Subject: Re: [PATCH vhost 01/22] virtio_ring: virtqueue_set_dma_premapped support disable

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 12 Oct 2023 05:15:52 -0400

On Wed, Oct 11, 2023 at 05:27:07PM +0800, Xuan Zhuo wrote:
> virtqueue_set_dma_premapped() adds a new parameter to disable the
> virtqueue premapped mode.
> 
> Signed-off-by: Xuan Zhuo <[email-masked]>
> ---
>  drivers/net/virtio_net.c     |  2 +-
>  drivers/virtio/virtio_ring.c | 11 ++++++++---
>  include/linux/virtio.h       |  2 +-
>  3 files changed, 10 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index fe7f314d65c9..6b5f47ebf9b2 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -737,7 +737,7 @@ static void virtnet_rq_set_premapped(struct virtnet_info *vi)
>  		return;
>  
>  	for (i = 0; i < vi->max_queue_pairs; i++) {
> -		if (virtqueue_set_dma_premapped(vi->rq[i].vq))
> +		if (virtqueue_set_dma_premapped(vi->rq[i].vq, true))
>  			continue;
>  
>  		vi->rq[i].do_dma = true;
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 51d8f3299c10..b3ded56722f4 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -2784,7 +2784,7 @@ EXPORT_SYMBOL_GPL(virtqueue_resize);
>   * 0: success.
>   * -EINVAL: vring does not use the dma api, so we can not enable premapped mode.
>   */
> -int virtqueue_set_dma_premapped(struct virtqueue *_vq)
> +int virtqueue_set_dma_premapped(struct virtqueue *_vq, bool mode)
>  {
>  	struct vring_virtqueue *vq = to_vvq(_vq);
>  	u32 num;
> @@ -2803,8 +2803,13 @@ int virtqueue_set_dma_premapped(struct virtqueue *_vq)
>  		return -EINVAL;
>  	}
>  
> -	vq->premapped = true;
> -	vq->do_unmap = false;
> +	if (mode) {
> +		vq->premapped = true;
> +		vq->do_unmap = false;
> +	} else {
> +		vq->premapped = false;
> +		vq->do_unmap = vq->use_dma_api;
> +	}
>  
>  	END_USE(vq);
>  
> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> index 4cc614a38376..1cf7b004348b 100644
> --- a/include/linux/virtio.h
> +++ b/include/linux/virtio.h
> @@ -81,7 +81,7 @@ bool virtqueue_enable_cb(struct virtqueue *vq);
>  
>  unsigned virtqueue_enable_cb_prepare(struct virtqueue *vq);
>  
> -int virtqueue_set_dma_premapped(struct virtqueue *_vq);
> +int virtqueue_set_dma_premapped(struct virtqueue *_vq, bool mode);
>  
>  bool virtqueue_poll(struct virtqueue *vq, unsigned);

Wait a sec I thought we never change premapped. If you make this
dynamic don't you need a bunch of locking?
Or maybe queue is empty when you change this?
If yes pls add a bunch of BUG_ON checks to make sure this is not misused.

> -- 
> 2.32.0.3.g01195cf9f