Re: [RFC v4 1/5] vhost: Refactor vhost_svq_add_split

[Eugenio Perez Martin]

On Thu, Dec 5, 2024 at 9:35 PM Sahil Siddiq <icegambit91@gmail.com> wrote:
>
> This commit refactors vhost_svq_add_split and
> vhost_svq_add to simplify their implementation
> and prepare for the addition of packed vqs in
> following commits.
>

Acked-by: Eugenio Pérez <eperezma@redhat.com>

> Signed-off-by: Sahil Siddiq <sahilcdq@proton.me>
> ---
> Changes v3 -> v4:
> - Split commit #1 in v3 into 2 commits.
> - Changes related to "vhost_svq_add_packed" are
>   now in commit #2.
>
>  hw/virtio/vhost-shadow-virtqueue.c | 102 ++++++++++++-----------------
>  1 file changed, 41 insertions(+), 61 deletions(-)
>
> diff --git a/hw/virtio/vhost-shadow-virtqueue.c 
> b/hw/virtio/vhost-shadow-virtqueue.c
> index 37aca8b431..bb7cf6d5db 100644
> --- a/hw/virtio/vhost-shadow-virtqueue.c
> +++ b/hw/virtio/vhost-shadow-virtqueue.c
> @@ -124,83 +124,48 @@ static bool vhost_svq_translate_addr(const 
> VhostShadowVirtqueue *svq,
>  }
>
>  /**
> - * Write descriptors to SVQ vring
> + * Write descriptors to SVQ split vring
>   *
>   * @svq: The shadow virtqueue
> - * @sg: Cache for hwaddr
> - * @iovec: The iovec from the guest
> - * @num: iovec length
> - * @more_descs: True if more descriptors come in the chain
> - * @write: True if they are writeable descriptors
> - *
> - * Return true if success, false otherwise and print error.
> + * @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 bool vhost_svq_vring_write_descs(VhostShadowVirtqueue *svq, hwaddr 
> *sg,
> -                                        const struct iovec *iovec, size_t 
> num,
> -                                        bool more_descs, bool write)
> +static void vhost_svq_add_split(VhostShadowVirtqueue *svq,
> +                                const struct iovec *out_sg, size_t out_num,
> +                                const struct iovec *in_sg, size_t in_num,
> +                                hwaddr *sgs, unsigned *head)
>  {
> +    unsigned avail_idx, n;
>      uint16_t i = svq->free_head, last = svq->free_head;
> -    unsigned n;
> -    uint16_t flags = write ? cpu_to_le16(VRING_DESC_F_WRITE) : 0;
> +    vring_avail_t *avail = svq->vring.avail;
>      vring_desc_t *descs = svq->vring.desc;
> -    bool ok;
> -
> -    if (num == 0) {
> -        return true;
> -    }
> +    size_t num = in_num + out_num;
>
> -    ok = vhost_svq_translate_addr(svq, sg, iovec, num);
> -    if (unlikely(!ok)) {
> -        return false;
> -    }
> +    *head = svq->free_head;
>
>      for (n = 0; n < num; n++) {
> -        if (more_descs || (n + 1 < num)) {
> -            descs[i].flags = flags | cpu_to_le16(VRING_DESC_F_NEXT);
> +        descs[i].flags = cpu_to_le16(n < out_num ? 0 : VRING_DESC_F_WRITE);
> +        if (n + 1 < num) {
> +            descs[i].flags |= cpu_to_le16(VRING_DESC_F_NEXT);
>              descs[i].next = cpu_to_le16(svq->desc_next[i]);
> +        }
> +
> +        descs[i].addr = cpu_to_le64(sgs[n]);
> +        if (n < out_num) {
> +            descs[i].len = cpu_to_le32(out_sg[n].iov_len);
>          } else {
> -            descs[i].flags = flags;
> +            descs[i].len = cpu_to_le32(in_sg[n - out_num].iov_len);
>          }
> -        descs[i].addr = cpu_to_le64(sg[n]);
> -        descs[i].len = cpu_to_le32(iovec[n].iov_len);
>
>          last = i;
>          i = cpu_to_le16(svq->desc_next[i]);
>      }
>
>      svq->free_head = le16_to_cpu(svq->desc_next[last]);
> -    return true;
> -}
> -
> -static bool vhost_svq_add_split(VhostShadowVirtqueue *svq,
> -                                const struct iovec *out_sg, size_t out_num,
> -                                const struct iovec *in_sg, size_t in_num,
> -                                unsigned *head)
> -{
> -    unsigned avail_idx;
> -    vring_avail_t *avail = svq->vring.avail;
> -    bool ok;
> -    g_autofree hwaddr *sgs = g_new(hwaddr, MAX(out_num, in_num));
> -
> -    *head = svq->free_head;
> -
> -    /* 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;
> -    }
> -
> -    ok = vhost_svq_vring_write_descs(svq, sgs, out_sg, out_num, in_num > 0,
> -                                     false);
> -    if (unlikely(!ok)) {
> -        return false;
> -    }
> -
> -    ok = vhost_svq_vring_write_descs(svq, sgs, in_sg, in_num, false, true);
> -    if (unlikely(!ok)) {
> -        return false;
> -    }
>
>      /*
>       * Put the entry in the available array (but don't update avail->idx 
> until
> @@ -214,7 +179,6 @@ static bool vhost_svq_add_split(VhostShadowVirtqueue *svq,
>      smp_wmb();
>      avail->idx = cpu_to_le16(svq->shadow_avail_idx);
>
> -    return true;
>  }
>
>  static void vhost_svq_kick(VhostShadowVirtqueue *svq)
> @@ -254,15 +218,31 @@ int vhost_svq_add(VhostShadowVirtqueue *svq, const 
> struct iovec *out_sg,
>      unsigned ndescs = in_num + out_num;
>      bool ok;
>
> +    /* We need some descriptors here */
> +    if (unlikely(!ndescs)) {
> +        qemu_log_mask(LOG_GUEST_ERROR,
> +                      "Guest provided element with no descriptors");
> +        return -EINVAL;
> +    }
> +
>      if (unlikely(ndescs > vhost_svq_available_slots(svq))) {
>          return -ENOSPC;
>      }
>
> -    ok = vhost_svq_add_split(svq, out_sg, out_num, in_sg, in_num, 
> &qemu_head);
> +    g_autofree hwaddr *sgs = g_new(hwaddr, ndescs);
> +    ok = vhost_svq_translate_addr(svq, sgs, out_sg, out_num);
>      if (unlikely(!ok)) {
>          return -EINVAL;
>      }
>
> +    ok = vhost_svq_translate_addr(svq, sgs + out_num, in_sg, in_num);
> +    if (unlikely(!ok)) {
> +        return -EINVAL;
> +    }
> +
> +    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;
>      svq->desc_state[qemu_head].ndescs = ndescs;
> --
> 2.47.0
>