Subject: Re: [PATCH vhost v10 07/10] virtio_ring: introduce helpers for premapped

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 22 Jun 2023 15:29:31 -0400

On Fri, Jun 02, 2023 at 05:22:03PM +0800, Xuan Zhuo wrote:
> This patch introduces three helpers for premapped mode.
> 
> * virtqueue_get_buf_premapped
> * virtqueue_detach_unused_buf_premapped
> 
> The above helpers work like the non-premapped funcs. But a cursor is
> passed.
> 
> virtqueue_detach is used to get the dma info of the last buf by
>   cursor.
> 
> Signed-off-by: Xuan Zhuo <[email-masked]>
> ---
>  drivers/virtio/virtio_ring.c | 83 ++++++++++++++++++++++++++++++++++++
>  include/linux/virtio.h       | 10 +++++
>  2 files changed, 93 insertions(+)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index cbc22daae7e1..6771b9661798 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -2555,6 +2555,66 @@ void *virtqueue_get_buf(struct virtqueue *_vq, unsigned int *len)
>  	return virtqueue_get_buf_ctx(_vq, len, NULL);
>  }
>  EXPORT_SYMBOL_GPL(virtqueue_get_buf);
> +
> +/**
> + * virtqueue_get_buf_premapped - get the next used buffer
> + * @_vq: the struct virtqueue we're talking about.
> + * @len: the length written into the buffer
> + * @ctx: extra context for the token
> + * @cursor: detach cursor
> + *
> + * If the device wrote data into the buffer, @len will be set to the
> + * amount written.  This means you don't need to clear the buffer
> + * beforehand to ensure there's no data leakage in the case of short
> + * writes.
> + *
> + * Caller must ensure we don't call this with other virtqueue
> + * operations at the same time (except where noted).
> + *
> + * This is used for the premapped vq. The cursor is passed by the dirver, that
> + * is used for virtqueue_detach. That will be initialized by virtio core
> + * internally.

initialized how?

> + *
> + * Returns NULL if there are no used buffers, or the "data" token
> + * handed to virtqueue_add_*().
> + */
> +void *virtqueue_get_buf_premapped(struct virtqueue *_vq, unsigned int *len,
> +				  void **ctx,
> +				  struct virtqueue_detach_cursor *cursor)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +
> +	return vq->packed_ring ? virtqueue_get_buf_ctx_packed(_vq, len, ctx, cursor) :
> +				 virtqueue_get_buf_ctx_split(_vq, len, ctx, cursor);
> +}
> +EXPORT_SYMBOL_GPL(virtqueue_get_buf_premapped);
> +
> +/**
> + * virtqueue_detach - get the dma info of last buf
> + * @_vq: the struct virtqueue we're talking about.
> + * @cursor: detach cursor
> + * @addr: the dma address
> + * @len: the length of the dma address
> + * @dir: the direction of the dma address
> + *
> + * This is used for the premapped vq. The cursor is initialized by
> + * virtqueue_get_buf_premapped or virtqueue_detach_unused_buf_premapped.
> + *
> + * Returns:
> + * -EAGAIN: there are more dma info, this function should be called more.

dma info is singular, so "there is".

I see you kept this idea of returning -EAGAIN on success.
Pls don't.

> + * -EINVAL: the process is done, should not call this function

here you really mean "a previous call returned 0"
we generally don't do defensive programming why do it here?

> + * 0: no more dma info

"no more" normally means "nothing was returned", not
"value returned and this was the last entry".

