Subject: Re: [PATCH vhost v10 05/10] virtio_ring: split-detach: support return dma info to driver

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 22 Jun 2023 15:36:41 -0400

On Fri, Jun 02, 2023 at 05:22:01PM +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. 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 | 118 ++++++++++++++++++++++++++++++++---
>  include/linux/virtio.h       |  11 ++++
>  2 files changed, 119 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index dc109fbc05a5..cdc4349f6066 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -754,8 +754,95 @@ static bool virtqueue_kick_prepare_split(struct virtqueue *_vq)
>  	return needs_kick;
>  }
>  
> -static void detach_buf_split(struct vring_virtqueue *vq, unsigned int head,
> -			     void **ctx)
> +static void detach_cursor_init_split(struct vring_virtqueue *vq,
> +				     struct virtqueue_detach_cursor *cursor, u16 head)
> +{
> +	struct vring_desc_extra *extra;
> +
> +	extra = &vq->split.desc_extra[head];
> +
> +	/* Clear data ptr. */
> +	vq->split.desc_state[head].data = NULL;
> +
> +	cursor->head = head;
> +	cursor->done = 0;
> +
> +	if (extra->flags & VRING_DESC_F_INDIRECT) {
> +		cursor->num = extra->len / sizeof(struct vring_desc);
> +		cursor->indirect = true;
> +		cursor->pos = 0;
> +
> +		vring_unmap_one_split(vq, head);
> +
> +		extra->next = vq->free_head;
> +
> +		vq->free_head = head;
> +
> +		/* Plus final descriptor */
> +		vq->vq.num_free++;
> +
> +	} else {
> +		cursor->indirect = false;
> +		cursor->pos = head;
> +	}
> +}
> +
> +static int virtqueue_detach_split(struct virtqueue *_vq, struct virtqueue_detach_cursor *cursor,
> +				  dma_addr_t *addr, u32 *len, enum dma_data_direction *dir)
> +{

I don't get it. This is generic split vq code? Why is it unconditionally
wasting time with cursors etc? Poking at split.desc_extra when not
necessary is also not really nice, will cause lots of cache misses.

And it looks like we duplicated a bunch of logic?

> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +	__virtio16 nextflag = cpu_to_virtio16(vq->vq.vdev, VRING_DESC_F_NEXT);
> +	int rc = -EAGAIN;
> +
> +	if (unlikely(cursor->done))
> +		return -EINVAL;
> +
> +	if (!cursor->indirect) {
> +		struct vring_desc_extra *extra;
> +		unsigned int i;
> +
> +		i = cursor->pos;
> +
> +		extra = &vq->split.desc_extra[i];
> +
> +		if (vq->split.vring.desc[i].flags & nextflag) {
> +			cursor->pos = extra->next;
> +		} else {
> +			extra->next = vq->free_head;
> +			vq->free_head = cursor->head;
> +			cursor->done = true;
> +			rc = 0;
> +		}
> +
> +		*addr = extra->addr;
> +		*len = extra->len;
> +		*dir = (extra->flags & VRING_DESC_F_WRITE) ? DMA_FROM_DEVICE : DMA_TO_DEVICE;
> +
> +		vq->vq.num_free++;
> +
> +	} else {
> +		struct vring_desc *indir_desc, *desc;
> +		u16 flags;
> +
> +		indir_desc = vq->split.desc_state[cursor->head].indir_desc;
> +		desc = &indir_desc[cursor->pos];
> +
> +		flags = virtio16_to_cpu(vq->vq.vdev, desc->flags);
> +		*addr = virtio64_to_cpu(vq->vq.vdev, desc->addr);
> +		*len = virtio32_to_cpu(vq->vq.vdev, 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 rc;
> +}
> +
> +static void detach_buf_split(struct vring_virtqueue *vq, unsigned int head)
>  {
>  	unsigned int i, j;
>  	__virtio16 nextflag = cpu_to_virtio16(vq->vq.vdev, VRING_DESC_F_NEXT);
> @@ -799,8 +886,6 @@ static void detach_buf_split(struct vring_virtqueue *vq, unsigned int head,
>  
>  		kfree(indir_desc);
>  		vq->split.desc_state[head].indir_desc = NULL;
> -	} else if (ctx) {
> -		*ctx = vq->split.desc_state[head].indir_desc;
>  	}
>  }
>  
> @@ -812,7 +897,8 @@ static bool more_used_split(const struct vring_virtqueue *vq)
>  
>  static void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq,
>  					 unsigned int *len,
> -					 void **ctx)
> +					 void **ctx,
> +					 struct virtqueue_detach_cursor *cursor)
>  {
>  	struct vring_virtqueue *vq = to_vvq(_vq);
>  	void *ret;
> @@ -852,7 +938,15 @@ static void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq,
>  
>  	/* detach_buf_split clears data, so grab it now. */
>  	ret = vq->split.desc_state[i].data;
> -	detach_buf_split(vq, i, ctx);
> +
> +	if (!vq->indirect && ctx)
> +		*ctx = vq->split.desc_state[i].indir_desc;
> +
> +	if (vq->premapped)
> +		detach_cursor_init_split(vq, cursor, i);
> +	else
> +		detach_buf_split(vq, i);
> +
>  	vq->last_used_idx++;
>  	/* If we expect an interrupt for the next entry, tell host
>  	 * by writing event index and flush out the write before
> @@ -961,7 +1055,8 @@ static bool virtqueue_enable_cb_delayed_split(struct virtqueue *_vq)
>  	return true;
>  }
>  
> -static void *virtqueue_detach_unused_buf_split(struct virtqueue *_vq)
> +static void *virtqueue_detach_unused_buf_split(struct virtqueue *_vq,
> +					       struct virtqueue_detach_cursor *cursor)
>  {
>  	struct vring_virtqueue *vq = to_vvq(_vq);
>  	unsigned int i;
> @@ -974,7 +1069,10 @@ static void *virtqueue_detach_unused_buf_split(struct virtqueue *_vq)
>  			continue;
>  		/* detach_buf_split clears data, so grab it now. */
>  		buf = vq->split.desc_state[i].data;
> -		detach_buf_split(vq, i, NULL);
> +		if (vq->premapped)
> +			detach_cursor_init_split(vq, cursor, i);
> +		else
> +			detach_buf_split(vq, i);
>  		vq->split.avail_idx_shadow--;
>  		vq->split.vring.avail->idx = cpu_to_virtio16(_vq->vdev,
>  				vq->split.avail_idx_shadow);
> @@ -2361,7 +2459,7 @@ 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) :
> -				 virtqueue_get_buf_ctx_split(_vq, len, ctx);
> +				 virtqueue_get_buf_ctx_split(_vq, len, ctx, NULL);
>  }
>  EXPORT_SYMBOL_GPL(virtqueue_get_buf_ctx);
>  
> @@ -2493,7 +2591,7 @@ 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) :
> -				 virtqueue_detach_unused_buf_split(_vq);
> +				 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 1fc0e1023bd4..eb4a4e4329aa 100644
> --- a/include/linux/virtio.h
> +++ b/include/linux/virtio.h
> @@ -38,6 +38,17 @@ struct virtqueue {
>  	void *priv;
>  };
>  
> +struct virtqueue_detach_cursor {
> +	unsigned indirect:1;
> +	unsigned done:1;
> +	unsigned hole:14;
> +
> +	/* for split head */
> +	unsigned head:16;
> +	unsigned num:16;
> +	unsigned pos:16;
> +};
> +

is cursor ever stored somewhere? If not don't use bitfields,
they cause many gcc versions to generate atrocious code.

>  int virtqueue_add_outbuf(struct virtqueue *vq,
>  			 struct scatterlist sg[], unsigned int num,
>  			 void *data,
> -- 
> 2.32.0.3.g01195cf9f