Re: [PATCH 4/6] virtio: virtqueue_ordered_flush - VIRTIO_F_IN_ORDER support

[Eugenio Perez Martin]

On Mon, May 6, 2024 at 5:06 PM Jonah Palmer <jonah.palmer@oracle.com> wrote:
>
> Add VIRTIO_F_IN_ORDER feature support for virtqueue_flush operations.
>
> The goal of the virtqueue_flush operation when the VIRTIO_F_IN_ORDER
> feature has been negotiated is to write elements to the used/descriptor
> ring in-order and then update used_idx.
>
> The function iterates through the VirtQueueElement used_elems array
> in-order starting at vq->used_idx. If the element is valid (filled), the
> element is written to the used/descriptor ring. This process continues
> until we find an invalid (not filled) element.
>
> If any elements were written, the used_idx is updated.
>
> Tested-by: Lei Yang <leiyang@redhat.com>
> Signed-off-by: Jonah Palmer <jonah.palmer@oracle.com>
> ---
>  hw/virtio/virtio.c | 75 +++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 74 insertions(+), 1 deletion(-)
>
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index 064046b5e2..0efed2c88e 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -1006,6 +1006,77 @@ static void virtqueue_packed_flush(VirtQueue *vq, 
> unsigned int count)
>      }
>  }
>
> +static void virtqueue_ordered_flush(VirtQueue *vq)
> +{
> +    unsigned int i = vq->used_idx;
> +    unsigned int ndescs = 0;
> +    uint16_t old = vq->used_idx;
> +    bool packed;
> +    VRingUsedElem uelem;
> +
> +    packed = virtio_vdev_has_feature(vq->vdev, VIRTIO_F_RING_PACKED);
> +
> +    if (packed) {
> +        if (unlikely(!vq->vring.desc)) {
> +            return;
> +        }
> +    } else if (unlikely(!vq->vring.used)) {
> +        return;
> +    }
> +
> +    /* First expected in-order element isn't ready, nothing to do */
> +    if (!vq->used_elems[i].filled) {
> +        return;
> +    }
> +
> +    /* Write first expected in-order element to used ring (split VQs) */
> +    if (!packed) {
> +        uelem.id = vq->used_elems[i].index;
> +        uelem.len = vq->used_elems[i].len;
> +        vring_used_write(vq, &uelem, i);
> +    }
> +
> +    ndescs += vq->used_elems[i].ndescs;
> +    i += ndescs;
> +    if (i >= vq->vring.num) {
> +        i -= vq->vring.num;
> +    }
> +
> +    /* Search for more filled elements in-order */
> +    while (vq->used_elems[i].filled) {
> +        if (packed) {
> +            virtqueue_packed_fill_desc(vq, &vq->used_elems[i], ndescs, 
> false);
> +        } else {
> +            uelem.id = vq->used_elems[i].index;
> +            uelem.len = vq->used_elems[i].len;
> +            vring_used_write(vq, &uelem, i);
> +        }
> +
> +        vq->used_elems[i].filled = false;
> +        ndescs += vq->used_elems[i].ndescs;
> +        i += ndescs;
> +        if (i >= vq->vring.num) {
> +            i -= vq->vring.num;
> +        }
> +    }
> +

I may be missing something, but you have split out the first case as a
special one, totally out of the while loop. Can't it be contained in
the loop checking !(packed && i == vq->used_idx)? That would avoid
code duplication.

A comment can be added in the line of "first entry of packed is
written the last so the guest does not see invalid descriptors".

> +    if (packed) {
> +        virtqueue_packed_fill_desc(vq, &vq->used_elems[vq->used_idx], 0, 
> true);
> +        vq->used_idx += ndescs;
> +        if (vq->used_idx >= vq->vring.num) {
> +            vq->used_idx -= vq->vring.num;
> +            vq->used_wrap_counter ^= 1;
> +            vq->signalled_used_valid = false;
> +        }
> +    } else {
> +        vring_used_idx_set(vq, i);
> +        if (unlikely((int16_t)(i - vq->signalled_used) < (uint16_t)(i - 
> old))) {
> +            vq->signalled_used_valid = false;
> +        }
> +    }
> +    vq->inuse -= ndescs;
> +}
> +
>  void virtqueue_flush(VirtQueue *vq, unsigned int count)
>  {
>      if (virtio_device_disabled(vq->vdev)) {
> @@ -1013,7 +1084,9 @@ void virtqueue_flush(VirtQueue *vq, unsigned int count)
>          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_flush(vq);
> +    } else if (virtio_vdev_has_feature(vq->vdev, VIRTIO_F_RING_PACKED)) {
>          virtqueue_packed_flush(vq, count);
>      } else {
>          virtqueue_split_flush(vq, count);
> --
> 2.39.3
>