Re: [PATCH v8 16/16] virtio_ring: introduce virtqueue_resize()

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

On Mon, Mar 14, 2022 at 05:34:55PM +0800, Xuan Zhuo wrote:
> Introduce virtqueue_resize() to implement the resize of vring.
> Based on these, the driver can dynamically adjust the size of the vring.
> For example: ethtool -G.
> 
> virtqueue_resize() implements resize based on the vq reset function. In
> case of failure to allocate a new vring, it will give up resize and use
> the original vring.
> 
> During this process, if the re-enable reset vq fails, the vq can no
> longer be used. Although the probability of this situation is not high.
> 
> The parameter recycle is used to recycle the buffer that is no longer
> used.
> 
> Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> ---
>  drivers/virtio/virtio_ring.c | 67 ++++++++++++++++++++++++++++++++++++
>  include/linux/virtio.h       |  3 ++
>  2 files changed, 70 insertions(+)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index fb0abf9a2f57..b1dde086a8a4 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -2528,6 +2528,73 @@ struct virtqueue *vring_create_virtqueue(
>  }
>  EXPORT_SYMBOL_GPL(vring_create_virtqueue);
>  
> +/**
> + * virtqueue_resize - resize the vring of vq
> + * @vq: the struct virtqueue we're talking about.
> + * @num: new ring num
> + * @recycle: callback for recycle the useless buffer
> + *
> + * When it is really necessary to create a new vring, it will set the current vq
> + * into the reset state. Then call the passed cb to recycle the buffer that is
> + * no longer used. Only after the new vring is successfully created, the old
> + * vring will be released.
> + *
> + * Caller must ensure we don't call this with other virtqueue operations
> + * at the same time (except where noted).
> + *
> + * Returns zero or a negative error.
> + * -ENOMEM: create new vring fail. But vq can still work
> + * -EBUSY:  reset/re-enable vq fail. vq may cannot work
> + * -ENOENT: not support resize
> + * -E2BIG/-EINVAL: param num error
> + */
> +int virtqueue_resize(struct virtqueue *vq, u32 num,
> +		     void (*recycle)(struct virtqueue *vq, void *buf))
> +{
> +	struct virtio_device *vdev = vq->vdev;
> +	void *buf;
> +	int err;
> +
> +	if (num > vq->num_max)
> +		return -E2BIG;
> +
> +	if (!num)
> +		return -EINVAL;
> +
> +	if (to_vvq(vq)->packed.vring.num == num)
> +		return 0;
> +
> +	if (!vq->vdev->config->reset_vq)
> +		return -ENOENT;
> +
> +	if (!vq->vdev->config->enable_reset_vq)
> +		return -ENOENT;
> +
> +	err = vq->vdev->config->reset_vq(vq);
> +	if (err) {
> +		if (err != -ENOENT)
> +			err = -EBUSY;
> +		return err;
> +	}
> +
> +	while ((buf = virtqueue_detach_unused_buf(vq)) != NULL)
> +		recycle(vq, buf);


So all this callback can do now is drop all buffers, and I think that is
not great.  Can we store them and invoke the callback after queue is
enabled?


> +
> +	if (virtio_has_feature(vdev, VIRTIO_F_RING_PACKED))
> +		err = virtqueue_resize_packed(vq, num);
> +	else
> +		err = virtqueue_resize_split(vq, num);
> +
> +	if (err)
> +		err = -ENOMEM;
> +
> +	if (vq->vdev->config->enable_reset_vq(vq))
> +		return -EBUSY;
> +
> +	return err;
> +}
> +EXPORT_SYMBOL_GPL(virtqueue_resize);
> +
>  /* Only available for split ring */
>  struct virtqueue *vring_new_virtqueue(unsigned int index,
>  				      unsigned int num,
> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> index d59adc4be068..c86ff02e0ca0 100644
> --- a/include/linux/virtio.h
> +++ b/include/linux/virtio.h
> @@ -91,6 +91,9 @@ dma_addr_t virtqueue_get_desc_addr(struct virtqueue *vq);
>  dma_addr_t virtqueue_get_avail_addr(struct virtqueue *vq);
>  dma_addr_t virtqueue_get_used_addr(struct virtqueue *vq);
>  
> +int virtqueue_resize(struct virtqueue *vq, u32 num,
> +		     void (*recycle)(struct virtqueue *vq, void *buf));
> +
>  /**
>   * virtio_device - representation of a device using virtio
>   * @index: unique position on the virtio bus
> -- 
> 2.31.0

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