Re: [PATCH] virtio: use shadow_avail_idx while checking number of heads

[Stefan Hajnoczi]

On Fri, 25 Aug 2023 at 13:04, Ilya Maximets <i.maximets@ovn.org> wrote:
>
> We do not need the most up to date number of heads, we only want to
> know if there is at least one.
>
> Use shadow variable as long as it is not equal to the last available
> index checked.  This avoids expensive qatomic dereference of the
> RCU-protected memory region cache as well as the memory access itself
> and the subsequent memory barrier.
>
> The change improves performance of the af-xdp network backend by 2-3%.
>
> Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
> ---
>  hw/virtio/virtio.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index 309038fd46..04bf7cc977 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -999,7 +999,15 @@ void virtqueue_push(VirtQueue *vq, const 
> VirtQueueElement *elem,
>  /* Called within rcu_read_lock().  */
>  static int virtqueue_num_heads(VirtQueue *vq, unsigned int idx)
>  {
> -    uint16_t num_heads = vring_avail_idx(vq) - idx;
> +    uint16_t num_heads;
> +
> +    if (vq->shadow_avail_idx != idx) {
> +        num_heads = vq->shadow_avail_idx - idx;
> +
> +        return num_heads;

This still needs to check num_heads > vq->vring.num and return -EINVAL
as is done below.

> +    }
> +
> +    num_heads = vring_avail_idx(vq) - idx;
>
>      /* Check it isn't doing very strange things with descriptor numbers. */
>      if (num_heads > vq->vring.num) {
> --
> 2.40.1
>
>