Re: [Qemu-devel] [PATCH v18 06/10] virtio_ring: add a new API, virtqueue_add_one_desc

[Michael S. Tsirkin]

On Wed, Nov 29, 2017 at 09:55:22PM +0800, Wei Wang wrote:
> Current virtqueue_add API implementation is based on the scatterlist
> struct, which uses kaddr. This is inadequate to all the use case of
> vring. For example:
> - Some usages don't use IOMMU, in this case the user can directly pass
>   in a physical address in hand, instead of going through the sg
>   implementation (e.g. the VIRTIO_BALLOON_F_SG feature)
> - Sometimes, a guest physical page may not have a kaddr (e.g. high
>   memory) but need to use vring (e.g. the VIRTIO_BALLOON_F_FREE_PAGE_VQ
>   feature)
> 
> The new API virtqueue_add_one_desc enables the caller to assign a vring
> desc with a physical address and len. Also, factor out the common code
> with virtqueue_add in vring_set_avail.
> 
> Signed-off-by: Wei Wang <address@hidden>
> Cc: Michael S. Tsirkin <address@hidden>

You previously managed without this patch, and it's preferable
IMHO since this patchset is already too big.

I don't really understand what is wrong with virtio_add_sgs + sg_set_page.
I don't think is assumes a kaddr.

> ---
>  drivers/virtio/virtio_ring.c | 94 
> +++++++++++++++++++++++++++++++++++---------
>  include/linux/virtio.h       |  6 +++
>  2 files changed, 81 insertions(+), 19 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index eb30f3e..0b87123 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -257,6 +257,79 @@ static struct vring_desc *alloc_indirect(struct 
> virtqueue *_vq,
>       return desc;
>  }
>  
> +static void vring_set_avail(struct virtqueue *_vq,
> +                         unsigned int i)
> +{
> +     struct vring_virtqueue *vq = to_vvq(_vq);
> +     unsigned int avail;
> +
> +     avail = vq->avail_idx_shadow & (vq->vring.num - 1);
> +     vq->vring.avail->ring[avail] = cpu_to_virtio16(_vq->vdev, i);
> +
> +     /*
> +      * Descriptors and available array need to be set before we expose the
> +      * new available array entries.
> +      */
> +     virtio_wmb(vq->weak_barriers);
> +     vq->avail_idx_shadow++;
> +     vq->vring.avail->idx = cpu_to_virtio16(_vq->vdev,
> +                                            vq->avail_idx_shadow);
> +     vq->num_added++;
> +
> +     pr_debug("Added buffer head %i to %p\n", i, vq);
> +
> +     /*
> +      * This is very unlikely, but theoretically possible.  Kick
> +      * just in case.
> +      */
> +     if (unlikely(vq->num_added == (1 << 16) - 1))
> +             virtqueue_kick(_vq);
> +}
> +
> +int virtqueue_add_one_desc(struct virtqueue *_vq,
> +                        uint64_t addr,
> +                        uint32_t len,
> +                        bool in_desc,
> +                        void *data)
> +{
> +     struct vring_virtqueue *vq = to_vvq(_vq);
> +     struct vring_desc *desc;
> +     unsigned int i;
> +
> +     START_USE(vq);
> +     BUG_ON(data == NULL);
> +
> +     if (unlikely(vq->broken)) {
> +             END_USE(vq);
> +             return -EIO;
> +     }
> +
> +     if (_vq->num_free < 1) {
> +             END_USE(vq);
> +             return -ENOSPC;
> +     }
> +
> +     i = vq->free_head;
> +     desc = &vq->vring.desc[i];
> +     desc->addr = cpu_to_virtio64(_vq->vdev, addr);
> +     desc->len = cpu_to_virtio32(_vq->vdev, len);
> +     if (in_desc)
> +             desc->flags = cpu_to_virtio16(_vq->vdev, VRING_DESC_F_WRITE);
> +     else
> +             desc->flags = 0;
> +     vq->desc_state[i].data = data;
> +     vq->desc_state[i].indir_desc = NULL;
> +     vq->free_head = virtio16_to_cpu(_vq->vdev, vq->vring.desc[i].next);
> +     _vq->num_free--;
> +
> +     vring_set_avail(_vq, i);
> +
> +     END_USE(vq);
> +
> +     return 0;
> +}
> +EXPORT_SYMBOL_GPL(virtqueue_add_one_desc);
> +
>  static inline int virtqueue_add(struct virtqueue *_vq,
>                               struct scatterlist *sgs[],
>                               unsigned int total_sg,
> @@ -269,7 +342,7 @@ static inline int virtqueue_add(struct virtqueue *_vq,
>       struct vring_virtqueue *vq = to_vvq(_vq);
>       struct scatterlist *sg;
>       struct vring_desc *desc;
> -     unsigned int i, n, avail, descs_used, uninitialized_var(prev), err_idx;
> +     unsigned int i, n, descs_used, uninitialized_var(prev), err_idx;
>       int head;
>       bool indirect;
>  
> @@ -395,26 +468,9 @@ static inline int virtqueue_add(struct virtqueue *_vq,
>       else
>               vq->desc_state[head].indir_desc = ctx;
>  
> -     /* Put entry in available array (but don't update avail->idx until they
> -      * do sync). */
> -     avail = vq->avail_idx_shadow & (vq->vring.num - 1);
> -     vq->vring.avail->ring[avail] = cpu_to_virtio16(_vq->vdev, head);
> -
> -     /* Descriptors and available array need to be set before we expose the
> -      * new available array entries. */
> -     virtio_wmb(vq->weak_barriers);
> -     vq->avail_idx_shadow++;
> -     vq->vring.avail->idx = cpu_to_virtio16(_vq->vdev, vq->avail_idx_shadow);
> -     vq->num_added++;
> -
> -     pr_debug("Added buffer head %i to %p\n", head, vq);
> +     vring_set_avail(_vq, head);
>       END_USE(vq);
>  
> -     /* This is very unlikely, but theoretically possible.  Kick
> -      * just in case. */
> -     if (unlikely(vq->num_added == (1 << 16) - 1))
> -             virtqueue_kick(_vq);
> -
>       return 0;
>  
>  unmap_release:
> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> index 988c735..1d89996 100644
> --- a/include/linux/virtio.h
> +++ b/include/linux/virtio.h
> @@ -35,6 +35,12 @@ struct virtqueue {
>       void *priv;
>  };
>  
> +int virtqueue_add_one_desc(struct virtqueue *_vq,
> +                        uint64_t addr,
> +                        uint32_t len,
> +                        bool in_desc,
> +                        void *data);
> +
>  int virtqueue_add_outbuf(struct virtqueue *vq,
>                        struct scatterlist sg[], unsigned int num,
>                        void *data,
> -- 
> 2.7.4