18:57:09.000000000 +0530
> @@ -156,6 +156,14 @@ static int vring_add_indirect(struct vri
>  	return head;
>  }
>  
> +int virtqueue_get_capacity(struct virtqueue *_vq)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +
> +	return vq->num_free;
> +}
> +EXPORT_SYMBOL_GPL(virtqueue_get_capacity);
> +
>  int virtqueue_add_buf_gfp(struct virtqueue *_vq,
>  			  struct scatterlist sg[],
>  			  unsigned int out,
> @@ -185,11 +193,6 @@ int virtqueue_add_buf_gfp(struct virtque
>  	if (vq->num_free < out + in) {
>  		pr_debug("Can't add buf len %i - avail = %i\n",
>  			 out + in, vq->num_free);
> -		/* FIXME: for historical reasons, we force a notify here if
> -		 * there are outgoing parts to the buffer.  Presumably the
> -		 * host should service the ring ASAP. */
> -		if (out)
> -			vq->notify(&vq->vq);
>  		END_USE(vq);
>  		return -ENOSPC;
>  	}

This will break qemu versions 0.13 and back.
I'm adding some new virtio ring flags, we'll be
able to reuse one of these to mean 'no need for
work around', I think.

-- 
MST