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

[Xuan Zhuo <xuanzhuo@linux.alibaba.com>]

On Mon, 10 Jan 2022 14:45:42 +0800, Jason Wang <jasowang@redhat.com> wrote:
>
> 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.
>

OK.

> 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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