netdev - Re: [PATCH vhost v13 05/12] virtio_ring: introduce virtqueue_dma

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Date: Mon, 14 Aug 2023 11:05:49 +0800
From: Jason Wang <jasowang@...hat.com>
To: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc: virtualization@...ts.linux-foundation.org, 
	"Michael S. Tsirkin" <mst@...hat.com>, "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Jesper Dangaard Brouer <hawk@...nel.org>, 
	John Fastabend <john.fastabend@...il.com>, netdev@...r.kernel.org, bpf@...r.kernel.org, 
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH vhost v13 05/12] virtio_ring: introduce virtqueue_dma_dev()

On Thu, Aug 10, 2023 at 8:31 PM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
>
> Added virtqueue_dma_dev() to get DMA device for virtio. Then the
> caller can do dma operation in advance. The purpose is to keep memory
> mapped across multiple add/get buf operations.
>
> Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> Acked-by: Jason Wang <jasowang@...hat.com>

So I think we don't have actual users for this in this series? Can we
simply have another independent patch for this?

> ---
>  drivers/virtio/virtio_ring.c | 17 +++++++++++++++++
>  include/linux/virtio.h       |  2 ++
>  2 files changed, 19 insertions(+)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index f9f772e85a38..bb3d73d221cd 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -2265,6 +2265,23 @@ int virtqueue_add_inbuf_ctx(struct virtqueue *vq,
>  }
>  EXPORT_SYMBOL_GPL(virtqueue_add_inbuf_ctx);
>
> +/**
> + * virtqueue_dma_dev - get the dma dev
> + * @_vq: the struct virtqueue we're talking about.
> + *
> + * Returns the dma dev. That can been used for dma api.
> + */
> +struct device *virtqueue_dma_dev(struct virtqueue *_vq)
> +{
> +       struct vring_virtqueue *vq = to_vvq(_vq);
> +
> +       if (vq->use_dma_api)
> +               return vring_dma_dev(vq);
> +       else
> +               return NULL;
> +}
> +EXPORT_SYMBOL_GPL(virtqueue_dma_dev);

One possible concern is that exporting things like NULL may result in
the switch in the caller (driver). I wonder if it's better to do
BUG_ON() in the path of NULL?

Thanks

> +
>  /**
>   * virtqueue_kick_prepare - first half of split virtqueue_kick call.
>   * @_vq: the struct virtqueue
> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> index 8add38038877..bd55a05eec04 100644
> --- a/include/linux/virtio.h
> +++ b/include/linux/virtio.h
> @@ -61,6 +61,8 @@ int virtqueue_add_sgs(struct virtqueue *vq,
>                       void *data,
>                       gfp_t gfp);
>
> +struct device *virtqueue_dma_dev(struct virtqueue *vq);
> +
>  bool virtqueue_kick(struct virtqueue *vq);
>
>  bool virtqueue_kick_prepare(struct virtqueue *vq);
> --
> 2.32.0.3.g01195cf9f
>