Re: [PATCH 4/6] virtio: split: virtqueue_add_split() support dma address

[Jason Wang <jasowang@redhat.com>]

在 2022/1/7 下午2:33, Xuan Zhuo 写道:
> virtqueue_add_split() only supports virtual addresses, dma is completed
> in virtqueue_add_split().
>
> In some scenarios (such as the AF_XDP scenario), the memory is allocated
> and DMA is completed in advance, so it is necessary for us to support
> passing the DMA address to virtqueue_add_split().
>
> This patch stipulates that if sg->dma_address is not NULL, use this
> address as the DMA address. And record this information in extra->flags,
> which can be skipped when executing dma unmap.
>
>      extra->flags |= VRING_DESC_F_PREDMA;


I think we need another name other than the VRING_DESC_F prefix since 
it's for the flag defined in the spec. May VIRTIO_DESC_F_PREDMA.

Thanks


>
> This relies on the previous patch, in the indirect scenario, for each
> desc allocated, an extra is allocated at the same time.
>
> Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> ---
>   drivers/virtio/virtio_ring.c | 28 ++++++++++++++++++++++++----
>   1 file changed, 24 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 7420741cb750..add8430d9678 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -66,6 +66,9 @@
>   #define LAST_ADD_TIME_INVALID(vq)
>   #endif
>   
> +/* This means the buffer dma is pre-alloc. Just used by vring_desc_extra */
> +#define VRING_DESC_F_PREDMA	(1 << 15)
> +
>   struct vring_desc_extra {
>   	dma_addr_t addr;		/* Descriptor DMA addr. */
>   	u32 len;			/* Descriptor length. */
> @@ -336,11 +339,19 @@ static inline struct device *vring_dma_dev(const struct vring_virtqueue *vq)
>   	return vq->vq.vdev->dev.parent;
>   }
>   
> +static inline bool sg_is_predma(struct scatterlist *sg)
> +{
> +	return !!sg->dma_address;
> +}
> +
>   /* Map one sg entry. */
>   static dma_addr_t vring_map_one_sg(const struct vring_virtqueue *vq,
>   				   struct scatterlist *sg,
>   				   enum dma_data_direction direction)
>   {
> +	if (sg_is_predma(sg))
> +		return sg_dma_address(sg);
> +
>   	if (!vq->use_dma_api)
>   		return (dma_addr_t)sg_phys(sg);
>   
> @@ -396,6 +407,9 @@ static unsigned int vring_unmap_one_split(const struct vring_virtqueue *vq,
>   				 (flags & VRING_DESC_F_WRITE) ?
>   				 DMA_FROM_DEVICE : DMA_TO_DEVICE);
>   	} else {
> +		if (flags & VRING_DESC_F_PREDMA)
> +			goto out;
> +
>   		dma_unmap_page(vring_dma_dev(vq),
>   			       extra->addr,
>   			       extra->len,
> @@ -441,7 +455,8 @@ static inline unsigned int virtqueue_add_desc_split(struct virtqueue *vq,
>   						    unsigned int i,
>   						    dma_addr_t addr,
>   						    unsigned int len,
> -						    u16 flags)
> +						    u16 flags,
> +						    bool predma)
>   {
>   	struct vring_virtqueue *vring = to_vvq(vq);
>   	struct vring_desc_extra *extra;
> @@ -468,6 +483,9 @@ static inline unsigned int virtqueue_add_desc_split(struct virtqueue *vq,
>   	extra->len = len;
>   	extra->flags = flags;
>   
> +	if (predma)
> +		extra->flags |= VRING_DESC_F_PREDMA;
> +
>   	return next;
>   }
>   
> @@ -547,7 +565,8 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
>   			 * table since it use stream DMA mapping.
>   			 */
>   			i = virtqueue_add_desc_split(_vq, in, i, addr, sg->length,
> -						     VRING_DESC_F_NEXT);
> +						     VRING_DESC_F_NEXT,
> +						     sg_is_predma(sg));
>   		}
>   	}
>   	for (; n < (out_sgs + in_sgs); n++) {
> @@ -563,7 +582,8 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
>   			i = virtqueue_add_desc_split(_vq, in, i, addr,
>   						     sg->length,
>   						     VRING_DESC_F_NEXT |
> -						     VRING_DESC_F_WRITE);
> +						     VRING_DESC_F_WRITE,
> +						     sg_is_predma(sg));
>   		}
>   	}
>   	/* Last one doesn't continue. */
> @@ -582,7 +602,7 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
>   
>   		virtqueue_add_desc_split(_vq, NULL, head, addr,
>   					 total_sg * sizeof(struct vring_desc),
> -					 VRING_DESC_F_INDIRECT);
> +					 VRING_DESC_F_INDIRECT, false);
>   	}
>   
>   	/* We're using some buffers from the free list. */

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