Re: [PATCH vhost v10 02/10] virtio_ring: introduce virtqueue_set_premapped()

[Jason Wang <jasowang@redhat.com>]

On Fri, Jun 2, 2023 at 5:22 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
>
> This helper allows the driver change the dma mode to premapped mode.
> Under the premapped mode, the virtio core do not do dma mapping
> internally.
>
> This just work when the use_dma_api is true. If the use_dma_api is false,
> the dma options is not through the DMA APIs, that is not the standard
> way of the linux kernel.
>
> Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> ---
>  drivers/virtio/virtio_ring.c | 40 ++++++++++++++++++++++++++++++++++++
>  include/linux/virtio.h       |  2 ++
>  2 files changed, 42 insertions(+)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 72ed07a604d4..2afdfb9e3e30 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -172,6 +172,9 @@ struct vring_virtqueue {
>         /* Host publishes avail event idx */
>         bool event;
>
> +       /* Do DMA mapping by driver */
> +       bool premapped;
> +
>         /* Head of free buffer list. */
>         unsigned int free_head;
>         /* Number we've added since last sync. */
> @@ -2059,6 +2062,7 @@ static struct virtqueue *vring_create_virtqueue_packed(
>         vq->packed_ring = true;
>         vq->dma_dev = dma_dev;
>         vq->use_dma_api = vring_use_dma_api(vdev);
> +       vq->premapped = false;
>
>         vq->indirect = virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC) &&
>                 !context;
> @@ -2548,6 +2552,7 @@ static struct virtqueue *__vring_new_virtqueue(unsigned int index,
>  #endif
>         vq->dma_dev = dma_dev;
>         vq->use_dma_api = vring_use_dma_api(vdev);
> +       vq->premapped = false;
>
>         vq->indirect = virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC) &&
>                 !context;
> @@ -2691,6 +2696,41 @@ int virtqueue_resize(struct virtqueue *_vq, u32 num,
>  }
>  EXPORT_SYMBOL_GPL(virtqueue_resize);
>
> +/**
> + * virtqueue_set_premapped - set the vring premapped mode
> + * @_vq: the struct virtqueue we're talking about.
> + *
> + * Enable the premapped mode of the vq.
> + *
> + * The vring in premapped mode does not do dma internally, so the driver must
> + * do dma mapping in advance. The driver must pass the dma_address through
> + * dma_address of scatterlist. When the driver got a used buffer from
> + * the vring, it has to unmap the dma address. So the driver must call
> + * virtqueue_get_buf_premapped()/virtqueue_detach_unused_buf_premapped().
> + *
> + * This must be called before adding any buf to vring.

And any old buffer should be detached?

> + * So this should be called immediately after init vq or vq reset.

Any way to detect and warn in this case? (not a must if it's too
expensive to do the check)

> + *
> + * 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.
> + * 0: success.
> + * -EINVAL: vring does not use the dma api, so we can not enable premapped mode.
> + */
> +int virtqueue_set_premapped(struct virtqueue *_vq)
> +{
> +       struct vring_virtqueue *vq = to_vvq(_vq);
> +
> +       if (!vq->use_dma_api)
> +               return -EINVAL;
> +
> +       vq->premapped = true;

I guess there should be a way to disable it. Would it be useful for
the case when AF_XDP sockets were destroyed?

Thanks


> +
> +       return 0;
> +}
> +EXPORT_SYMBOL_GPL(virtqueue_set_premapped);
> +
>  /* 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 b93238db94e3..1fc0e1023bd4 100644
> --- a/include/linux/virtio.h
> +++ b/include/linux/virtio.h
> @@ -78,6 +78,8 @@ bool virtqueue_enable_cb(struct virtqueue *vq);
>
>  unsigned virtqueue_enable_cb_prepare(struct virtqueue *vq);
>
> +int virtqueue_set_premapped(struct virtqueue *_vq);
> +
>  bool virtqueue_poll(struct virtqueue *vq, unsigned);
>
>  bool virtqueue_enable_cb_delayed(struct virtqueue *vq);
> --
> 2.32.0.3.g01195cf9f
>

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