[PATCH vhost v2 06/12] virtio_ring: split-indirect: support premapped

Jason Wang
2023-03-14T07:57:06Z

On Wed, Mar 8, 2023 at 2:44?PM Xuan Zhuo <xuanzhuo at linux.alibaba.com>
wrote:
>
> virtio core only supports virtual addresses, dma is completed in virtio
> core.
>
> In some scenarios (such as the AF_XDP), the memory is allocated
> and DMA is completed in advance, so it is necessary for us to support
> passing the DMA address to virtio core.
>
> Drives can use sg->dma_address to pass the mapped dma address to virtio
> core. If one sg->dma_address is used then all sgs must use
sg->dma_address,
> otherwise all dma_address must be null.
>
> On the indirect path, if dma_address is used, desc_state.indir_desc will
> be mixed with VRING_INDIRECT_PREMAPPED. So when do unmap, we can pass it.

It's better to mention why indirect descriptors can't be done in the
same way with direct descriptors.

Btw, if we change the semantics of desc_extra.dma_addr and
desc_state.indir_desc, we should add comments to definitions of those
structures.

>
> Signed-off-by: Xuan Zhuo <xuanzhuo at linux.alibaba.com>
> ---
>  drivers/virtio/virtio_ring.c | 28 ++++++++++++++++++++++------
>  1 file changed, 22 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 66a071e3bdef..11827d2e56a8 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -231,6 +231,18 @@ static void vring_free(struct virtqueue *_vq);
>   * Helpers.
>   */
>
> +#define VRING_INDIRECT_PREMAPPED  BIT(0)
> +
> +#define desc_mix_dma_map(do_map, desc) \
> +       (do_map ? desc : (typeof(desc))((unsigned long)(desc) |
VRING_INDIRECT_PREMAPPED))
> +
> +#define desc_rm_dma_map(desc) \
> +       ((typeof(desc))((unsigned long)(desc) &
~VRING_INDIRECT_PREMAPPED))
> +
> +#define desc_map_inter(desc) \
> +       !((unsigned long)(desc) & VRING_INDIRECT_PREMAPPED)
> +
> +
>  #define to_vvq(_vq) container_of(_vq, struct vring_virtqueue, vq)
>
>  static inline bool virtqueue_use_indirect(struct vring_virtqueue *vq,
> @@ -725,7 +737,7 @@ static inline int virtqueue_add_split(struct virtqueue
*_vq,
>         /* Store token and indirect buffer state. */
>         vq->split.desc_state[head].data = data;
>         if (indirect)
> -               vq->split.desc_state[head].indir_desc = desc;
> +               vq->split.desc_state[head].indir_desc =
desc_mix_dma_map(do_map, desc);

So using indir_desc is kind of hacky (since we don't use indirect for
rx with extra context).

But at least I think we should seeka way to use the same metadata for
both direct and indirect descriptors.

E.g can we make them all to use indir_desc?

Thanks

>         else
>                 vq->split.desc_state[head].indir_desc = ctx;
>
> @@ -820,22 +832,26 @@ static void detach_buf_split(struct vring_virtqueue
*vq, unsigned int head,
>         vq->vq.num_free++;
>
>         if (vq->indirect) {
> -               struct vring_desc *indir_desc > -
vq->split.desc_state[head].indir_desc;
> +               struct vring_desc *mix =
vq->split.desc_state[head].indir_desc;
> +               struct vring_desc *indir_desc;
>                 u32 len;
>
>                 /* Free the indirect table, if any, now that it's
unmapped. */
> -               if (!indir_desc)
> +               if (!mix)
>                         return;
>
> +               indir_desc = desc_rm_dma_map(mix);
> +
>                 len = vq->split.desc_extra[head].len;
>
>                 BUG_ON(!(vq->split.desc_extra[head].flags &
>                                 VRING_DESC_F_INDIRECT));
>                 BUG_ON(len == 0 || len % sizeof(struct vring_desc));
>
> -               for (j = 0; j < len / sizeof(struct vring_desc); j++)
> -                       vring_unmap_one_split_indirect(vq,
&indir_desc[j]);
> +               if (desc_map_inter(mix)) {
> +                       for (j = 0; j < len / sizeof(struct vring_desc);
j++)
> +                               vring_unmap_one_split_indirect(vq,
&indir_desc[j]);
> +               }
>
>                 kfree(indir_desc);
>                 vq->split.desc_state[head].indir_desc = NULL;
> --
> 2.32.0.3.g01195cf9f
>