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

[Jason Wang <jasowang@redhat.com>]

On Fri, Jun 2, 2023 at 5:22 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> 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 second thought, can we simply offload the tracking to the driver
itself? This looks the way many other modern NIC drivers did.

In pre mapped mode, the DMA address is in fact told by the driver
itself so it should have sufficient knowledge. And in some cases, the
driver wants to optimize/merge/delay the unampping so the DMA
addresses returned by the virtio core are not even interested in those
cases.

Thanks



>
> 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 <xuanzhuo@linux.alibaba.com>
> ---
>  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)
> +{
> +       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;
> +};
> +
>  int virtqueue_add_outbuf(struct virtqueue *vq,
>                          struct scatterlist sg[], unsigned int num,
>                          void *data,
> --
> 2.32.0.3.g01195cf9f
>

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