Re: [PATCH 3/6] virtio: virtqueue_ordered_fill - VIRTIO_F_IN_ORDER support

[Eugenio Perez Martin]

On Mon, May 6, 2024 at 5:05 PM Jonah Palmer <jonah.palmer@oracle.com> wrote:
>
> Add VIRTIO_F_IN_ORDER feature support for virtqueue_fill operations.
>
> The goal of the virtqueue_fill operation when the VIRTIO_F_IN_ORDER
> feature has been negotiated is to search for this now-used element,
> set its length, and mark the element as filled in the VirtQueue's
> used_elems array.
>
> By marking the element as filled, it will indicate that this element is
> ready to be flushed, so long as the element is in-order.
>
> Tested-by: Lei Yang <leiyang@redhat.com>
> Signed-off-by: Jonah Palmer <jonah.palmer@oracle.com>
> ---
>  hw/virtio/virtio.c | 26 +++++++++++++++++++++++++-
>  1 file changed, 25 insertions(+), 1 deletion(-)
>
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index e6eb1bb453..064046b5e2 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -873,6 +873,28 @@ static void virtqueue_packed_fill(VirtQueue *vq, const 
> VirtQueueElement *elem,
>      vq->used_elems[idx].ndescs = elem->ndescs;
>  }
>
> +static void virtqueue_ordered_fill(VirtQueue *vq, const VirtQueueElement 
> *elem,
> +                                   unsigned int len)
> +{
> +    unsigned int i = vq->used_idx;
> +
> +    /* Search for element in vq->used_elems */
> +    while (i != vq->last_avail_idx) {
> +        /* Found element, set length and mark as filled */
> +        if (vq->used_elems[i].index == elem->index) {
> +            vq->used_elems[i].len = len;
> +            vq->used_elems[i].filled = true;
> +            break;
> +        }
> +
> +        i += vq->used_elems[i].ndescs;
> +
> +        if (i >= vq->vring.num) {
> +            i -= vq->vring.num;
> +        }
> +    }

This has a subtle problem: ndescs and elems->id are controlled by the
guest, so it could make QEMU to loop forever looking for the right
descriptor. For each iteration, the code must control that the
variable "i" will be different for the next iteration, and that there
will be no more than vq->last_avail_idx - vq->used_idx iterations.

Apart of that, I think it makes more sense to split the logical
sections of the function this way:
/* declarations */
i = vq->used_idx

/* Search for element in vq->used_elems */
while (vq->used_elems[i].index != elem->index &&
vq->used_elems[i].index i != vq->last_avail_idx && ...) {
...
}

/* Set length and mark as filled */
vq->used_elems[i].len = len;
vq->used_elems[i].filled = true;
---

But I'm ok either way.

> +}
> +
>  static void virtqueue_packed_fill_desc(VirtQueue *vq,
>                                         const VirtQueueElement *elem,
>                                         unsigned int idx,
> @@ -923,7 +945,9 @@ void virtqueue_fill(VirtQueue *vq, const VirtQueueElement 
> *elem,
>          return;
>      }
>
> -    if (virtio_vdev_has_feature(vq->vdev, VIRTIO_F_RING_PACKED)) {
> +    if (virtio_vdev_has_feature(vq->vdev, VIRTIO_F_IN_ORDER)) {
> +        virtqueue_ordered_fill(vq, elem, len);
> +    } else if (virtio_vdev_has_feature(vq->vdev, VIRTIO_F_RING_PACKED)) {
>          virtqueue_packed_fill(vq, elem, len, idx);
>      } else {
>          virtqueue_split_fill(vq, elem, len, idx);
> --
> 2.39.3
>