Subject: Re: [PATCH vhost v10 06/10] virtio_ring: packed-detach: support return dma info to driver

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 2 Jun 2023 07:40:46 -0400

On Fri, Jun 02, 2023 at 05:22:02PM +0800, Xuan Zhuo wrote:
> Under the premapped mode, the driver needs to unmap the DMA address
> after receiving the buffer. The virtio core records the DMA address,
> so the driver needs a way to get the dma info from the virtio core.
> 
> A straightforward approach is to pass an array to the virtio core when
> calling virtqueue_get_buf(). However, it is not feasible when there are
> multiple DMA addresses in the descriptor chain, and the array size is
> unknown.
> 
> To solve this problem, a helper be introduced. After calling
> virtqueue_get_buf(), the driver can call the helper to
> retrieve a dma info. If the helper function returns -EAGAIN, it means
> that there are more DMA addresses to be processed, and the driver should
> call the helper function again.

Please, keep error codes for when an actual error occurs.
A common pattern would be:
	<0 - error
	0 - success, done
	>0 - success, more to do

> To keep track of the current position in
> the chain, a cursor must be passed to the helper function, which is
> initialized by virtqueue_get_buf().
> 
> Some processes are done inside this helper, so this helper MUST be
> called under the premapped mode.
> 
> Signed-off-by: Xuan Zhuo <[email-masked]>
> ---
>  drivers/virtio/virtio_ring.c | 105 ++++++++++++++++++++++++++++++++---
>  include/linux/virtio.h       |   9 ++-
>  2 files changed, 103 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index cdc4349f6066..cbc22daae7e1 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -1695,8 +1695,85 @@ static bool virtqueue_kick_prepare_packed(struct virtqueue *_vq)
>  	return needs_kick;
>  }
>  
> +static void detach_cursor_init_packed(struct vring_virtqueue *vq,
> +				      struct virtqueue_detach_cursor *cursor, u16 id)
> +{
> +	struct vring_desc_state_packed *state = NULL;
> +	u32 len;
> +
> +	state = &vq->packed.desc_state[id];
> +
> +	/* Clear data ptr. */
> +	state->data = NULL;
> +
> +	vq->packed.desc_extra[state->last].next = vq->free_head;
> +	vq->free_head = id;
> +	vq->vq.num_free += state->num;
> +
> +	/* init cursor */
> +	cursor->curr = id;
> +	cursor->done = 0;
> +	cursor->pos = 0;
> +
> +	if (vq->packed.desc_extra[id].flags & VRING_DESC_F_INDIRECT) {
> +		len = vq->split.desc_extra[id].len;
> +
> +		cursor->num = len / sizeof(struct vring_packed_desc);
> +		cursor->indirect = true;
> +
> +		vring_unmap_extra_packed(vq, &vq->packed.desc_extra[id]);
> +	} else {
> +		cursor->num = state->num;
> +		cursor->indirect = false;
> +	}
> +}
> +
> +static int virtqueue_detach_packed(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);
> +
> +	if (unlikely(cursor->done))
> +		return -EINVAL;
> +
> +	if (!cursor->indirect) {
> +		struct vring_desc_extra *extra;
> +
> +		extra = &vq->packed.desc_extra[cursor->curr];
> +		cursor->curr = extra->next;
> +
> +		*addr = extra->addr;
> +		*len = extra->len;
> +		*dir = (extra->flags & VRING_DESC_F_WRITE) ? DMA_FROM_DEVICE : DMA_TO_DEVICE;
> +
> +		if (++cursor->pos == cursor->num) {
> +			cursor->done = true;
> +			return 0;
> +		}
> +	} else {
> +		struct vring_packed_desc *indir_desc, *desc;
> +		u16 flags;
> +
> +		indir_desc = vq->packed.desc_state[cursor->curr].indir_desc;
> +		desc = &indir_desc[cursor->pos];
> +
> +		flags = le16_to_cpu(desc->flags);
> +		*addr = le64_to_cpu(desc->addr);
> +		*len = le32_to_cpu(desc->len);
> +		*dir = (flags & VRING_DESC_F_WRITE) ?  DMA_FROM_DEVICE : DMA_TO_DEVICE;
