Re: [PATCH v2 09/19] vhost: Track number of descs in SVQElement

[Jason Wang]

On Fri, Jul 15, 2022 at 12:32 AM Eugenio Pérez <eperezma@redhat.com> wrote:
>
> Since CVQ will be able to modify elements, the number of descriptors in
> the guest may not match with the number of descriptors exposed. Track
> separately.
>
> Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> ---
>  hw/virtio/vhost-shadow-virtqueue.h |  6 ++++++
>  hw/virtio/vhost-shadow-virtqueue.c | 10 +++++-----
>  2 files changed, 11 insertions(+), 5 deletions(-)
>
> diff --git a/hw/virtio/vhost-shadow-virtqueue.h 
> b/hw/virtio/vhost-shadow-virtqueue.h
> index f35d4b8f90..143c86a568 100644
> --- a/hw/virtio/vhost-shadow-virtqueue.h
> +++ b/hw/virtio/vhost-shadow-virtqueue.h
> @@ -17,6 +17,12 @@
>
>  typedef struct SVQElement {
>      VirtQueueElement elem;
> +
> +    /*
> +     * Number of descriptors exposed to the device. May or may not match
> +     * guest's
> +     */
> +    unsigned int ndescs;
>  } SVQElement;

Can we simplify things furtherly by moving ndscs into a dedicated array at svq?

Then we don't need to bother with introducing SVQElement.

Thanks

>
>  /* Shadow virtqueue to relay notifications */
> diff --git a/hw/virtio/vhost-shadow-virtqueue.c 
> b/hw/virtio/vhost-shadow-virtqueue.c
> index 442ca3cbd3..3b112c4ec8 100644
> --- a/hw/virtio/vhost-shadow-virtqueue.c
> +++ b/hw/virtio/vhost-shadow-virtqueue.c
> @@ -243,10 +243,10 @@ static int vhost_svq_add(VhostShadowVirtqueue *svq, 
> const struct iovec *out_sg,
>                            size_t in_num, SVQElement *svq_elem)
>  {
>      unsigned qemu_head;
> -    unsigned ndescs = in_num + out_num;
> +    svq_elem->ndescs = in_num + out_num;
>      bool ok;
>
> -    if (unlikely(ndescs > vhost_svq_available_slots(svq))) {
> +    if (unlikely(svq_elem->ndescs > vhost_svq_available_slots(svq))) {
>          return -ENOSPC;
>      }
>
> @@ -393,7 +393,7 @@ static SVQElement *vhost_svq_get_buf(VhostShadowVirtqueue 
> *svq,
>      SVQElement *elem;
>      const vring_used_t *used = svq->vring.used;
>      vring_used_elem_t used_elem;
> -    uint16_t last_used, last_used_chain, num;
> +    uint16_t last_used, last_used_chain;
>
>      if (!vhost_svq_more_used(svq)) {
>          return NULL;
> @@ -420,8 +420,8 @@ static SVQElement *vhost_svq_get_buf(VhostShadowVirtqueue 
> *svq,
>      }
>
>      elem = svq->ring_id_maps[used_elem.id];
> -    num = elem->elem.in_num + elem->elem.out_num;
> -    last_used_chain = vhost_svq_last_desc_of_chain(svq, num, used_elem.id);
> +    last_used_chain = vhost_svq_last_desc_of_chain(svq, elem->ndescs,
> +                                                   used_elem.id);
>      svq->desc_next[last_used_chain] = svq->free_head;
>      svq->free_head = used_elem.id;
>
> --
> 2.31.1
>