Re: [RFC v3 1/3] vhost: Introduce packed vq and add buffer elements

[Eugenio Perez Martin]

On Fri, Aug 2, 2024 at 1:22 PM 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 v2 -> v3:
> * vhost-shadow-virtqueue.c
>   - Move parts common to "vhost_svq_add_split" and
>     "vhost_svq_add_packed" to "vhost_svq_add".
>   (vhost_svq_add_packed):
>   - Refactor to minimize duplicate code between
>     this and "vhost_svq_add_split"
>   - Fix code style issues.
>   (vhost_svq_add_split):
>   - Merge with "vhost_svq_vring_write_descs()"
>   - Refactor to minimize duplicate code between
>     this and "vhost_svq_add_packed"
>   (vhost_svq_add):
>   - Refactor to minimize duplicate code between
>     split and packed version of "vhost_svq_add"
>
>  hw/virtio/vhost-shadow-virtqueue.c | 174 +++++++++++++++++++----------
>  1 file changed, 115 insertions(+), 59 deletions(-)
>
> diff --git a/hw/virtio/vhost-shadow-virtqueue.c 
> b/hw/virtio/vhost-shadow-virtqueue.c
> index fc5f408f77..4c308ee53d 100644
> --- a/hw/virtio/vhost-shadow-virtqueue.c
> +++ b/hw/virtio/vhost-shadow-virtqueue.c
> @@ -124,97 +124,132 @@ 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;
> +
> +    /*
> +     * Put the entry in the available array (but don't update avail->idx 
> until
> +     * they do sync).
> +     */
> +    avail_idx = svq->shadow_avail_idx & (svq->vring.num - 1);
> +    avail->ring[avail_idx] = cpu_to_le16(*head);
> +    svq->shadow_avail_idx++;
> +
> +    /* Update the avail index after write the descriptor */
> +    smp_wmb();
> +    avail->idx = cpu_to_le16(svq->shadow_avail_idx);
>  }
>

I think this code is already in a very good shape. But actual testing
is needed before acks.

As a suggestion, we can split it into:
1) Refactor in vhost_svq_translate_addr to support out_num+in_num. No
functional change.
2) Refactor vhost_svq_add_split to extract common code into
vhost_svq_add. No functional change.
3) Adding packed code.

How to split or merge the patches is not a well-defined thing, so I'm
happy with this patch if you think the reactor is not worth it.

> -static bool vhost_svq_add_split(VhostShadowVirtqueue *svq,
> +/**
> + * 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,
> -                                unsigned *head)
> +                                hwaddr *sgs, 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));
> +    uint16_t id, curr, i, head_flags = 0;
> +    size_t num = out_num + in_num;
> +    unsigned n;
>
> -    *head = svq->free_head;
> +    struct vring_packed_desc *descs = svq->vring_packed.vring.desc;
>
> -    /* 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;
> -    }
> +    *head = svq->vring_packed.next_avail_idx;
> +    i = *head;
> +    id = svq->free_head;
> +    curr = id;
>
> -    ok = vhost_svq_vring_write_descs(svq, sgs, out_sg, out_num, in_num > 0,
> -                                     false);
> -    if (unlikely(!ok)) {
> -        return false;
> +    /* 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;
> +        }
>      }
>
> -    ok = vhost_svq_vring_write_descs(svq, sgs, in_sg, in_num, false, true);
> -    if (unlikely(!ok)) {
> -        return false;
> +    if (i <= *head) {
> +        svq->vring_packed.avail_wrap_counter ^= 1;
>      }
>
> +    svq->vring_packed.next_avail_idx = i;
> +    svq->free_head = curr;
> +
>      /*
> -     * Put the entry in the available array (but don't update avail->idx 
> until
> -     * they do sync).
> +     * A driver MUST NOT make the first descriptor in the list
> +     * available before all subsequent descriptors comprising
> +     * the list are made available.
>       */
> -    avail_idx = svq->shadow_avail_idx & (svq->vring.num - 1);
> -    avail->ring[avail_idx] = cpu_to_le16(*head);
> -    svq->shadow_avail_idx++;
> -
> -    /* Update the avail index after write the descriptor */
>      smp_wmb();
> -    avail->idx = cpu_to_le16(svq->shadow_avail_idx);
> -
> -    return true;
> +    svq->vring_packed.vring.desc[*head].flags = head_flags;
>  }
>
>  static void vhost_svq_kick(VhostShadowVirtqueue *svq)
> @@ -254,15 +289,36 @@ 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;
> +    }
> +
> +    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;
>      svq->desc_state[qemu_head].ndescs = ndescs;
> --
> 2.45.2
>