> +
> +		if (++cursor->pos == cursor->num) {
> +			kfree(indir_desc);
> +			cursor->done = true;
> +			return 0;
> +		}
> +	}
> +
> +	return -EAGAIN;
> +}
> +
>  static void detach_buf_packed(struct vring_virtqueue *vq,
> -			      unsigned int id, void **ctx)
> +			      unsigned int id)
>  {
>  	struct vring_desc_state_packed *state = NULL;
>  	struct vring_packed_desc *desc;
> @@ -1736,8 +1813,6 @@ static void detach_buf_packed(struct vring_virtqueue *vq,
>  		}
>  		kfree(desc);
>  		state->indir_desc = NULL;
> -	} else if (ctx) {
> -		*ctx = state->indir_desc;
>  	}
>  }
>  
> @@ -1768,7 +1843,8 @@ static bool more_used_packed(const struct vring_virtqueue *vq)
>  
>  static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq,
>  					  unsigned int *len,
> -					  void **ctx)
> +					  void **ctx,
> +					  struct virtqueue_detach_cursor *cursor)
>  {
>  	struct vring_virtqueue *vq = to_vvq(_vq);
>  	u16 last_used, id, last_used_idx;
> @@ -1808,7 +1884,14 @@ static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq,
>  
>  	/* detach_buf_packed clears data, so grab it now. */
>  	ret = vq->packed.desc_state[id].data;
> -	detach_buf_packed(vq, id, ctx);
> +
> +	if (!vq->indirect && ctx)
> +		*ctx = vq->packed.desc_state[id].indir_desc;
> +
> +	if (vq->premapped)
> +		detach_cursor_init_packed(vq, cursor, id);
> +	else
> +		detach_buf_packed(vq, id);
>  
>  	last_used += vq->packed.desc_state[id].num;
>  	if (unlikely(last_used >= vq->packed.vring.num)) {
> @@ -1960,7 +2043,8 @@ static bool virtqueue_enable_cb_delayed_packed(struct virtqueue *_vq)
>  	return true;
>  }
>  
> -static void *virtqueue_detach_unused_buf_packed(struct virtqueue *_vq)
> +static void *virtqueue_detach_unused_buf_packed(struct virtqueue *_vq,
> +						struct virtqueue_detach_cursor *cursor)
>  {
>  	struct vring_virtqueue *vq = to_vvq(_vq);
>  	unsigned int i;
> @@ -1973,7 +2057,10 @@ static void *virtqueue_detach_unused_buf_packed(struct virtqueue *_vq)
>  			continue;
>  		/* detach_buf clears data, so grab it now. */
>  		buf = vq->packed.desc_state[i].data;
> -		detach_buf_packed(vq, i, NULL);
> +		if (vq->premapped)
> +			detach_cursor_init_packed(vq, cursor, i);
> +		else
> +			detach_buf_packed(vq, i);
>  		END_USE(vq);
>  		return buf;
>  	}
> @@ -2458,7 +2545,7 @@ void *virtqueue_get_buf_ctx(struct virtqueue *_vq, unsigned int *len,
>  {
>  	struct vring_virtqueue *vq = to_vvq(_vq);
>  
> -	return vq->packed_ring ? virtqueue_get_buf_ctx_packed(_vq, len, ctx) :
> +	return vq->packed_ring ? virtqueue_get_buf_ctx_packed(_vq, len, ctx, NULL) :
>  				 virtqueue_get_buf_ctx_split(_vq, len, ctx, NULL);
>  }
>  EXPORT_SYMBOL_GPL(virtqueue_get_buf_ctx);
> @@ -2590,7 +2677,7 @@ void *virtqueue_detach_unused_buf(struct virtqueue *_vq)
>  {
>  	struct vring_virtqueue *vq = to_vvq(_vq);
>  
> -	return vq->packed_ring ? virtqueue_detach_unused_buf_packed(_vq) :
> +	return vq->packed_ring ? virtqueue_detach_unused_buf_packed(_vq, NULL) :
>  				 virtqueue_detach_unused_buf_split(_vq, NULL);
>  }
>  EXPORT_SYMBOL_GPL(virtqueue_detach_unused_buf);
> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> index eb4a4e4329aa..7f137c7a9034 100644
> --- a/include/linux/virtio.h
> +++ b/include/linux/virtio.h
> @@ -43,8 +43,13 @@ struct virtqueue_detach_cursor {
>  	unsigned done:1;
>  	unsigned hole:14;
>  
> -	/* for split head */
> -	unsigned head:16;
> +	union {
> +		/* for split head */
> +		unsigned head:16;
> +
> +		/* for packed id */
> +		unsigned curr:16;
> +	};
>  	unsigned num:16;
>  	unsigned pos:16;
>  };
> -- 
> 2.32.0.3.g01195cf9f