Re: [RFC v2 1/3] vhost: Introduce packed vq and add buffer elements

[Eugenio Perez Martin]

On Fri, Jul 26, 2024 at 11:58 AM Sahil Siddiq <icegambit91@gmail.com> wrote:
>
> This is the first patch 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.
>
> Signed-off-by: Sahil Siddiq <sahilcdq@proton.me>
> ---
> Changes v1 -> v2:
> * Split commit from RFC v1 into two commits.
> * vhost-shadow-virtqueue.c
>   (vhost_svq_add_packed):
>   - Merge with "vhost_svq_vring_write_descs_packed()"
>   - Remove "num == 0" check
>
>  hw/virtio/vhost-shadow-virtqueue.c | 93 +++++++++++++++++++++++++++++-
>  1 file changed, 92 insertions(+), 1 deletion(-)
>
> diff --git a/hw/virtio/vhost-shadow-virtqueue.c 
> b/hw/virtio/vhost-shadow-virtqueue.c
> index fc5f408f77..c7b7e0c477 100644
> --- a/hw/virtio/vhost-shadow-virtqueue.c
> +++ b/hw/virtio/vhost-shadow-virtqueue.c
> @@ -217,6 +217,91 @@ static bool vhost_svq_add_split(VhostShadowVirtqueue 
> *svq,
>      return true;
>  }
>
> +static bool vhost_svq_add_packed(VhostShadowVirtqueue *svq,
> +                                const struct iovec *out_sg, size_t out_num,
> +                                const struct iovec *in_sg, size_t in_num,
> +                                unsigned *head)
> +{
> +    bool ok;
> +    uint16_t head_flags = 0;
> +    g_autofree hwaddr *sgs = g_new(hwaddr, out_num + in_num);
> +
> +    *head = svq->vring_packed.next_avail_idx;
> +
> +    /* We need some descriptors here */
> +    if (unlikely(!out_num && !in_num)) {
> +        qemu_log_mask(LOG_GUEST_ERROR,
> +                      "Guest provided element with no descriptors");
> +        return false;
> +    }
> +
> +    uint16_t id, curr, i;
> +    unsigned n;
> +    struct vring_packed_desc *descs = svq->vring_packed.vring.desc;
> +
> +    i = *head;
> +    id = svq->free_head;
> +    curr = id;
> +
> +    size_t num = out_num + in_num;
> +
> +    ok = vhost_svq_translate_addr(svq, sgs, out_sg, out_num);
> +    if (unlikely(!ok)) {
> +        return false;
> +    }
> +
> +    ok = vhost_svq_translate_addr(svq, sgs + out_num, in_sg, in_num);
> +    if (unlikely(!ok)) {
> +        return false;
> +    }
> +

(sorry I missed this from the RFC v1) I think all of the above should
be in the caller, isn't it? It is duplicated with split.

Also, declarations should be at the beginning of blocks per QEMU
coding style [1].

The rest looks good to me by visual inspection.

> +    /* 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;
> +
> +    return true;
> +}
> +
>  static void vhost_svq_kick(VhostShadowVirtqueue *svq)
>  {
>      bool needs_kick;
> @@ -258,7 +343,13 @@ int vhost_svq_add(VhostShadowVirtqueue *svq, const 
> struct iovec *out_sg,
>          return -ENOSPC;
>      }
>
> -    ok = vhost_svq_add_split(svq, out_sg, out_num, in_sg, in_num, 
> &qemu_head);
> +    if (virtio_vdev_has_feature(svq->vdev, VIRTIO_F_RING_PACKED)) {
> +        ok = vhost_svq_add_packed(svq, out_sg, out_num,
> +                                  in_sg, in_num, &qemu_head);
> +    } else {
> +        ok = vhost_svq_add_split(svq, out_sg, out_num,
> +                                 in_sg, in_num, &qemu_head);
> +    }
>      if (unlikely(!ok)) {
>          return -EINVAL;
>      }
> --
> 2.45.2
>

[1] https://www.qemu.org/docs/master/devel/style.html#declarations