[PATCH vhost 04/17] virtio_ring: split: remove double check of the unmap ops

[Jason Wang]

On Tue, Jan 30, 2024 at 7:42 PM Xuan Zhuo <xuanzhuo at linux.alibaba.com> wrote:
>
> In the functions vring_unmap_one_split and
> vring_unmap_one_split_indirect,
> multiple checks are made whether unmap is performed and whether it is
> INDIRECT.
>
> These two functions are usually called in a loop, and we should put the
> check outside the loop.
>
> And we unmap the descs with VRING_DESC_F_INDIRECT on the same path with
> other descs, that make the thing more complex. If we distinguish the
> descs with VRING_DESC_F_INDIRECT before unmap, thing will be clearer.
>
> 1. only one desc of the desc table is used, we do not need the loop
> 2. the called unmap api is difference from the other desc
> 3. the vq->premapped is not needed to check
> 4. the vq->indirect is not needed to check
> 5. the state->indir_desc must not be null
>
> Signed-off-by: Xuan Zhuo <xuanzhuo at linux.alibaba.com>
> ---
>  drivers/virtio/virtio_ring.c | 80 ++++++++++++++++++------------------
>  1 file changed, 39 insertions(+), 41 deletions(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index dd03bc5a81fe..2b41fdbce975 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -452,9 +452,6 @@ static void vring_unmap_one_split_indirect(const struct vring_virtqueue *vq,
>  {
>         u16 flags;
>
> -       if (!vring_need_unmap_buffer(vq))
> -               return;
> -
>         flags = virtio16_to_cpu(vq->vq.vdev, desc->flags);
>
>         dma_unmap_page(vring_dma_dev(vq),
> @@ -472,27 +469,12 @@ static unsigned int vring_unmap_one_split(const struct vring_virtqueue *vq,
>
>         flags = extra[i].flags;
>
> -       if (flags & VRING_DESC_F_INDIRECT) {
> -               if (!vq->use_dma_api)
> -                       goto out;
> -
> -               dma_unmap_single(vring_dma_dev(vq),
> -                                extra[i].addr,
> -                                extra[i].len,
> -                                (flags & VRING_DESC_F_WRITE) ?
> -                                DMA_FROM_DEVICE : DMA_TO_DEVICE);
> -       } else {
> -               if (!vring_need_unmap_buffer(vq))
> -                       goto out;
> -
> -               dma_unmap_page(vring_dma_dev(vq),
> -                              extra[i].addr,
> -                              extra[i].len,
> -                              (flags & VRING_DESC_F_WRITE) ?
> -                              DMA_FROM_DEVICE : DMA_TO_DEVICE);
> -       }
> +       dma_unmap_page(vring_dma_dev(vq),
> +                      extra[i].addr,
> +                      extra[i].len,
> +                      (flags & VRING_DESC_F_WRITE) ?
> +                      DMA_FROM_DEVICE : DMA_TO_DEVICE);
>
> -out:
>         return extra[i].next;
>  }
>
> @@ -660,7 +642,7 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
>                         vq, desc, total_sg * sizeof(struct vring_desc),
>                         DMA_TO_DEVICE);
>                 if (vring_mapping_error(vq, addr)) {
> -                       if (vq->premapped)
> +                       if (!vring_need_unmap_buffer(vq))
>                                 goto free_indirect;
>
>                         goto unmap_release;
> @@ -713,6 +695,9 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
>         return 0;
>
>  unmap_release:
> +
> +       WARN_ON(!vring_need_unmap_buffer(vq));
> +
>         err_idx = i;
>
>         if (indirect)
> @@ -774,34 +759,42 @@ 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);
> +       u16 flags;
>
>         /* Clear data ptr. */
>         vq->split.desc_state[head].data = NULL;
> +       flags = vq->split.desc_extra[head].flags;
>
>         /* Put back on free list: unmap first-level descriptors and find end */
>         i = head;
>
> -       while (vq->split.vring.desc[i].flags & nextflag) {
> -               vring_unmap_one_split(vq, i);
> -               i = vq->split.desc_extra[i].next;
> -               vq->vq.num_free++;
> -       }
> -
> -       vring_unmap_one_split(vq, i);
> -       vq->split.desc_extra[i].next = vq->free_head;
> -       vq->free_head = head;
> +       if (!(flags & VRING_DESC_F_INDIRECT)) {

So during add we do:

        if (!indirect && vring_need_unmap_buffer(vq))
                vq->split.desc_extra[prev & (vq->split.vring.num - 1)].flags &=
                        ~VRING_DESC_F_NEXT;

Then using flags here unconditionally seems not reliable.

I post a patch to store flags unconditionally at:

https://lore.kernel.org/all/20220224122655-mutt-send-email-mst@kernel.org/

> +               while (vq->split.vring.desc[i].flags & nextflag) {
> +                       if (vring_need_unmap_buffer(vq))
> +                               vring_unmap_one_split(vq, i);
> +                       i = vq->split.desc_extra[i].next;
> +                       vq->vq.num_free++;
> +               }
>
> -       /* Plus final descriptor */
> -       vq->vq.num_free++;
> +               if (vring_need_unmap_buffer(vq))
> +                       vring_unmap_one_split(vq, i);
>
> -       if (vq->indirect) {
> +               if (ctx)
> +                       *ctx = vq->split.desc_state[head].indir_desc;
> +       } else {
>                 struct vring_desc *indir_desc =
>                                 vq->split.desc_state[head].indir_desc;
>                 u32 len;
>
> -               /* Free the indirect table, if any, now that it's unmapped. */
> -               if (!indir_desc)
> -                       return;
> +               if (vq->use_dma_api) {
> +                       struct vring_desc_extra *extra = vq->split.desc_extra;
> +
> +                       dma_unmap_single(vring_dma_dev(vq),
> +                                        extra[i].addr,
> +                                        extra[i].len,
> +                                        (flags & VRING_DESC_F_WRITE) ?
> +                                        DMA_FROM_DEVICE : DMA_TO_DEVICE);
> +               }

Note that there's a following

BUG_ON(!(vq->split.desc_extra[head].flags &
                                VRING_DESC_F_INDIRECT));

Which I think we can remove.

Thanks