Re: [RFC v4 2/5] vhost: Write descriptors to packed svq

[Eugenio Perez Martin]

On Thu, Dec 5, 2024 at 9:35 PM Sahil Siddiq <icegambit91@gmail.com> wrote:
>
> This commit is the first in a series to add support for packed
> virtqueues in vhost_shadow_virtqueue.
>
> This patch implements the insertion of available buffers in the
> descriptor area. It takes into account descriptor chains, but does
> not consider indirect descriptors.
>
> Also validate svq-specific features that vdpa supports.
>
> Signed-off-by: Sahil Siddiq <sahilcdq@proton.me>
> ---
> Changes v3 -> v4:
> - Split commit #1 in v3 into 2 commits.
> - vhost-shadow-virtqueue.c
>   (vhost_svq_valid_features): Add enums.
>
>  hw/virtio/vhost-shadow-virtqueue.c | 83 +++++++++++++++++++++++++++++-
>  1 file changed, 81 insertions(+), 2 deletions(-)
>
> diff --git a/hw/virtio/vhost-shadow-virtqueue.c 
> b/hw/virtio/vhost-shadow-virtqueue.c
> index bb7cf6d5db..6eee01ab3c 100644
> --- a/hw/virtio/vhost-shadow-virtqueue.c
> +++ b/hw/virtio/vhost-shadow-virtqueue.c
> @@ -33,6 +33,9 @@ bool vhost_svq_valid_features(uint64_t features, Error 
> **errp)
>           ++b) {
>          switch (b) {
>          case VIRTIO_F_ANY_LAYOUT:
> +        case VIRTIO_F_RING_PACKED:
> +        case VIRTIO_F_RING_RESET:
> +        case VIRTIO_RING_F_INDIRECT_DESC:
>          case VIRTIO_RING_F_EVENT_IDX:

This is good, but it should be added in the last commit. Otherwise
we're enabling packed vq without the code to handle it.

The rest looks good to me.

>              continue;
>
> @@ -178,7 +181,78 @@ static void vhost_svq_add_split(VhostShadowVirtqueue 
> *svq,
>      /* Update the avail index after write the descriptor */
>      smp_wmb();
>      avail->idx = cpu_to_le16(svq->shadow_avail_idx);
> +}
>
> +/**
> + * Write descriptors to SVQ packed vring
> + *
> + * @svq: The shadow virtqueue
> + * @out_sg: The iovec to the guest
> + * @out_num: Outgoing iovec length
> + * @in_sg: The iovec from the guest
> + * @in_num: Incoming iovec length
> + * @sgs: Cache for hwaddr
> + * @head: Saves current free_head
> + */
> +static void vhost_svq_add_packed(VhostShadowVirtqueue *svq,
> +                                const struct iovec *out_sg, size_t out_num,
> +                                const struct iovec *in_sg, size_t in_num,
> +                                hwaddr *sgs, unsigned *head)
> +{
> +    uint16_t id, curr, i, head_flags = 0;
> +    size_t num = out_num + in_num;
> +    unsigned n;
> +
> +    struct vring_packed_desc *descs = svq->vring_packed.vring.desc;
> +
> +    *head = svq->vring_packed.next_avail_idx;
> +    i = *head;
> +    id = svq->free_head;
> +    curr = id;
> +
> +    /* Write descriptors to SVQ packed vring */
> +    for (n = 0; n < num; n++) {
> +        uint16_t flags = cpu_to_le16(svq->vring_packed.avail_used_flags |
> +                                     (n < out_num ? 0 : VRING_DESC_F_WRITE) |
> +                                     (n + 1 == num ? 0 : VRING_DESC_F_NEXT));
> +        if (i == *head) {
> +            head_flags = flags;
> +        } else {
> +            descs[i].flags = flags;
> +        }
> +
> +        descs[i].addr = cpu_to_le64(sgs[n]);
> +        descs[i].id = id;
> +        if (n < out_num) {
> +            descs[i].len = cpu_to_le32(out_sg[n].iov_len);
> +        } else {
> +            descs[i].len = cpu_to_le32(in_sg[n - out_num].iov_len);
> +        }
> +
> +        curr = cpu_to_le16(svq->desc_next[curr]);
> +
> +        if (++i >= svq->vring_packed.vring.num) {
> +            i = 0;
> +            svq->vring_packed.avail_used_flags ^=
> +                    1 << VRING_PACKED_DESC_F_AVAIL |
> +                    1 << VRING_PACKED_DESC_F_USED;
> +        }
> +    }
> +
> +    if (i <= *head) {
> +        svq->vring_packed.avail_wrap_counter ^= 1;
> +    }
> +
> +    svq->vring_packed.next_avail_idx = i;
> +    svq->free_head = curr;
> +
> +    /*
> +     * A driver MUST NOT make the first descriptor in the list
> +     * available before all subsequent descriptors comprising
> +     * the list are made available.
> +     */
> +    smp_wmb();
> +    svq->vring_packed.vring.desc[*head].flags = head_flags;
>  }
>
>  static void vhost_svq_kick(VhostShadowVirtqueue *svq)
> @@ -240,8 +314,13 @@ int vhost_svq_add(VhostShadowVirtqueue *svq, const 
> struct iovec *out_sg,
>          return -EINVAL;
>      }
>
> -    vhost_svq_add_split(svq, out_sg, out_num, in_sg,
> -                        in_num, sgs, &qemu_head);
> +    if (virtio_vdev_has_feature(svq->vdev, VIRTIO_F_RING_PACKED)) {
> +        vhost_svq_add_packed(svq, out_sg, out_num, in_sg,
> +                             in_num, sgs, &qemu_head);
> +    } else {
> +        vhost_svq_add_split(svq, out_sg, out_num, in_sg,
> +                            in_num, sgs, &qemu_head);
> +    }
>
>      svq->num_free -= ndescs;
>      svq->desc_state[qemu_head].elem = elem;
> --
> 2.47.0
>