> + */
> +int virtqueue_detach(struct virtqueue *_vq, struct virtqueue_detach_cursor *cursor,
> +		     dma_addr_t *addr, u32 *len, enum dma_data_direction *dir)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +
> +	return vq->packed_ring ? virtqueue_detach_packed(_vq, cursor, addr, len, dir) :
> +				 virtqueue_detach_split(_vq, cursor, addr, len, dir);
> +}
> +EXPORT_SYMBOL_GPL(virtqueue_detach);
> +
>  /**
>   * virtqueue_disable_cb - disable callbacks
>   * @_vq: the struct virtqueue we're talking about.
> @@ -2682,6 +2742,29 @@ void *virtqueue_detach_unused_buf(struct virtqueue *_vq)
>  }
>  EXPORT_SYMBOL_GPL(virtqueue_detach_unused_buf);
>  
> +/**
> + * virtqueue_detach_unused_buf_premapped - detach first unused buffer
> + * @_vq: the struct virtqueue we're talking about.
> + * @cursor: detach cursor
> + *
> + * This is used for the premapped vq. The cursor is passed by the dirver, that
> + * is used for virtqueue_detach. That will be initialized by virtio core
> + * internally.
> + *
> + * Returns NULL or the "data" token handed to virtqueue_add_*().
> + * This is not valid on an active queue; it is useful for device
> + * shutdown or the reset queue.
> + */
> +void *virtqueue_detach_unused_buf_premapped(struct virtqueue *_vq,
> +					    struct virtqueue_detach_cursor *cursor)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +
> +	return vq->packed_ring ? virtqueue_detach_unused_buf_packed(_vq, cursor) :
> +				 virtqueue_detach_unused_buf_split(_vq, cursor);
> +}
> +EXPORT_SYMBOL_GPL(virtqueue_detach_unused_buf_premapped);
> +
>  static inline bool more_used(const struct vring_virtqueue *vq)
>  {
>  	return vq->packed_ring ? more_used_packed(vq) : more_used_split(vq);
> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> index 7f137c7a9034..0a11c5b32fe5 100644
> --- a/include/linux/virtio.h
> +++ b/include/linux/virtio.h
> @@ -3,6 +3,7 @@
>  #define _LINUX_VIRTIO_H
>  /* Everything a virtio driver needs to work with any particular virtio
>   * implementation. */
> +#include <linux/dma-mapping.h>
>  #include <linux/types.h>
>  #include <linux/scatterlist.h>
>  #include <linux/spinlock.h>
> @@ -88,6 +89,10 @@ void *virtqueue_get_buf(struct virtqueue *vq, unsigned int *len);
>  void *virtqueue_get_buf_ctx(struct virtqueue *vq, unsigned int *len,
>  			    void **ctx);
>  
> +void *virtqueue_get_buf_premapped(struct virtqueue *_vq, unsigned int *len,
> +				  void **ctx,
> +				  struct virtqueue_detach_cursor *cursor);
> +
>  void virtqueue_disable_cb(struct virtqueue *vq);
>  
>  bool virtqueue_enable_cb(struct virtqueue *vq);
> @@ -101,6 +106,8 @@ bool virtqueue_poll(struct virtqueue *vq, unsigned);
>  bool virtqueue_enable_cb_delayed(struct virtqueue *vq);
>  
>  void *virtqueue_detach_unused_buf(struct virtqueue *vq);
> +void *virtqueue_detach_unused_buf_premapped(struct virtqueue *_vq,
> +					    struct virtqueue_detach_cursor *cursor);
>  
>  unsigned int virtqueue_get_vring_size(const struct virtqueue *vq);
>  
> @@ -114,6 +121,9 @@ dma_addr_t virtqueue_get_used_addr(const struct virtqueue *vq);
>  int virtqueue_resize(struct virtqueue *vq, u32 num,
>  		     void (*recycle)(struct virtqueue *vq, void *buf));
>  
> +int virtqueue_detach(struct virtqueue *_vq, struct virtqueue_detach_cursor *cursor,
> +		     dma_addr_t *addr, u32 *len, enum dma_data_direction *dir);
> +
>  /**
>   * struct virtio_device - representation of a device using virtio
>   * @index: unique position on the virtio bus
> -- 
> 2.32.0.3.g01195cf9f