On 8/25/23 19:04, Ilya Maximets 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.maxim...@ovn.org>
> ---
Kind reminder.
Best regards, Ilya Maximets.
> 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;
> + }
> +
> + num_heads = vring_avail_idx(vq) - idx;
>
> /* Check it isn't doing very strange things with descriptor numbers. */
> if (num_heads > vq->vring.num) {
